The disease attacks the brain, leaving some victims in a statue-like condition, speechless and motionless. Between 1915 and 1926, an epidemic
of encephalitis lethargica spread around the world. Nearly five million
people were affected, a third of whom died in the acute stages. Many of
those who survived never returned to their pre-existing "aliveness".
They
would be conscious and aware – yet not fully awake; they would sit
motionless and speechless all day in their chairs, totally lacking
energy, impetus, initiative, motive, appetite, affect or desire; they
registered what went on about them without active attention, and with
profound indifference. They neither conveyed nor felt the feeling of
life; they were as insubstantial as ghosts, and as passive as zombies.
No recurrence of the epidemic has since been reported, though isolated cases continue to occur.
Signs and symptoms
Encephalitis lethargica is characterized by high fever, sore throat, headache, lethargy, double vision, delayed physical and mental response, sleep inversion and catatonia. In severe cases, patients may enter a coma-like state (akinetic mutism). Patients may also experience abnormal eye movements ("oculogyric crises"), Parkinsonism, upper body weakness, muscular pains, tremors, neck rigidity, and behavioral changes including psychosis. Klazomania (a vocal tic) is sometimes present.
Cause
Encephalitis lethargica. Its sequelae and treatment - Constantin Von Economo, 1931: front page
The causes of encephalitis lethargica are uncertain.
Some studies have explored its origins in an autoimmune response, and, separately or in relation to an immune response, links to pathologies of infectious disease — viral and bacterial, e.g., in the case of influenza, where a link with encephalitis is clear. Postencephalitic parkinsonism was clearly documented to have followed an outbreak of encephalitis lethargica following 1918 influenza pandemic; evidence for viral causation of the Parkinson's symptoms is circumstantial (epidemiologic, and finding influenza antigens in encephalitis lethargica patients), while evidence arguing against this cause is of the negative sort (e.g., lack of viral RNA in postencephalitic parkinsonian brain material).
In reviewing the relationship between influenza and encephalitis
lethargica (EL), McCall and coworkers conclude, as of 2008, that while
"the case against influenza [is] less decisive than currently perceived…
there is little direct evidence supporting influenza in the etiology of
EL," and that "[a]lmost 100 years after the EL epidemic, its etiology
remains enigmatic."
Hence, while opinions on the relationship of encephalitis lethargica to
influenza remain divided, the preponderance of literature appears
skeptical.
German neurologist Felix Stern, who examined hundreds of
encephalitis lethargica patients during the 1920s, pointed out that the
encephalitis lethargica typically evolved over time. The early symptom
would be dominated by sleepiness or wakefulness. A second symptom would
lead to an oculogyric crisis.
The third symptom would be recovery, followed by a Parkinson-like
symptom. If patients of Stern followed this course of disease, he
diagnosed them with encephalitis lethargica. Stern suspected
encephalitis lethargica to be close to polio without evidence.
Nevertheless, he experimented with the convalescent serum of survivors
of the first acute symptom. He vaccinated patients with early stage
symptoms and told them that it might be successful. Stern is author of
the 1920s definitive book Die Epidemische Encephalitis (1920 and 2nd ed. 1928). Stern was driven to suicide during the Holocaust by the German state, his research forgotten.
In 2010, in a substantial Oxford University Press compendium reviewing
the historic and contemporary views on EL, its editor, Joel Vilensky of
the Indiana University School of Medicine,
quotes Pool, writing in 1930, who states, "we must confess that
etiology is still obscure, the causative agent still unknown, the
pathological riddle still unsolved…", and goes on to offer the following
conclusion, as of that publication date:
Does
the present volume solve the "riddle" of EL, which… has been referred
to as the greatest medical mystery of the 20th century? Unfortunately,
no: but inroads are certainly made here pertaining to diagnosis,
pathology, and even treatment."
Subsequent to publication of this compendium, an enterovirus was discovered in encephalitis lethargica cases from the epidemic. In 2012, Oliver Sacks acknowledged this virus as the probable cause of the disease. Other sources have suggested Diplococcus as a cause.
Diagnosis
There
have been several proposed diagnostic criteria for encephalitis
lethargica. One, which has been widely accepted, includes an acute or
subacute encephalitic illness where all other known causes of
encephalitis have been excluded. Another diagnostic criterion, suggested
more recently, says that the diagnosis of encephalitis lethargica "may
be considered if the patient’s condition cannot be attributed to any
other known neurological condition and that they show the following
signs: Influenza-like signs; hypersomnolence (hypersomnia), wakeability, ophthalmoplegia (paralysis of the muscles that control the movement of the eye), and psychiatric changes."
The Great Encephalitis Pandemic (1915-1926)
In the winter of 1916–1917, a "new" illness suddenly appeared in Vienna
and other cities, and rapidly spread world-wide over the next three
years. Earlier reports appeared throughout Europe as early as the winter
of 1915–1916, but communication about the disease was slow and chaotic,
given the varied manifestation of symptoms and difficulties
disseminating information in wartime. Until Constantin von Economo identified a unique pattern of damage among the brains of deceased patients and introduced the unifying name encephalitis lethargica, reports of the protean disease came in under a range of names: botulism, toxic ophthalmoplegia, epidemic stupor, epidemic lethargic encephalitis, acute polioencephalitis, Heine-Medin disease, bulbar paralysis, hystero-epilepsy, acute dementia, and sometimes just "an obscure disease with cerebral symptoms." Just ten days before von Economo's breakthrough in Vienna, Jean-René Cruchet described forty cases of "subacute encephalomyelitis" in France.
In the ten years that the pandemic raged, nearly five million
people's lives were taken or ravaged. Encephalitis lethargica assumed
its most virulent form between October 1918 and January 1919. The
pandemic disappeared in 1927 as abruptly and mysteriously as it first
appeared. The great encephalitis pandemic coincided with the 1918 influenza pandemic,
and it is likely that the influenza virus potentiated the effects of
the encephalitis virus or lowered resistance to it in a catastrophic
way.
Aftermath of the Pandemic (1927-1967)
In the aftermath of the pandemic
(between 1927 and 1967), many surviving patients seemed to make a
complete recovery and return to their normal lives. However, the
majority of them subsequently developed neurological or psychiatric
disorders, often after years or decades of seemingly perfect health.
Post-encephalitic syndromes varied widely: sometimes they proceeded
rapidly, leading to profound disability or death; sometimes very slowly;
sometimes they progressed to a certain point and then stayed at this
point for years or decades; and sometimes, following their initial
onslaught, they remitted and disappeared.
Modern treatment approaches to encephalitis lethargica include
immunomodulating therapies, and treatments to remediate specific
symptoms.
There is little evidence so far of a consistent effective treatment for the initial stages, though some patients given steroids have seen improvement. The disease becomes progressive, with evidence of brain damage similar to Parkinson's disease.
Treatment is then symptomatic. Levodopa (L-DOPA) and other anti-Parkinson drugs often produce dramatic responses; however, most people given L-DOPA experience improvements that are short lived.
Notable cases
Notable cases include:
Muriel "Kit" Richardson (née Hewitt), first wife of actor Sir Ralph Richardson, died of the condition in October 1942, having first shown symptoms in 1927–28.
There is speculation that Adolf Hitler
may have had encephalitis lethargica when he was a young adult (in
addition to the more substantial case for Parkinsonism in his later
years).
Mervyn Peake (1911–1968), author of the Gormenghast
books, began his decline towards death which was initially attributed
to encephalitis lethargica with Parkinson's disease-like symptoms,
although others have later suggested his decline in health and eventual
death may have been due to Lewy body dementia.
