2
the insect body-plan
The combination of modular body-segments and jointed appendages sets the arthropods apart from all other invertebrates. In the case of insects, this fundamental arrangement has been modified in countless different ways, providing insects with the means to pursue varied ways of life and exploit all kinds of opportunities to diversify.
2.3 • Segmentation and appendages
Insect body-shapes vary enormously, but always comprise three main sections—the head, thorax (to which the legs and wings are attached), and abdomen.
exoskeleton
The adult insect’s body is supported and protected by an exoskeleton, a more or less rigid outer shell that offers protection from the elements and helps prevent water loss.
In vertebrates, the body’s support system comes from the bones within—the endoskeleton. An insect’s exoskeleton provides support from the outside, at the expense of some bodily flexibility. However, although the exoskeleton’s constituent parts are rigid, the joints between the segments allow the insect to move its body-sections—in some cases very freely. It also fractures at certain joints in immature insects prior to being shed (molted), which enables the insect to grow.
The outer covering, or integument, of an insect is known as a cuticle. It is an efficient multipurpose casing for the body, giving it the necessary stiffness to resist damage from encounters with environmental hazards, and preventing dehydration. It typically has two layers—an outer waxy covering (the epicuticle) that provides waterproofing, and the inner layer (procuticle). The procuticle is made from a soft, flexible, protein-based connective tissue called chitin. Underneath the cuticle is the epidermis, a single layer of cells that produces the proteins that form the cuticle.
In some insects (especially those that live in damp environments), the procuticle remains single-layered and soft over part or all of the body. However, in most adult insects at least part of the procuticle develops a hard outer layer (exocuticle) of more rigid proteins. The development of the exocuticle is known as sclerotization. Body parts with particularly well-developed sclerotization include the “armor” of some beetles, and the biting mouthparts of dragonflies. Most insects have a well-sclerotized head capsule, enclosing all of the segments of the head. This capsule is shed as a single piece when an insect larva molts.
The larval Stag Beetle has a soft cuticle, as it lives in the shelter of decaying wood. The adult has a hardened, sclerotized cuticle.
The layers of an insect’s outer covering: The exocuticle, or outer part of the procuticle, is not present in all insects.
hair and scales
Many insects have hairy or even furry bodies. The hairs, or setae, each form from one individual specialized cell (a trichogen cell) within the epidermis—the hair grows outward through the cuticle. The function of setae varies between species and includes protection or camouflage, heat conservation, and storage facilities (the pollen baskets of bees are formed from clumps of setae). They can even provide self-defense (the setae of some moth caterpillars contain a toxin that irritates the skin or mouth of any would-be predator).
Caddisflies, butterflies, and moths have modified setae, which are expanded into wide, platelike scales. Layers of scales cover the bodies and most noticeably the wings of moths and butterflies—they provide protection, play a role in heat absorption, and their microstructure interferes with light waves to produce the diversity of iridescent color found in many members of this group. The wing scales often get dislodged, but the membrane below is still flight-capable.
Most insects have quite plentiful hairs, or setae, on at least some parts of their bodies.
the three body-sections
One of the first things we learn about insects’ bodies, besides the fact that they have six legs, is that their bodies comprise three distinct sections—the head, the thorax, and the abdomen.
A close look at a wasp or an ant makes clear that its head, thorax, and abdomen are very distinct, the attachments between the three parts being marked by a pronounced narrowing of the body. In other insect groups the distinctions are less obvious, but still apparent on examination.
Like almost all freely moving animals, insects have their primary sensory and eating equipment at their front ends, to gather information about whatever they are approaching (and then, if it is food, to consume it). The head usually bears two obvious compound eyes and a pair of antennae. There will also be a set of mouthparts, which vary greatly across different groups of insects in the way they are shaped and how they work. In a few cases, adult insects have no functional mouthparts and do not feed at all during their short lives. The antennae and mouthparts are all modified versions of the uniramous (a single, unbranched series of segments) appendages that grow in pairs from the segments of the head, and are segmented, like the legs.
