Insect anatomy
A- Head
B- Thorax
C- Abdomen
Insects (Class Insecta) are a major group of arthropods and the most diverse group of animals on the
Earth, with over a million described species—more than all other animal groups combined.[1] Insects may be found in nearly all environments
on the planet, although only a small number of species occur in the oceans where crustaceans tend to predominate instead. There are approximately 5,000 dragonfly species, 2,000 praying mantis, 20,000 grasshopper, 170,000 butterfly and moth, 120,000 fly, 82,000 true bug, 360,000 beetle, and 110,000
bee, wasp and ant species described to date. Estimates of the total number of current
species, including those not yet known to science, range from two million to fifty million, with newer studies favouring a lower
figure of about six to ten million.[1][2][3] Adult modern insects range in size from a 0.139 mm (0.00547 in) fairyfly
(Dicopomorpha echmepterygis) to a 55.5 cm (21.9 in) long stick insect (Phobaeticus serratipes).[4] The heaviest documented insect was a Giant Weta of 70
g, 2½ oz), but other possible candidates include the
Goliath beetles Goliathus goliatus,
Goliathus regius and Cerambycid beetles such
as Titanus giganteus, though no one is certain which is truly the heaviest.[4]
The study of insects (from Latin insectus, meaning "cut into sections") is called
entomology, from the Greek εντομος, also meaning "cut
into sections".[5]
Morphology
Insects possess segmented bodies supported by an exoskeleton, a hard outer covering made
mostly of chitin. The segments of the body are organized into three regions, or tagmata; a head, a thorax, and an abdomen. The head supports a pair of sensory antennae, a pair of
compound eyes, one to three simple eyes ("ocelli") and
three sets of variously modified appendages that form the mouthparts. The thorax has
six legs (one pair each for the prothorax, mesothorax and the
metathorax segments making up the thorax) and two or four wings (if present in the species).
The abdomen (made up of eleven segments some of which may be reduced or fused) has most of the digestive, respiratory, excretory and reproductive internal structures.
Nervous system
Their nervous system can be divided into a brain and a ventral nerve cord. The head capsule (made up of six fused segments) has six pairs of
ganglia. The first three pairs are fused into the brain, while the three following pairs are
fused into a structure 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. And some, like the house fly
Musca domestica, have all the body ganglia fused into a single large thoracic
ganglion.
Respiration and circulation
Insect respiration is accomplished without lungs, but instead insects possess a system of
internal tubes and sacs through which gases either diffuse or are actively pumped, delivering oxygen directly to body tissues
(see Invertebrate trachea). Since oxygen is delivered directly, the circulatory
system is not used to carry oxygen, and is therefore greatly reduced; it has no closed vessels (i.e., no veins or arteries), consisting of little more than a single, perforated dorsal tube
which pulses peristaltically, and in doing so helps circulate the hemolymph inside the body cavity.
Exoskeleton
Most higher insects have two pairs of wings located on the second and third thoracic
segments. Insects are the only invertebrates to have developed flight, and this has played an important part in their success. The winged insects, and their wingless
relatives, make up the subclass Pterygota. Insect
flight is not very well understood, relying heavily on turbulent aerodynamic effects. The primitive insect groups use
muscles that act directly on the wing structure. The more advanced groups making up the Neoptera have foldable wings and their muscles act on the thorax wall and power the wings indirectly. These
muscles are able to contract multiple times for each single nerve impulse, allowing the wings to beat faster than would
ordinarily be possible (see insect flight).
Their 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 another layer under it called the procuticle. This is chitinous and much thicker than the epicuticle and has two layers, the outer being the
exocuticle while the inner is the endocuticle. The tough and flexible endocuticle is built from numerous layers of fibrous chitin
and proteins, criss-crossing each others in a sandwich pattern, while the exocuticle is rigid and sclerotized. The exocuticle is greatly reduced in many soft-bodied insects, especially the larval stages (e.g., caterpillars).
Development
Hoverflies mating in flight
Most insects hatch from eggs, but others are ovoviviparous or viviparous, and all undergo a series of
moults as they develop and grow in size. This manner of growth is necessitated by the inelastic
exoskeleton. Moulting is a process by which the individual escapes the confines of the exoskeleton in order to increase in size,
then grows a new and larger outer covering. In some insects, the young are called nymphs and are similar in form to the
adult except that the wings are not developed until the adult stage. This is called incomplete metamorphosis and insects showing this are termed hemimetabolous. Holometabolous insects show
complete metamorphosis, which distinguishes the Endopterygota and includes many of
the most successful insect groups. In these species, an egg hatches to produce a larva,
which is generally worm-like in form, and can be divided into five different forms; eruciform (caterpillar-like), scarabaeiform
(grublike), campodeiform (elongated, flattened, and active), elateriform (wireworm-like) and vermiform (maggot-like). The larva
grows and eventually becomes a pupa, a stage sealed within a cocoon in some species. There are three types of pupae; obtect (the pupa is compact with the legs and other
appendages enclosed), exarate (where the pupa has the legs and other appendages free and extended) and coarctate (where the pupa
develops inside the larval skin). In the pupal stage, the insect undergoes considerable change in form to emerge as an adult, or
imago. Butterflies are an example of an insect that undergoes complete metamorphosis. Some
insects have even evolved hypermetamorphosis.