Insects or Insecta (from Latininsectum) are hexapodinvertebrates and the largest group within the arthropodphylum. Definitions and circumscriptions vary; usually, insects comprise a class within the Arthropoda. As used here, the term Insecta is synonymous with Ectognatha. Insects have a chitinousexoskeleton, a three-part body (head, thorax and abdomen), three pairs of jointed legs, compound eyes and one pair of antennae. Insects are the most diverse group of animals; they include more than a million described species and represent more than half of all known living organisms. The total number of extant species is estimated at between six and ten million; potentially over 90% of the animal life forms on Earth are insects. Insects may be found in nearly all environments, although only a small number of species reside in the oceans, which are dominated by another arthropod group, crustaceans.
Nearly all insects hatch from eggs. Insect growth is constrained by the inelastic exoskeleton and development involves a series of molts. The immature stages often differ from the adults in structure, habit and habitat, and can include a passive pupal stage in those groups that undergo four-stage metamorphosis. Insects that undergo three-stage metamorphosis lack a pupal stage and adults develop through a series of nymphal stages. The higher level relationship of the insects is unclear. Fossilized insects of enormous size have been found from the Paleozoic Era, including giant dragonflies with wingspans of 55 to 70 cm (22 to 28 in). The most diverse insect groups appear to have coevolved with flowering plants.
Adult insects typically move about by walking, flying, or
sometimes swimming. As it allows for rapid yet stable movement, many
insects adopt a tripedal gait in which they walk with their legs
touching the ground in alternating triangles, composed of the front
& rear on one side with the middle on the other side. Insects are
the only invertebrates to have evolved flight, and all flying insects
derive from one common ancestor. Many insects spend at least part of
their lives under water, with larval adaptations that include gills, and some adult insects are aquatic and have adaptations for swimming. Some species, such as water striders, are capable of walking on the surface of water. Insects are mostly solitary, but some, such as certain bees, ants and termites, are social and live in large, well-organized colonies. Some insects, such as earwigs, show maternal care, guarding their eggs and young. Insects can communicate with each other in a variety of ways. Male moths can sense the pheromones of female moths over great distances. Other species communicate with sounds: cricketsstridulate, or rub their wings together, to attract a mate and repel other males. Lampyridbeetles communicate with light.
Humans regard certain insects as pests, and attempt to control them using insecticides, and a host of other techniques. Some insects damage crops by feeding on sap, leaves, fruits, or wood. Some species are parasitic, and may vector diseases. Some insects perform complex ecological roles; blow-flies, for example, help consume carrion but also spread diseases. Insect pollinators
are essential to the life cycle of many flowering plant species on
which most organisms, including humans, are at least partly dependent;
without them, the terrestrial portion of the biosphere would be
devastated. Many insects are considered ecologically beneficial as predators and a few provide direct economic benefit. Silkworms produce silk and honey bees produce honey
and both have been domesticated by humans. Insects are consumed as food
in 80% of the world's nations, by people in roughly 3000 ethnic groups. Human activities also have effects on insect biodiversity.
Etymology
The word "insect" comes from the Latin word insectum, meaning "with a notched or divided body", or literally "cut into", from the neuter singular perfect passive participle of insectare, "to cut into, to cut up", from in- "into" and secare "to cut"; because insects appear "cut into" three sections. A calque of Greekἔντομον [éntomon], "cut into sections", Pliny the Elder introduced the Latin designation as a loan-translation of the Greek word ἔντομος (éntomos) or "insect" (as in entomology), which was Aristotle's
term for this class of life, also in reference to their "notched"
bodies. "Insect" first appears documented in English in 1601 in Holland's translation of Pliny. Translations of Aristotle's term also form the usual word for "insect" in Welsh (trychfil, from trychu "to cut" and mil, "animal"), Serbo-Croatian (zareznik, from rezati, "to cut"), Russian (насекомоеnasekomoje, from seč'/-sekat', "to cut"), etc.
Definitions
The precise definition of the taxon Insecta and the equivalent
English name "insect" varies; three alternative definitions are shown in
the table.
In the broadest circumscription, Insecta sensu lato consists of all hexapods.
Traditionally, insects defined in this way were divided into
"Apterygota" (the first five groups in the table)—the wingless
insects—and Pterygota—the winged insects. However, modern phylogenetic studies have shown that "Apterygota" is not monophyletic,
and so does not form a good taxon. A narrower circumscription restricts
insects to those hexapods with external mouthparts, and comprises only
the last three groups in the table. In this sense, Insecta sensu stricto is equivalent to Ectognatha.
In the narrowest circumscription, insects are restricted to hexapods
that are either winged or descended from winged ancestors. Insecta sensu strictissimo is then equivalent to Pterygota. For the purposes of this article, the middle definition is used; insects consist of two wingless taxa, Archaeognatha (jumping bristletails) and Zygentoma (silverfish), plus the winged or secondarily wingless Pterygota.
Phylogeny and evolution
Evolution has produced enormous variety in insects. Pictured are some possible shapes of antennae.
The evolutionary relationship of insects to other animal groups remains unclear.
Insects form a single clade, closely related to crustaceans and myriapods.
Other terrestrial arthropods, such as centipedes, millipedes, scorpions, and spiders,
are sometimes confused with insects since their body plans can appear
similar, sharing (as do all arthropods) a jointed exoskeleton. However,
upon closer examination, their features differ significantly; most
noticeably, they do not have the six-legged characteristic of adult
insects.
The higher-level phylogeny of the arthropods continues to be a matter of debate and research. In 2008, researchers at Tufts University
uncovered what they believe is the world's oldest known full-body
impression of a primitive flying insect, a 300-million-year-old specimen
from the Carboniferous period. The oldest definitive insect fossil is the DevonianRhyniognatha hirsti, from the 396-million-year-old Rhynie chert. It may have superficially resembled a modern-day silverfish
insect. This species already possessed dicondylic mandibles (two
articulations in the mandible), a feature associated with winged
insects, suggesting that wings may already have evolved at this time.
Thus, the first insects probably appeared earlier, in the Silurian period.
Four super radiations of insects have occurred: beetles (from about 300 million years ago), flies (from about 250 million years ago), moths and wasps (both from about 150 million years ago). These four groups account for the majority of described species. The flies and moths along with the fleas evolved from the Mecoptera.
The origins of insect flight
remain obscure, since the earliest winged insects currently known
appear to have been capable fliers. Some extinct insects had an
additional pair of winglets attaching to the first segment of the
thorax, for a total of three pairs. As of 2009, no evidence suggests the
insects were a particularly successful group of animals before they
evolved to have wings.
Late Carboniferous and Early Permian insect orders include both extant groups, their stem groups, and a number of Paleozoic
groups, now extinct. During this era, some giant dragonfly-like forms
reached wingspans of 55 to 70 cm (22 to 28 in), making them far larger
than any living insect. This gigantism may have been due to higher atmospheric oxygen
levels that allowed increased respiratory efficiency relative to today.
The lack of flying vertebrates could have been another factor. Most
extinct orders of insects developed during the Permian period that began
around 270 million years ago. Many of the early groups became extinct
during the Permian-Triassic extinction event, the largest mass extinction in the history of the Earth, around 252 million years ago.
The remarkably successful Hymenoptera appeared as long as 146 million years ago in the Cretaceous period, but achieved their wide diversity more recently in the Cenozoic era, which began 66 million years ago. A number of highly successful insect groups evolved in conjunction with flowering plants, a powerful illustration of coevolution.
Many modern insect genera developed during the Cenozoic. Insects from this period on are often found preserved in amber, often in perfect condition. The body plan, or morphology, of such specimens is thus easily compared with modern species. The study of fossilized insects is called paleoentomology.
Phylogeny
Taxonomy
Traditional morphology-based or appearance-based systematics have usually given the Hexapoda the rank of superclass, and identified four groups within it: insects (Ectognatha), springtails (Collembola), Protura, and Diplura, the latter three being grouped together as the Entognatha
on the basis of internalized mouth parts. Supraordinal relationships
have undergone numerous changes with the advent of methods based on
evolutionary history and genetic data. A recent theory is that the
Hexapoda are polyphyletic
(where the last common ancestor was not a member of the group), with
the entognath classes having separate evolutionary histories from the
Insecta. Many of the traditional appearance-based taxa have been shown to be paraphyletic, so rather than using ranks like subclass, superorder, and infraorder, it has proved better to use monophyletic
groupings (in which the last common ancestor is a member of the group).