The three segments that form an insect’s thorax, from front to back, are the prothorax, the mesothorax, and the metathorax. Each bears a pair of legs (though one or more pairs may be rudimentary in certain species), used for walking, running, swimming, jumping, or just perching. The thorax is also the attachment point for the wings in flying insects, with the front pair attached to the mesothorax and the hind pair to the metathorax. The thoracic segments also contain musculature to drive the wings.
Wasps and related insects show very distinct body-sections, but all other insects’ bodies are similarly divided into head, thorax, and abdomen.
The segments at the thoracic end of the abdomen lack appendages, but the segmented structure is usually evident, and often even accentuated by contrastingly colored markings. Some insects have 11 or 12 abdominal segments, but it varies—more recently evolved groups have fewer. The insect’s reproductive organs, which may have visible external parts, are found near the abdomen tip, while the last segment has a pair of appendages (cerci). These are prominent in some groups (as in the “forceps” of ear wigs) and often have a reproductive function—for example, male dragonflies use their cerci (or “claspers”) to grip the female prior to mating.
Earwigs have prominent cerci—the paired appendages on the last segment of the abdomen.
Damselflies have very long abdomens, and boxlike thoraxes that bear their four large wings and long legs.
Stoneflies’ wings lie flat along their abdomens when they are at rest.
Mayflies lack functional mouthparts in adulthood, so they have small and relatively simple heads.
joining up
The neck, or cervix, that links the head to the thorax is membranous, with muscles to control head movement. Many insects can move their heads freely. The junction between thorax and abdomen is less flexible, as the first abdominal segment (the propodeum) is fused to the metathorax. The slim “waist” of some insects, such as wasps, is formed by a particularly narrow propodeum, giving these insects more range of movement between the thorax and the abdomen.
segmentation and appendages
In all arthropods, the body is made up of repeated segments, joined together front to back. That is very apparent in a caterpillar, but much less so in an adult butterfly.
The standard, most primitive arthropod body is made up of a repeated series of near-identical body-segments, each of which bears a pair of jointed appendages. In some arthropods, the appendages are biramous (split into two branches), and in most such cases one of the branches of a biramous appendage functions as a leg and the other as a gill. This structure can be seen in crustaceans, such as crayfish. However, in insects the appendages do not have the second, gill branch, and are classed as uniramous (single-branched).
Very few insect species show, when adult, a continuous body structure made of repeated, similar-looking segments. Instead, the body-segments (if apparent at all) are arranged in distinct groups, and their shape is modified according to their function and position in the body. Even the insect’s head was originally formed of five to seven distinct segments, although these are not apparent on an intact adult insect as they are fused into a continuous capsule.
The abdominal segments do not have appendages. Those of the thorax and head do bear appendages but these are uniramous. The appendages of the thorax function as legs, while those on the head form feeding and sensory apparatus.
The trilobite genus Kolihapeltis was notable for the long spines that project from the head segments.
Stylonurus was a genus of chelicerates—the arthropod group that contains spiders and scorpions. The body and leg segmentation was relatively simple.
The insect’s leg, like its body, has a segmented structure, with movable joints between some of the segments. Its main parts are the coxa (the first segment, where it joins the body—typically very short), the trochanter (second segment, also small and functioning like a hip joint in vertebrates), the femur (the third and usually largest and thickest segment), the tibia (the fourth and often longest segment, articulating with the femur in a flexible, knee-like joint), and the tarsus (formed of several small segments, or tarsi, giving some flexibility) and forming the last joint. The last tarsal segment, the pretarsus, supports the claws.
The segments of an insect’s leg are adapted to form sturdy midsections that connect via flexible joints.
internal order
The body-segments also show some repeated structural features with regard to their internal anatomy—this can be seen with aspects of the nervous system and circulatory and respiratory systems, for example. The insect body-plan is sometimes described as modular because of that repeated arrangement, with each segment having some degree of autonomy from its neighbors.
Many caterpillars show obvious body-segmentation, and the three pairs of true legs at the front of the body are jointed.
Adult insects of the more-recently evolved groups, such as this chrysid wasp, lack a uniformly segmented appearance. The head and thorax in particular look like single structures.
limb structure
Insects use their legs for much more than locomotion, including such behaviors as gripping prey or a mate, grooming, and performing social displays. Whatever the legs’ function, though, all share the same basic plan.