Some insects (parastic wasps) show polyembryony where a single fertilized egg can divide
into many and in some cases thousands of separate embryos. Other developmental and reproductive variations include
haplodiploidy, polymorphism, paedomorphosis (metathetely and prothetely),
sexual dimorphism, parthenogenesis and more
rarely hermaphroditism.
Behaviour
Platymeris biguttata ("Twin-spotted assassin bug") with prey
Many insects possess very sensitive and/or 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 kilometres. 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, and
it is by no means universal; the general pattern, however, is that if an insect can produce sound, then it can also hear sound,
though the range of frequencies they can hear is often quite narrow (and may in fact be limited to only the frequency that they
themselves produce). Some nocturnal moths can perceive the ultrasonic
emissions of bats, a mechanism which helps them avoid predation. Certain predatory and parasitic
insects can detect the characteristic sounds made by their prey/hosts. Bloodsucking insects have special sensory structures that
can detect infrared emissions, and use them to home in on their hosts.
Sensillae: sensory structures on insects
Most insects 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 actively 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 of bees and various types of wasps.
A few such insects also have a well-developed number sense, among the solitary wasps that provision with a single species of
prey. 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 it is always the same for each sex of larvae. 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. She can in other
words distinguish between both the numbers five and ten in the caterpillars she is providing and which cell contains a male or a
female.
Social behaviour
A termite mound made by the cathedral termite
Social insects, such as the termites, ants and many bees and wasps, are the most familiar
species of eusocial animal. 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 (i.e., where a behaviour is used to represent and convey specific information about something in the
environment), called the "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.
Only those insects which live in nests or colonies demonstrate any true capacity for fine-scale spatial orientation or
"homing" - this can be quite sophisticated, however, and allow an insect to return unerringly to a single hole a few millimetres
in diameter among a mass of thousands of apparently identical holes all clustered together, after a trip of up to several
kilometres' distance, and (in cases where an insect hibernates) as long as a year after last
viewing the area (a phenomenon known as philopatry). A few insects migrate, but this is a larger-scale form of navigation, and often involves only large, general regions (e.g., the overwintering
areas of the Monarch butterfly).
Light production and vision
A few insects, notably the beetles of the family Lampyridae have evolved light generating
organs. They are also able to control this light generation to produce flashes and some species use the light to attract
mates.
Most insects except some species of cave dwelling crickets are able to perceive light and dark. Many species have acute vision
capable of detecting minute movements. The eyes include simple eyes or ocelli as well as
compound eyes of varying sizes. Many species are able to detect light in the infrared,
ultraviolet as well as the visible light wavelengths. Colour vision has been demonstrated in many species.
Sound production and hearing
Insects were the earliest organisms to produce sounds and to sense them. Soundmaking in insects is achieved mostly by
mechanical action of appendages. In the grasshoppers and crickets this is achieved by
stridulation. The cicadas have the loudest sounds among the
insects and have special modifications to their body and musculature to produce and amplify sounds. Some species such as the
African cicada, Brevisana brevis have been measured at 106.7 decibels at a distance of 50 cm (20 in).[4] Some insects, such as the hawk moth, can hear ultrasound and
take evasive action when they sense detection by bats. Some moths produce clicks and these were earlier thought to have a role in
jamming the bat echolocation, but it was subsequently found that these are produced mostly by unpalatable moths to warn the bats,
just as warning colouration is used visually.[6] These calls are also made by other moths involved in mimicry.[7]
Very low sounds are also produced in various species of Lepidoptera, Coleoptera and Hymenoptera, mostly through the use of wing movement or
friction at the joints of appendages.
Most soundmaking insects also have tympanal organs that can perceive airborne sounds. Most insects are also able to sense
vibrations transmitted by the substrate. Communication with substrate-borne vibrational
signals is widespread among insects because of the size constraints in producing air-borne sounds.[8] 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.[9] The mechanisms of production of vibrational signals are just as diverse as those for producing sound
in insects.
The Madagascar hissing cockroach has the ability to press air through
the spiracles to make a hissing noise, and the Death's-head Hawkmoth makes a
squeaking noise by forcing air out of their pharynx.
Chemical communication
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. 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.
Locomotion
Flight
-
Insects are the only group of invertebrates to have developed flight. The evolution of insect wings has been a subject of
debate. Some proponents suggest that the wings are para-notal in origin while others have suggested they are modified gills. 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 percentage of oxygen in the
atmosphere found from ice core-samples was as high as 35% compared to the current 21%. The respiratory system of insects
constrains their size, however the high oxygen in the atmosphere allowed larger sizes.[10] 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.