The following represents the best-supported monophyletic groupings for
the Insecta.
Insects can be divided into two groups historically treated as
subclasses: wingless insects, known as Apterygota, and winged insects,
known as Pterygota. The Apterygota consist of the primitively wingless
order of the silverfish (Zygentoma). Archaeognatha make up the
Monocondylia based on the shape of their mandibles, while Zygentoma and Pterygota are grouped together as Dicondylia. The Zygentoma themselves possibly are not monophyletic, with the family Lepidotrichidae being a sister group to the Dicondylia (Pterygota and the remaining Zygentoma).
Paleoptera and Neoptera are the winged orders of insects differentiated by the presence of hardened body parts called sclerites,
and in the Neoptera, muscles that allow their wings to fold flatly over
the abdomen. Neoptera can further be divided into incomplete
metamorphosis-based (Polyneoptera and Paraneoptera)
and complete metamorphosis-based groups. It has proved difficult to
clarify the relationships between the orders in Polyneoptera because of
constant new findings calling for revision of the taxa. For example, the
Paraneoptera have turned out to be more closely related to the
Endopterygota than to the rest of the Exopterygota. The recent molecular
finding that the traditional louse orders Mallophaga and Anoplura are derived from within Psocoptera has led to the new taxon Psocodea. Phasmatodea and Embiidina have been suggested to form the Eukinolabia. Mantodea, Blattodea, and Isoptera are thought to form a monophyletic group termed Dictyoptera.
The Exopterygota likely are paraphyletic in regard to the
Endopterygota. Matters that have incurred controversy include
Strepsiptera and Diptera grouped together as Halteria based on a
reduction of one of the wing pairs—a position not well-supported in the
entomological community.
The Neuropterida are often lumped or split on the whims of the
taxonomist. Fleas are now thought to be closely related to boreid
mecopterans. Many questions remain in the basal relationships among endopterygote orders, particularly the Hymenoptera.
The study of the classification or taxonomy of any insect is called systematic entomology.
If one works with a more specific order or even a family, the term may
also be made specific to that order or family, for example systematic dipterology.
Evolutionary relationships
Insects are prey for a variety of organisms, including terrestrial
vertebrates. The earliest vertebrates on land existed 400 million years
ago and were large amphibious piscivores. Through gradual evolutionary change, insectivory was the next diet type to evolve.
Insects were among the earliest terrestrial herbivores and acted as major selection agents on plants. Plants evolved chemical defenses against this herbivory
and the insects, in turn, evolved mechanisms to deal with plant toxins.
Many insects make use of these toxins to protect themselves from their
predators. Such insects often advertise their toxicity using warning
colors. This successful evolutionary pattern has also been used by mimics.
Over time, this has led to complex groups of coevolved species.
Conversely, some interactions between plants and insects, like pollination, are beneficial to both organisms. Coevolution has led to the development of very specific mutualisms in such systems.
Diversity
A pie chart of described eukaryote species, showing just over half of these to be insects
Estimates on the total number of insect species, or those within specific orders,
often vary considerably. Globally, averages of these estimates suggest
there are around 1.5 million beetle species and 5.5 million insect
species, with about 1 million insect species currently found and
described.
Between 950,000–1,000,000 of all described species are insects,
so over 50% of all described eukaryotes (1.8 million) are insects (see
illustration). With only 950,000 known non-insects, if the actual number
of insects is 5.5 million, they may represent over 80% of the total.
As only about 20,000 new species of all organisms are described each
year, most insect species may remain undescribed, unless the rate of
species descriptions greatly increases. Of the 24 orders of insects,
four dominate in terms of numbers of described species; at least 670,000
identified species belong to Coleoptera, Diptera, Hymenoptera or Lepidoptera.
As of 2017, at least 66 insect species extinctions had been recorded
in the previous 500 years, which generally occurred on oceanic islands. Declines in insect abundance have been attributed to artificial lighting, land use changes such as urbanization or agricultural use, pesticide use, and invasive species.
Studies summarized in a 2019 review suggested a large proportion of
insect species are threatened with extinction in the 21st century.
Though ecologist Manu Sanders notes the 2019 review was biased by
mostly excluding data showing increases or stability in insect
population, with the studies limited to specific geographic areas and
specific groups of species.
Claims of pending mass insect extinctions or "insect apocalypse" based
on a subset of these studies have been popularized in news reports, but
often extrapolate beyond the study data or hyperbolize study findings.
For some insect groups such as some butterflies, bees, and beetles,
declines in abundance and diversity have been documented in European
studies. Other areas have shown increases in some insect species,
although trends in most regions are currently unknown. It is difficult
to assess long-term trends in insect abundance or diversity because
historical measurements are generally not known for many species. Robust
data to assess at-risk areas or species is especially lacking for
arctic and tropical regions and a majority of the southern hemisphere.
Insects have segmented bodies supported by exoskeletons, the hard outer covering made mostly of chitin. The segments of the body are organized into three distinctive but interconnected units, or tagmata: a head, a thorax and an abdomen. The head supports a pair of sensory antennae, a pair of compound eyes, zero to three simple eyes (or ocelli) and three sets of variously modified appendages that form the mouthparts.
The thorax is made up of three segments: the prothorax, mesothorax and
the metathorax. Each thoracic segment supports one pair of legs. The
meso- and metathoracic segments may each have a pair of wings,
depending on the insect. The abdomen consists of eleven segments,
though in a few species of insects, these segments may be fused together
or reduced in size. The abdomen also contains most of the digestive, respiratory, excretory and reproductive internal structures. Considerable variation and many adaptations in the body parts of insects occur, especially wings, legs, antenna and mouthparts.
Segmentation
The head is enclosed in a hard, heavily sclerotized, unsegmented, exoskeletal head capsule, or epicranium,
which contains most of the sensing organs, including the antennae,
ocellus or eyes, and the mouthparts. Of all the insect orders,
Orthoptera displays the most features found in other insects, including
the sutures and sclerites. Here, the vertex, or the apex (dorsal region), is situated between the compound eyes for insects with a hypognathous and opisthognathous head. In prognathous insects, the vertex is not found between the compound eyes, but rather, where the ocelli
are normally. This is because the primary axis of the head is rotated
90° to become parallel to the primary axis of the body. In some species,
this region is modified and assumes a different name.
The thorax is a tagma composed of three sections, the prothorax, mesothorax and the metathorax.
The anterior segment, closest to the head, is the prothorax, with the
major features being the first pair of legs and the pronotum. The middle
segment is the mesothorax, with the major features being the second
pair of legs and the anterior wings. The third and most posterior
segment, abutting the abdomen, is the metathorax, which features the
third pair of legs and the posterior wings. Each segment is dilineated
by an intersegmental suture. Each segment has four basic regions. The
dorsal surface is called the tergum (or notum) to distinguish it from the abdominal terga.
The two lateral regions are called the pleura (singular: pleuron) and
the ventral aspect is called the sternum. In turn, the notum of the
prothorax is called the pronotum, the notum for the mesothorax is called
the mesonotum and the notum for the metathorax is called the metanotum.
Continuing with this logic, the mesopleura and metapleura, as well as
the mesosternum and metasternum, are used.
The abdomen
is the largest tagma of the insect, which typically consists of 11–12
segments and is less strongly sclerotized than the head or thorax. Each
segment of the abdomen is represented by a sclerotized tergum and
sternum. Terga are separated from each other and from the adjacent
sterna or pleura by membranes. Spiracles are located in the pleural
area. Variation of this ground plan includes the fusion of terga or
terga and sterna to form continuous dorsal or ventral shields or a
conical tube. Some insects bear a sclerite in the pleural area called a
laterotergite. Ventral sclerites are sometimes called laterosternites.
During the embryonic stage of many insects and the postembryonic stage
of primitive insects, 11 abdominal segments are present. In modern
insects there is a tendency toward reduction in the number of the
abdominal segments, but the primitive number of 11 is maintained during
embryogenesis. Variation in abdominal segment number is considerable. If
the Apterygota are considered to be indicative of the ground plan for
pterygotes, confusion reigns: adult Protura have 12 segments, Collembola
have 6. The orthopteran family Acrididae has 11 segments, and a fossil
specimen of Zoraptera has a 10-segmented abdomen.
Exoskeleton
The insect outer skeleton, the cuticle, is made up of two layers: the epicuticle, which is a thin and waxy water resistant outer layer and contains no chitin, and a lower layer called the procuticle.
The procuticle is chitinous and much thicker than the epicuticle and
has two layers: an outer layer known as the exocuticle and an inner
layer known as the endocuticle. The tough and flexible endocuticle is
built from numerous layers of fibrous chitin and proteins,
criss-crossing each other in a sandwich pattern, while the exocuticle is
rigid and hardened. The exocuticle is greatly reduced in many insects during their larval stages, e.g., caterpillars. It is also reduced in soft-bodied adult insects.
Insects are the only invertebrates to have developed active flight capability, and this has played an important role in their success.
Their flight muscles are able to contract multiple times for each
single nerve impulse, allowing the wings to beat faster than would
ordinarily be possible.
Having their muscles attached to their exoskeletons is efficient and allows more muscle connections.
Internal
Nervous system
The nervous system of an insect can be divided into a brain and a ventral nerve cord. The head capsule is made up of six fused segments, each with either a pair of ganglia,
or a cluster of nerve cells outside of the brain. The first three pairs
of ganglia are fused into the brain, while the three following pairs
are fused into a structure of three pairs of ganglia under the insect's esophagus, called the subesophageal ganglion.
The thoracic
segments have one ganglion on each side, which are connected into a
pair, one pair per segment. This arrangement is also seen in the abdomen
but only in the first eight segments. Many species of insects have
reduced numbers of ganglia due to fusion or reduction. Some cockroaches have just six ganglia in the abdomen, whereas the wasp Vespa crabro has only two in the thorax and three in the abdomen. Some insects, like the house fly Musca domestica, have all the body ganglia fused into a single large thoracic ganglion.
At least a few insects have nociceptors, cells that detect and transmit signals responsible for the sensation of pain. This was discovered in 2003 by studying the variation in reactions of larvae of the common fruitfly Drosophila
to the touch of a heated probe and an unheated one. The larvae reacted
to the touch of the heated probe with a stereotypical rolling behavior
that was not exhibited when the larvae were touched by the unheated
probe. Although nociception has been demonstrated in insects, there is no consensus that insects feel pain consciously.
Insects are capable of learning.
Digestive system
An insect uses its digestive system to extract nutrients and other substances from the food it consumes. Most of this food is ingested in the form of macromolecules and other complex substances like proteins, polysaccharides, fats and nucleic acids. These macromolecules must be broken down by catabolic reactions into smaller molecules like amino acids and simple sugars before being used by cells of the body for energy, growth, or reproduction. This break-down process is known as digestion.
There is extensive variation among different orders, life stages, and even castes in the digestive system of insects.
This is the result of extreme adaptations to various lifestyles. The
present description focus on a generalized composition of the digestive
system of an adult orthopteroid insect, which is considered basal to
interpreting particularities of other groups.
The main structure of an insect's digestive system is a long enclosed tube called the alimentary canal, which runs lengthwise through the body. The alimentary canal directs food unidirectionally from the mouth to the anus.
It has three sections, each of which performs a different process of
digestion. In addition to the alimentary canal, insects also have paired
salivary glands and salivary reservoirs. These structures usually
reside in the thorax, adjacent to the foregut. The salivary glands
(element 30 in numbered diagram) in an insect's mouth produce saliva.
The salivary ducts lead from the glands to the reservoirs and then
forward through the head to an opening called the salivarium, located
behind the hypopharynx. By moving its mouthparts (element 32 in numbered
diagram) the insect can mix its food with saliva. The mixture of saliva
and food then travels through the salivary tubes into the mouth, where
it begins to break down. Some insects, like flies, have extra-oral digestion.
Insects using extra-oral digestion expel digestive enzymes onto their
food to break it down. This strategy allows insects to extract a
significant proportion of the available nutrients from the food source. The gut is where almost all of insects' digestion takes place. It can be divided into the foregut, midgut and hindgut.
Foregut
Stylized diagram of insect digestive tract showing malpighian tubule, from an insect of the order Orthoptera
The first section of the alimentary canal is the foregut (element 27 in numbered diagram), or stomodaeum. The foregut is lined with a cuticular lining made of chitin and proteins as protection from tough food. The foregut includes the buccal cavity (mouth), pharynx, esophagus and crop and proventriculus (any part may be highly modified), which both store food and signify when to continue passing onward to the midgut.
Digestion starts in buccal cavity (mouth) as partially chewed food is broken down by saliva from the salivary glands. As the salivary glands produce fluid and carbohydrate-digesting enzymes (mostly amylases),
strong muscles in the pharynx pump fluid into the buccal cavity,
lubricating the food like the salivarium does, and helping blood
feeders, and xylem and phloem feeders.
From there, the pharynx passes food to the esophagus, which could
be just a simple tube passing it on to the crop and proventriculus, and
then onward to the midgut, as in most insects. Alternately, the foregut
may expand into a very enlarged crop and proventriculus, or the crop
could just be a diverticulum, or fluid-filled structure, as in some Diptera species.
Bumblebee defecating. Note the contraction of the abdomen to provide internal pressure
Midgut
Once food leaves the crop, it passes to the midgut
(element 13 in numbered diagram), also known as the mesenteron, where
the majority of digestion takes place. Microscopic projections from the
midgut wall, called microvilli,
increase the surface area of the wall and allow more nutrients to be
absorbed; they tend to be close to the origin of the midgut. In some
insects, the role of the microvilli and where they are located may vary.
For example, specialized microvilli producing digestive enzymes may
more likely be near the end of the midgut, and absorption near the
origin or beginning of the midgut.
Hindgut
In the hindgut (element 16 in numbered diagram), or proctodaeum, undigested food particles are joined by uric acid
to form fecal pellets. The rectum absorbs 90% of the water in these
fecal pellets, and the dry pellet is then eliminated through the anus
(element 17), completing the process of digestion. Envaginations at the
anterior end of the hindgut form the Malpighian tubules, which form the
main excretory system of insects.
Excretory system
Insects may have one to hundreds of Malpighian tubules
(element 20). These tubules remove nitrogenous wastes from the
hemolymph of the insect and regulate osmotic balance. Wastes and solutes
are emptied directly into the alimentary canal, at the junction between
the midgut and hindgut.[34]:71–72, 78–80
Reproductive system
The reproductive system of female insects consist of a pair of ovaries, accessory glands, one or more spermathecae, and ducts connecting these parts. The ovaries are made up of a number of egg tubes, called ovarioles,
which vary in size and number by species. The number of eggs that the
insect is able to make vary by the number of ovarioles with the rate
that eggs can develop being also influenced by ovariole design. Female
insects are able make eggs, receive and store sperm, manipulate sperm
from different males, and lay eggs. Accessory glands or glandular parts
of the oviducts produce a variety of substances for sperm maintenance,
transport and fertilization, as well as for protection of eggs. They can
produce glue and protective substances for coating eggs or tough
coverings for a batch of eggs called oothecae. Spermathecae are tubes or sacs in which sperm can be stored between the time of mating and the time an egg is fertilized.
For males, the reproductive system is the testis, suspended in the body cavity by tracheae and the fat body.