In addition to powered flight, many of the smaller insects are also dispersed by winds. These include the aphids which are often transported long distances by low-level jet streams.[11]
Walking
Many adult insects use six legs for walking and have adopted a tripedal gait. The tripedal
gait allows for rapid walking whilst always having a stable stance and has been studied extensively in cockroaches. 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, whilst 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 speed and is illustrated in the gif animation of a 7-spot
ladybird (Coccinellidae, Coccinella
septempunctata). However, this type of locomotion is not rigid and insects can adapt a variety of gaits; for example,
when moving slowly, turning, or avoiding obstacles, four or more feet may be touching the ground. Insects can also adapt their
gait to cope with the loss of one or more limbs.
Cockroaches are amongst the fastest insect runners and at full speed actually adopt a
bipedal run to reach a high velocity in proportion to their body size. As Cockroaches move
extremely rapidly, they need recording at several hundred frames per second to reveal their gait. More sedate locomotion is also
studied by scientists in stick insects Phasmatodea.
A few insects have evolved to walk on the surface of the water, especially the bugs of the family, Gerridae, also known as water striders. A few species in the genus Halobates even live on the surface of open oceans, a habitat that has few insect species.
Insect walking is of particular interest as an alternative form of locomotion to the use of wheels for robots
(Robot locomotion).
Swimming
The backswimmer
Notonecta glauca underwater, showing the paddle like hindleg
adaptation
A large number of insects live either parts or the whole of their lives underwater. In many orders the immature stages are
spent in water while the adults are either aerial or terrestrial in habit. A few species spend a part of their adult life either
under or over water.
Many of these species have adaptations to help in locomotion under water. The water beetles and water bugs have legs adapted
into paddle like structures. Some Odonate larvae, such as dragonfly naiads, propel themselves
rapidly by expelling water forcibly out of the rectal chamber.[12]
Some species like the water striders are capable of walking on the surface of water. Some
others such as the Rove beetle Velia are known to emit salivary secretions that
reduce surface tension making it possible for them to move on the surface of water by Marangoni propulsion (also described using
the german term Entspannungsschwimmen).[13][14]
Evolution
Evolution has produced astonishing variety in insects. Pictured are some of the possible shapes of
antennae.
-
The relationships of insects to other animal groups remain unclear. Although more traditionally grouped with millipedes and
centipedes, evidence has emerged favoring closer evolutionary ties with the crustaceans. In
the Pancrustacea theory insects, together with Remipedia
and Malacostraca, make up a natural clade.
The oldest definitive insect fossil is the Devonian Rhyniognatha hirsti, estimated at
396-407 million years old.[15] This
species already possessed dicondylic mandibles, 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.[15]
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. So far, there is nothing that suggests that the insects were a
particularly successful group of animals before they got their wings.
Late Carboniferous and Early Permian insect orders
include both several current very long-lived groups and a number of Paleozoic forms. During this era, some giant dragonfly-like
forms reached wingspans of 55 to 70 cm, (22-28 in) making them far larger than any living insect. Also their nymphs
must have had a very impressive size. 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 extant orders of insects developed during the Permian era 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 Hymenopterans appeared in the Cretaceous but achieved their
diversity more recently, in the Cenozoic. A number of highly-successful insect groups evolved
in conjunction with flowering plants, a powerful illustration of co-evolution.
Many modern insect genera developed during the Cenozoic; insects from this period on are
often found preserved in amber, often in perfect condition. Such specimens are
easily compared with modern species. The study of fossilized insects is called paleoentomology.
Coevolution
- See also: Coevolution
Insects were among the earliest terrestrial herbivores and they 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. And such
insects advertise their toxicity using warning colours. This successful evolutionary pattern has also been utilized by
mimics. Over time, this has led to complex groups of co-evolved species. Conversely, some interactions between plants and insects are beneficial (see
pollination), and coevolution has led to the development of very specific mutualisms in such systems.
Systematics
This is a list of the orders and higher taxa of insects.
Within the subphylum Hexapoda, that consists of four
groups in total, the springtails (Collembola) are often treated as insects; however some
authors treat them as distinct from the insects in having a different evolutionary origin. This may also be the case for the rest
of the members of the Entognatha; Protura and
Diplura.
The true insects, those of the Class Insecta, are distinguished from all other arthropods in part by having
ectognathous, or exposed, mouthparts and eleven abdominal segments. The true insects are therefore sometimes also referred
to as the Ectognatha. Many insect groups are winged as adults. The exopterygote part
of the Neoptera are sometimes divided into Orthopteroida (cerci
present) and Hemipteroida (cerci absent), also called lower and higher Exopterygota; a full
classification is given below.
Subclass Apterygota
-
Subclass Pterygota
-
-
- Superorder Exopterygota[verification needed]
- Proposed superorder Dictyoptera
- Proposed superorder Paraneoptera
- Superorder Endopterygota
- Proposed superorder Mecopteroidea/Antliophora