Most male insects have a pair of testes, inside of which are sperm
tubes or follicles that are enclosed within a membranous sac. The
follicles connect to the vas deferens by the vas efferens, and the two
tubular vasa deferentia connect to a median ejaculatory duct that leads
to the outside. A portion of the vas deferens is often enlarged to form
the seminal vesicle, which stores the sperm before they are discharged
into the female. The seminal vesicles have glandular linings that
secrete nutrients for nourishment and maintenance of the sperm. The
ejaculatory duct is derived from an invagination of the epidermal cells
during development and, as a result, has a cuticular lining. The
terminal portion of the ejaculatory duct may be sclerotized to form the
intromittent organ, the aedeagus. The remainder of the male reproductive
system is derived from embryonic mesoderm, except for the germ cells,
or spermatogonia, which descend from the primordial pole cells very early during embryogenesis.
Respiratory system
The tube-like heart (green) of the mosquito Anopheles gambiae extends horizontally across the body, interlinked with the diamond-shaped wing muscles (also green) and surrounded by pericardial cells (red). Blue depicts cell nuclei.
Insect respiration is accomplished without lungs. Instead, the insect respiratory system
uses a system of internal tubes and sacs through which gases either
diffuse or are actively pumped, delivering oxygen directly to tissues
that need it via their trachea
(element 8 in numbered diagram). In most insects, air is taken in
through openings on the sides of the abdomen and thorax called spiracles.
The respiratory system is an important factor that limits the
size of insects. As insects get larger, this type of oxygen transport is
less efficient and thus the heaviest insect currently weighs less than
100 g. However, with increased atmospheric oxygen levels, as were
present in the late Paleozoic, larger insects were possible, such as dragonflies with wingspans of more than two feet.
There are many different patterns of gas exchange demonstrated by different groups of insects. Gas exchange patterns in insects can range from continuous and diffusive ventilation, to discontinuous gas exchange. During continuous gas exchange, oxygen is taken in and carbon dioxide
is released in a continuous cycle. In discontinuous gas exchange,
however, the insect takes in oxygen while it is active and small amounts
of carbon dioxide are released when the insect is at rest. Diffusive ventilation is simply a form of continuous gas exchange that occurs by diffusion
rather than physically taking in the oxygen. Some species of insect
that are submerged also have adaptations to aid in respiration. As
larvae, many insects have gills that can extract oxygen dissolved in
water, while others need to rise to the water surface to replenish air
supplies, which may be held or trapped in special structures.
Circulatory system
Because oxygen is delivered directly to tissues via tracheoles, the
circulatory system is not used to carry oxygen, and is therefore greatly
reduced. The insect circulatory system is open; it has no veins or arteries, and instead consists of little more than a single, perforated dorsal tube that pulses peristaltically.
This dorsal blood vessel (element 14) is divided into two sections: the
heart and aorta. The dorsal blood vessel circulates the hemolymph, arthropods' fluid analog of blood, from the rear of the body cavity forward. Hemolymph is composed of plasma in which hemocytes
are suspended. Nutrients, hormones, wastes, and other substances are
transported throughout the insect body in the hemolymph. Hemocytes
include many types of cells that are important for immune responses,
wound healing, and other functions. Hemolymph pressure may be increased
by muscle contractions or by swallowing air into the digestive system to
aid in moulting. Hemolymph is also a major part of the open circulatory system of other arthropods, such as spiders and crustaceans.
The majority of insects hatch from eggs. The fertilization and development takes place inside the egg, enclosed by a shell (chorion)
that consists of maternal tissue. In contrast to eggs of other
arthropods, most insect eggs are drought resistant. This is because
inside the chorion two additional membranes develop from embryonic
tissue, the amnion and the serosa. This serosa secretes a cuticle rich in chitin that protects the embryo against desiccation. In Schizophora however the serosa does not develop, but these flies lay their eggs in damp places, such as rotting matter. Some species of insects, like the cockroach Blaptica dubia, as well as juvenile aphids and tsetse flies, are ovoviviparous. The eggs of ovoviviparous animals develop entirely inside the female, and then hatch immediately upon being laid. Some other species, such as those in the genus of cockroaches known as Diploptera, are viviparous, and thus gestate inside the mother and are born alive. Some insects, like parasitic wasps, show polyembryony, where a single fertilized egg divides into many and in some cases thousands of separate embryos. Insects may be univoltine, bivoltine or multivoltine, i.e. they may have one, two or many broods (generations) in a year.
The different forms of the male (top) and female (bottom) tussock mothOrgyia recens is an example of sexual dimorphism in insects.
Other developmental and reproductive variations include haplodiploidy, polymorphism, paedomorphosis or peramorphosis, sexual dimorphism, parthenogenesis and more rarely hermaphroditism. In haplodiploidy, which is a type of sex-determination system, the offspring's sex is determined by the number of sets of chromosomes an individual receives. This system is typical in bees and wasps. Polymorphism is where a species may have different morphs or forms, as in the oblong winged katydid, which has four different varieties: green, pink and yellow or tan. Some insects may retain phenotypes
that are normally only seen in juveniles; this is called
paedomorphosis. In peramorphosis, an opposite sort of phenomenon,
insects take on previously unseen traits after they have matured into
adults. Many insects display sexual dimorphism, in which males and
females have notably different appearances, such as the moth Orgyia recens as an exemplar of sexual dimorphism in insects.
Some insects use parthenogenesis, a process in which the female can reproduce and give birth without having the eggs fertilized by a male.
Many aphids undergo a form of parthenogenesis, called cyclical
parthenogenesis, in which they alternate between one or many generations
of asexual and sexual reproduction.
In summer, aphids are generally female and parthenogenetic; in the
autumn, males may be produced for sexual reproduction. Other insects
produced by parthenogenesis are bees, wasps and ants, in which they
spawn males. However, overall, most individuals are female, which are
produced by fertilization. The males are haploid and the females are diploid. More rarely, some insects display hermaphroditism, in which a given individual has both male and female reproductive organs.
Insect life-histories show adaptations to withstand cold and dry
conditions. Some temperate region insects are capable of activity during
winter, while some others migrate to a warmer climate or go into a
state of torpor. Still other insects have evolved mechanisms of diapause that allow eggs or pupae to survive these conditions.
Metamorphosis
Metamorphosis
in insects is the biological process of development all insects must
undergo. There are two forms of metamorphosis: incomplete metamorphosis
and complete metamorphosis.
Incomplete metamorphosis
Hemimetabolous insects, those with incomplete metamorphosis, change gradually by undergoing a series of molts.
An insect molts when it outgrows its exoskeleton, which does not
stretch and would otherwise restrict the insect's growth. The molting
process begins as the insect's epidermis secretes a new epicuticle
inside the old one. After this new epicuticle is secreted, the
epidermis releases a mixture of enzymes that digests the endocuticle and
thus detaches the old cuticle. When this stage is complete, the insect
makes its body swell by taking in a large quantity of water or air,
which makes the old cuticle split along predefined weaknesses where the
old exocuticle was thinnest.
Immature insects that go through incomplete metamorphosis are called nymphs or in the case of dragonflies and damselflies, also naiads.
Nymphs are similar in form to the adult except for the presence of
wings, which are not developed until adulthood. With each molt, nymphs
grow larger and become more similar in appearance to adult insects.
Holometabolism, or complete metamorphosis, is where the insect changes in four stages, an egg or embryo, a larva, a pupa and the adult or imago. In these species, an egg hatches to produce a larva,
which is generally worm-like in form. This worm-like form can be one of
several varieties: eruciform (caterpillar-like), scarabaeiform
(grub-like), campodeiform (elongated, flattened and active), elateriform
(wireworm-like) or vermiform (maggot-like). The larva grows and
eventually becomes a pupa, a stage marked by reduced movement and often sealed within a cocoon.
There are three types of pupae: obtect, exarate or coarctate. Obtect
pupae are compact, with the legs and other appendages enclosed. Exarate
pupae have their legs and other appendages free and extended. Coarctate
pupae develop inside the larval skin.
Insects undergo considerable change in form during the pupal stage, and
emerge as adults. Butterflies are a well-known example of insects that
undergo complete metamorphosis, although most insects use this life
cycle. Some insects have evolved this system to hypermetamorphosis.
Complete metamorphosis is a trait of the most diverse insect group, the Endopterygota. Endopterygota includes 11 Orders, the largest being Diptera (flies), Lepidoptera (butterflies and moths), and Hymenoptera (bees, wasps, and ants), and Coleoptera (beetles). This form of development is exclusive to insects and not seen in any other arthropods.
Senses and communication
Many insects possess very sensitive and specialized organs of perception. Some insects such as bees can perceive ultraviolet wavelengths, or detect polarized light, while the antennae of male moths can detect the pheromones of female moths over distances of many kilometers. The yellow paper wasp (Polistes versicolor)
is known for its wagging movements as a form of communication within
the colony; it can waggle with a frequency of 10.6±2.1 Hz (n=190). These
wagging movements can signal the arrival of new material into the nest
and aggression between workers can be used to stimulate others to
increase foraging expeditions.
There is a pronounced tendency for there to be a trade-off between
visual acuity and chemical or tactile acuity, such that most insects
with well-developed eyes have reduced or simple antennae, and vice
versa. There are a variety of different mechanisms by which insects
perceive sound; while the patterns are not universal, insects can
generally hear sound if they can produce it. Different insect species
can have varying hearing,
though most insects can hear only a narrow range of frequencies related
to the frequency of the sounds they can produce. Mosquitoes have been
found to hear up to 2 kHz, and some grasshoppers can hear up to 50 kHz.
Certain predatory and parasitic insects can detect the characteristic
sounds made by their prey or hosts, respectively. For instance, some
nocturnal moths can perceive the ultrasonic emissions of bats, which helps them avoid predation. Insects that feed on blood have special sensory structures that can detect infrared emissions, and use them to home in on their hosts.
Some insects display a rudimentary sense of numbers,
such as the solitary wasps that prey upon a single species. The mother
wasp lays her eggs in individual cells and provides each egg with a
number of live caterpillars on which the young feed when hatched. Some
species of wasp always provide five, others twelve, and others as high
as twenty-four caterpillars per cell. The number of caterpillars is
different among species, but always the same for each sex of larva. The
male solitary wasp in the genus Eumenes
is smaller than the female, so the mother of one species supplies him
with only five caterpillars; the larger female receives ten caterpillars
in her cell.
Light production and vision
Most insects have compound eyes and two antennae.
A few insects, such as members of the families Poduridae and Onychiuridae (Collembola), Mycetophilidae (Diptera) and the beetle families Lampyridae, Phengodidae, Elateridae and Staphylinidae are bioluminescent. The most familiar group are the fireflies,
beetles of the family Lampyridae. Some species are able to control this
light generation to produce flashes. The function varies with some
species using them to attract mates, while others use them to lure prey.
Cave dwelling larvae of Arachnocampa (Mycetophilidae, fungus gnats) glow to lure small flying insects into sticky strands of silk.
Some fireflies of the genus Photurismimic the flashing of female Photinus species to attract males of that species, which are then captured and devoured. The colors of emitted light vary from dull blue (Orfelia fultoni, Mycetophilidae) to the familiar greens and the rare reds (Phrixothrix tiemanni, Phengodidae).
Most insects, except some species of cave crickets,
are able to perceive light and dark. Many species have acute vision
capable of detecting minute movements. The eyes may include simple eyes
or ocelli as well as compound eyes of varying sizes. Many species are able to detect light in the infrared, ultraviolet and the visible light wavelengths. Color vision has been demonstrated in many species and phylogenetic analysis suggests that UV-green-blue trichromacy existed from at least the Devonian period between 416 and 359 million years ago.
Sound production and hearing
Insects were the earliest organisms to produce and sense sounds.
Insects make sounds mostly by mechanical action of appendages. In grasshoppers and crickets, this is achieved by stridulation. Cicadas make the loudest sounds among the insects by producing and amplifying sounds with special modifications to their body to form tymbals and associated musculature. The African cicadaBrevisana brevis has been measured at 106.7 decibels at a distance of 50 cm (20 in). Some insects, such as the Helicoverpa zea moths, hawk moths and Hedylid butterflies, can hear ultrasound and take evasive action when they sense that they have been detected by bats. Some moths produce ultrasonic clicks that were once thought to have a role in jamming bat echolocation. The ultrasonic clicks were subsequently found to be produced mostly by unpalatable moths to warn bats, just as warning colorations are used against predators that hunt by sight. Some otherwise palatable moths have evolved to mimic these calls.
More recently, the claim that some moths can jam bat sonar has been
revisited. Ultrasonic recording and high-speed infrared videography of
bat-moth interactions suggest the palatable tiger moth really does
defend against attacking big brown bats using ultrasonic clicks that jam
bat sonar.
Very low sounds are also produced in various species of Coleoptera, Hymenoptera, Lepidoptera, Mantodea and Neuroptera.
These low sounds are simply the sounds made by the insect's movement.
Through microscopic stridulatory structures located on the insect's
muscles and joints, the normal sounds of the insect moving are amplified
and can be used to warn or communicate with other insects. Most
sound-making insects also have tympanal organs that can perceive airborne sounds. Some species in Hemiptera, such as the corixids (water boatmen), are known to communicate via underwater sounds. Most insects are also able to sense vibrations transmitted through surfaces.
Communication using surface-borne vibrational signals is more
widespread among insects because of size constraints in producing
air-borne sounds.
Insects cannot effectively produce low-frequency sounds, and
high-frequency sounds tend to disperse more in a dense environment (such
as foliage), so insects living in such environments communicate primarily using substrate-borne vibrations. The mechanisms of production of vibrational signals are just as diverse as those for producing sound in insects.
Some species use vibrations for communicating within members of
the same species, such as to attract mates as in the songs of the shield bugNezara viridula. Vibrations can also be used to communicate between entirely different species; lycaenid (gossamer-winged butterfly) caterpillars, which are myrmecophilous (living in a mutualistic association with ants) communicate with ants in this way. The Madagascar hissing cockroach has the ability to press air through its spiracles to make a hissing noise as a sign of aggression; the death's-head hawkmoth
makes a squeaking noise by forcing air out of their pharynx when
agitated, which may also reduce aggressive worker honey bee behavior
when the two are in close proximity.
Chemical communication
Chemical communications in animals rely on a variety of aspects
including taste and smell. Chemoreception is the physiological response
of a sense organ (i.e. taste or smell) to a chemical stimulus where the
chemicals act as signals to regulate the state or activity of a cell. A
semiochemical is a message-carrying chemical that is meant to attract,
repel, and convey information. Types of semiochemicals include
pheromones and kairomones. One example is the butterfly Phengaris arionwhich uses chemical signals as a form of mimicry to aid in predation.
In addition to the use of sound for communication, a wide range of insects have evolved chemical means for communication. These chemicals, termed semiochemicals, are often derived from plant metabolites include those meant to attract, repel and provide other kinds of information. Pheromones, a type of semiochemical, are used for attracting mates of the opposite sex, for aggregating conspecific
individuals of both sexes, for deterring other individuals from
approaching, to mark a trail, and to trigger aggression in nearby
individuals. Allomones benefit their producer by the effect they have upon the receiver. Kairomones
benefit their receiver instead of their producer. Synomones benefit the
producer and the receiver. While some chemicals are targeted at
individuals of the same species, others are used for communication
across species. The use of scents is especially well known to have
developed in social insects.
Social insects, such as termites, ants and many bees and wasps, are the most familiar species of eusocial animals.
They live together in large well-organized colonies that may be so
tightly integrated and genetically similar that the colonies of some
species are sometimes considered superorganisms. It is sometimes argued that the various species of honey bee
are the only invertebrates (and indeed one of the few non-human groups)
to have evolved a system of abstract symbolic communication where a
behavior is used to represent and convey specific information about something in the environment. In this communication system, called dance language,
the angle at which a bee dances represents a direction relative to the
sun, and the length of the dance represents the distance to be flown. Though perhaps not as advanced as honey bees, bumblebees also potentially have some social communication behaviors. Bombus terrestris,
for example, exhibit a faster learning curve for visiting unfamiliar,
yet rewarding flowers, when they can see a conspecific foraging on the
same species.
Only insects that live in nests or colonies demonstrate any true
capacity for fine-scale spatial orientation or homing. This can allow an
insect to return unerringly to a single hole a few millimeters in
diameter among thousands of apparently identical holes clustered
together, after a trip of up to several kilometers' distance. In a
phenomenon known as philopatry, insects that hibernate have shown the ability to recall a specific location up to a year after last viewing the area of interest. A few insects seasonally migrate large distances between different geographic regions (e.g., the overwintering areas of the monarch butterfly).
Care of young
The eusocial insects build nests, guard eggs, and provide food for offspring full-time (see Eusociality).
Most insects, however, lead short lives as adults, and rarely interact
with one another except to mate or compete for mates. A small number
exhibit some form of parental care,
where they will at least guard their eggs, and sometimes continue
guarding their offspring until adulthood, and possibly even feeding
them. Another simple form of parental care is to construct a nest (a
burrow or an actual construction, either of which may be simple or
complex), store provisions in it, and lay an egg upon those provisions.
The adult does not contact the growing offspring, but it nonetheless
does provide food. This sort of care is typical for most species of bees
and various types of wasps.
Basic
motion of the insect wing in insect with an indirect flight mechanism
scheme of dorsoventral cut through a thorax segment with a wings b joints c dorsoventral muscles d longitudinal muscles.
Insects are the only group of invertebrates to have developed flight. The evolution of insect wings has been a subject of debate. Some entomologists suggest that the wings are from paranotal lobes, or extensions from the insect's exoskeleton called the nota, called the paranotal theory. Other theories are based on a pleural
origin. These theories include suggestions that wings originated from
modified gills, spiracular flaps or as from an appendage of the epicoxa.
The epicoxal theory suggests the insect wings are modified epicoxal exites, a modified appendage at the base of the legs or coxa. In the Carboniferous age, some of the Meganeura
dragonflies had as much as a 50 cm (20 in) wide wingspan. The
appearance of gigantic insects has been found to be consistent with high
atmospheric oxygen. The respiratory system of insects constrains their
size, however the high oxygen in the atmosphere allowed larger sizes. The largest flying insects today are much smaller and include several moth species such as the Atlas moth and the white witch (Thysania agrippina).
Insect flight has been a topic of great interest in aerodynamics
due partly to the inability of steady-state theories to explain the
lift generated by the tiny wings of insects. But insect wings are in
motion, with flapping and vibrations, resulting in churning and eddies, and the misconception that physics says "bumblebees can't fly" persisted throughout most of the twentieth century.
Unlike birds, many small insects are swept along by the prevailing winds although many of the larger insects are known to make migrations. Aphids are known to be transported long distances by low-level jet streams. As such, fine line patterns associated with converging winds within weather radar imagery, like the WSR-88D radar network, often represent large groups of insects.
Spatial
and temporal stepping pattern of walking desert ants performing an
alternating tripod gait. Recording rate: 500 fps, Playback rate: 10 fps.
Many adult insects use six legs for walking and have adopted a tripedalgait. The tripedal gait allows for rapid walking while always having a stable stance and has been studied extensively in cockroaches and ants.
The legs are used in alternate triangles touching the ground. For the
first step, the middle right leg and the front and rear left legs are in
contact with the ground and move the insect forward, while the front
and rear right leg and the middle left leg are lifted and moved forward
to a new position. When they touch the ground to form a new stable
triangle the other legs can be lifted and brought forward in turn and so
on.
The purest form of the tripedal gait is seen in insects moving at high
speeds. However, this type of locomotion is not rigid and insects can
adapt a variety of gaits. For example, when moving slowly, turning,
avoiding obstacles, climbing or slippery surfaces, four (tetrapod) or
more feet (wave-gait) may be touching the ground. Insects can also adapt their gait to cope with the loss of one or more limbs.
Cockroaches are among the fastest insect runners and, at full
speed, adopt a bipedal run to reach a high velocity in proportion to
their body size. As cockroaches move very quickly, they need to be video
recorded at several hundred frames per second to reveal their gait.
More sedate locomotion is seen in the stick insects or walking sticks (Phasmatodea). A few insects have evolved to walk on the surface of the water, especially members of the Gerridae family, commonly known as water striders. A few species of ocean-skaters in the genus Halobates even live on the surface of open oceans, a habitat that has few insect species.
Use in robotics
Insect walking is of particular interest as an alternative form of locomotion in robots. The study of insects and bipeds has a significant impact on possible robotic methods of transport. This may allow new robots to be designed that can traverse terrain that robots with wheels may be unable to handle.
Swimming
The backswimmer Notonecta glauca underwater, showing its paddle-like hindleg adaptation
A large number of insects live either part or the whole of their
lives underwater. In many of the more primitive orders of insect, the
immature stages are spent in an aquatic environment. Some groups of
insects, like certain water beetles, have aquatic adults as well.
Many of these species have adaptations to help in under-water
locomotion. Water beetles and water bugs have legs adapted into
paddle-like structures. Dragonfly naiads use jet propulsion, forcibly expelling water out of their rectal chamber. Some species like the water striders
are capable of walking on the surface of water. They can do this
because their claws are not at the tips of the legs as in most insects,
but recessed in a special groove further up the leg; this prevents the
claws from piercing the water's surface film. Other insects such as the Rove beetleStenus
are known to emit pygidial gland secretions that reduce surface tension
making it possible for them to move on the surface of water by Marangoni propulsion (also known by the German term Entspannungsschwimmen).
Ecology
Insect ecology is the scientific study of how insects, individually or as a community, interact with the surrounding environment or ecosystem.
Insects play one of the most important roles in their ecosystems, which
includes many roles, such as soil turning and aeration, dung burial,
pest control, pollination and wildlife nutrition. An example is the beetles, which are scavengers that feed on dead animals and fallen trees and thereby recycle biological materials into forms found useful by other organisms. These insects, and others, are responsible for much of the process by which topsoil is created.
Defense and predation
Perhaps one of the most well-known examples of mimicry, the viceroy butterfly (top) appears very similar to the monarch butterfly (bottom).
Insects are mostly soft bodied, fragile and almost defenseless
compared to other, larger lifeforms. The immature stages are small, move
slowly or are immobile, and so all stages are exposed to predation and parasitism. Insects then have a variety of defense strategies to avoid being attacked by predators or parasitoids. These include camouflage, mimicry, toxicity and active defense.
Camouflage is an important defense strategy, which involves the use of coloration or shape to blend into the surrounding environment.
This sort of protective coloration is common and widespread among
beetle families, especially those that feed on wood or vegetation, such
as many of the leaf beetles (family Chrysomelidae) or weevils.
In some of these species, sculpturing or various colored scales or
hairs cause the beetle to resemble bird dung or other inedible objects.
Many of those that live in sandy environments blend in with the
coloration of the substrate. Most phasmids are known for effectively replicating the forms of sticks and leaves, and the bodies of some species (such as O. macklotti and Palophus centaurus) are covered in mossy or lichenous
outgrowths that supplement their disguise. Very rarely, a species may
have the ability to change color as their surroundings shift (Bostra scabrinota). In a further behavioral adaptation to supplement crypsis,
a number of species have been noted to perform a rocking motion where
the body is swayed from side to side that is thought to reflect the
movement of leaves or twigs swaying in the breeze. Another method by
which stick insects avoid predation and resemble twigs is by feigning
death (catalepsy),
where the insect enters a motionless state that can be maintained for a
long period. The nocturnal feeding habits of adults also aids
Phasmatodea in remaining concealed from predators.
Another defense that often uses color or shape to deceive potential enemies is mimicry. A number of longhorn beetles (family Cerambycidae) bear a striking resemblance to wasps, which helps them avoid predation even though the beetles are in fact harmless. Batesian and Müllerianmimicry
complexes are commonly found in Lepidoptera. Genetic polymorphism and
natural selection give rise to otherwise edible species (the mimic)
gaining a survival advantage by resembling inedible species (the model).
Such a mimicry complex is referred to as Batesian. One of the most famous examples, where the viceroy butterfly was long believed to be a Batesian mimic of the inedible monarch,
was later disproven, as the viceroy is more toxic than the monarch, and
this resemblance is now considered to be a case of Müllerian mimicry.
In Müllerian mimicry, inedible species, usually within a taxonomic
order, find it advantageous to resemble each other so as to reduce the
sampling rate by predators who need to learn about the insects'
inedibility. Taxa from the toxic genus Heliconius form one of the most well known Müllerian complexes.
Chemical defense is another important defense found among species
of Coleoptera and Lepidoptera, usually being advertised by bright
colors, such as the monarch butterfly.
They obtain their toxicity by sequestering the chemicals from the
plants they eat into their own tissues. Some Lepidoptera manufacture
their own toxins. Predators that eat poisonous butterflies and moths may
become sick and vomit violently, learning not to eat those types of
species; this is actually the basis of Müllerian mimicry. A predator who
has previously eaten a poisonous lepidopteran may avoid other species
with similar markings in the future, thus saving many other species as
well. Some ground beetles of the family Carabidae can spray chemicals from their abdomen with great accuracy, to repel predators.
Pollination is the process by which pollen is transferred in the reproduction of plants, thereby enabling fertilisation and sexual reproduction.
Most flowering plants require an animal to do the transportation. While
other animals are included as pollinators, the majority of pollination
is done by insects. Because insects usually receive benefit for the pollination in the form of energy rich nectar it is a grand example of mutualism.
The various flower traits (and combinations thereof) that
differentially attract one type of pollinator or another are known as pollination syndromes.
These arose through complex plant-animal adaptations. Pollinators find
flowers through bright colorations, including ultraviolet, and
attractant pheromones. The study of pollination by insects is known as anthecology.
Parasitism
Many insects are parasites of other insects such as the parasitoid wasps. These insects are known as entomophagous parasites.
They can be beneficial due to their devastation of pests that can
destroy crops and other resources. Many insects have a parasitic
relationship with humans such as the mosquito. These insects are known
to spread diseases such as malaria and yellow fever and because of such, mosquitoes indirectly cause more deaths of humans than any other animal.
Many insects are considered pests by humans. Insects commonly regarded as pests include those that are parasitic (e.g.lice, bed bugs), transmit diseases (mosquitoes, flies), damage structures (termites), or destroy agricultural goods (locusts, weevils). Many entomologists are involved in various forms of pest control, as in research for companies to produce insecticides, but increasingly rely on methods of biological pest control,
or biocontrol. Biocontrol uses one organism to reduce the population
density of another organism—the pest—and is considered a key element of integrated pest management.
Despite the large amount of effort focused at controlling
insects, human attempts to kill pests with insecticides can backfire. If
used carelessly, the poison can kill all kinds of organisms in the
area, including insects' natural predators, such as birds, mice and
other insectivores. The effects of DDT's use exemplifies how some insecticides can threaten wildlife beyond intended populations of pest insects.
In beneficial roles
Because they help flowering plants to cross-pollinate, some insects are critical to agriculture. This European honey bee is gathering nectar while pollen collects on its body.
Although pest insects attract the most attention, many insects are beneficial to the environment and to humans. Some insects, like wasps, bees, butterflies and ants, pollinateflowering plants. Pollination is a mutualistic relationship between plants and insects. As insects gather nectar from different plants of the same species, they also spread pollen from plants on which they have previously fed. This greatly increases plants' ability to cross-pollinate, which maintains and possibly even improves their evolutionary fitness. This ultimately affects humans since ensuring healthy crops is critical to agriculture.
As well as pollination ants help with seed distribution of plants. This
helps to spread the plants, which increases plant diversity. This leads
to an overall better environment. A serious environmental problem is the decline of populations of pollinator insects, and a number of species of insects are now cultured primarily for pollination management in order to have sufficient pollinators in the field, orchard or greenhouse at bloom time. Another solution, as shown in Delaware, has been to raise native plants to help support native pollinators like L. vierecki. Insects also produce useful substances such as honey, wax, lacquer and silk. Honey bees
have been cultured by humans for thousands of years for honey, although
contracting for crop pollination is becoming more significant for beekeepers. The silkworm has greatly affected human history, as silk-driven trade established relationships between China and the rest of the world.
Insectivorous
insects, or insects that feed on other insects, are beneficial to
humans if they eat insects that could cause damage to agriculture and
human structures. For example, aphids feed on crops and cause problems for farmers, but ladybugs feed on aphids, and can be used as a means to significantly reduce pest aphid populations. While birds
are perhaps more visible predators of insects, insects themselves
account for the vast majority of insect consumption. Ants also help
control animal populations by consuming small vertebrates. Without predators to keep them in check, insects can undergo almost unstoppable population explosions.
Insects are also used in medicine, for example fly larvae (maggots) were formerly used to treat wounds to prevent or stop gangrene,
as they would only consume dead flesh. This treatment is finding modern
usage in some hospitals. Recently insects have also gained attention as
potential sources of drugs and other medicinal substances. Adult insects, such as crickets and insect larvae of various kinds, are also commonly used as fishing bait.
In research
The common fruitfly Drosophila melanogaster is one of the most widely used organisms in biological research.
Insects play important roles in biological research. For example, because of its small size, short generation time and high fecundity, the common fruit fly Drosophila melanogaster is a model organism for studies in the genetics of higher eukaryotes. D. melanogaster has been an essential part of studies into principles like genetic linkage, interactions between genes, chromosomal genetics, development, behavior and evolution. Because genetic systems are well conserved among eukaryotes, understanding basic cellular processes like DNA replication or transcription in fruit flies can help to understand those processes in other eukaryotes, including humans. The genome of D. melanogaster was sequenced
in 2000, reflecting the organism's important role in biological
research. It was found that 70% of the fly genome is similar to the
human genome, supporting the evolution theory.
As food
In some cultures, insects, especially deep-friedcicadas, are considered to be delicacies,
whereas in other places they form part of the normal diet. Insects have
a high protein content for their mass, and some authors suggest their
potential as a major source of protein in human nutrition. In most first-world countries, however, entomophagy (the eating of insects), is taboo.
Since it is impossible to entirely eliminate pest insects from the human
food chain, insects are inadvertently present in many foods, especially
grains. Food safety laws in many countries do not prohibit insect parts in food, but rather limit their quantity. According to cultural materialist anthropologist Marvin Harris, the eating of insects is taboo in cultures that have other protein sources such as fish or livestock.
Due to the abundance of insects and a worldwide concern of food shortages, the Food and Agriculture Organization of the United Nations
considers that the world may have to, in the future, regard the
prospects of eating insects as a food staple. Insects are noted for
their nutrients, having a high content of protein, minerals and fats and
are eaten by one-third of the global population.
As foddder
Processed maggots can be used as fodder for farmed animals such as chicken, fish and pigs.
Many species of insects are sold and kept as pets.
In culture
Scarab beetles held religious and cultural symbolism in Old Egypt, Greece and some shamanistic Old World cultures. The ancient Chinese regarded cicadas as symbols of rebirth or immortality. In Mesopotamian literature, the epic poem of Gilgamesh has allusions to Odonata that signify the impossibility of immortality. Among the Aborigines of Australia of the Arrernte language groups, honey ants and witchety grubs served as personal clan totems. In the case of the 'San' bush-men of the Kalahari, it is the praying mantis that holds much cultural significance including creation and zen-like patience in waiting.
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