Mantises use their long, barbed forelimbs to catch and grip prey.
As we have seen, insects’ legs are segmented and have five main sections, each formed by a single segment except the outermost one (the tarsus), which is formed from a “stack” of several small tarsi. The five sections are attached to each other through flexible hinge joints. The integument of the limbs is the same as for the body, comprising a cuticle that overlies the epidermis. Legs often have plentiful setae. The dense long hairs on the tarsi of diving beetles and bugs function as swimming “flippers,” while the front legs of dragonflies and damselflies have rows of long, stiff setae that form a sort of net to help capture prey on the wing.
The last section of the tarsus, the pretarsus, usually has one or two small hooked claws, which help the insect hang on to the substrate as it walks or rests. Other structures that may be found on this segment include hairy adhesive pads or lobes, which provide grip on flat surfaces.
In addition to the main sections, legs may have additional projections. The forelimbs of praying mantises, for example, have sharp spines and serrations for piercing prey. In burrowing or burying insects, such as mole crickets or scarab beetles, the tibia or tarsus has flat, hard projections for digging into the earth.
shaped for life
The relative size and shape of insects’ leg sections can provide clues to the way they live. Long but sturdy legs indicate a fast runner, while more delicate legs suggest an insect that perches or climbs. Jumping insects usually have hind legs that are much longer than the front or middle pair, and show disproportionately large femurs to power their leaps. Digging insects have thickened front legs, and predators often also have larger and longer front legs. Swimmers may have one or more pairs of specialized, fin- or oar-like swimming legs.
The hefty, clawed front legs of mole crickets are used for digging in soft earth.
Millipedes and other myriapods are arthropods, like insects, but have a simpler body-plan and far more pairs of legs.
Most beetles can fly, but have strong flexible legs and travel more on foot than in the air.
Worker ants lack wings, but their long legs enable them to climb easily and run quickly.
Phasmids use their long, delicate legs to climb through plant foliage.
the wings and elytra
Most insects are winged in their adult form and can fly. Insect wings are often breathtakingly beautiful in their structure, perhaps even more so when given a closer look.
The power of flight has allowed insects to spread and thrive throughout even the most inhospitable habitats on Earth. Some spend almost their whole adult lives on the wing and fly with extraordinary efficiency and agility, while for others the wings are weak and are an “emergency” option to escape a sudden threat. Despite this diversity, all winged insects are thought to share a single common ancestor, and their wings show many commonalities.
Most insect groups have two pairs of transparent wings, formed from a fine membrane and supported by a branching network of veins, which may be nearly invisible or dark and prominent. There is typically a particularly long and sturdy vein (the costa) at (or almost at) the leading edge of the wing—the insect can usually fly even with considerable wing damage as long as the costa is intact. The exact pattern of wing venation is important when it comes to telling apart some confusingly similar insect families.
In some groups, including the bees, wasps, and mayflies, the hind wings are considerably smaller than the forewings. Among the grasshoppers the forewings are long and narrow, with a thick and leathery texture. When the insect is resting the forewings protect the hind wings, which are membranous and shorter but very broad. In the true flies (Diptera), the hind wings are reduced to small, club-like structures (halteres).
Ladybugs and other beetles keep their hind wings folded up under their elytra. They can only fly once the elytra are opened and the wings unfurled.
The dark smudges on the forewings of the male Gatekeeper Butterfly are formed by androconia—special scent-producing scales.
Butterflies use their wings to regulate their temperature, opening them to warm up and closing them to prevent chilling or overheating.
Caddisflies have dense setae on their wings, and, as we have seen, those of butterflies and moths have layers of overlapping scales. These include, in males of some species, specialized scent-releasing scales (androconia)—the insect wafts this scent toward a female during attempts to mate. Some membranous-winged insects have patches of pigment on their wings, forming patterns that may be shown off in wing-flicking courtship displays.
elytra
The elytra, or wing cases, of beetles are modified forewings. They are heavily sclerotized, thick, and strong, protecting the hind wings, which are folded beneath, as well as the abdomen. When the insect takes flight, its elytra lift and part to allow the hind wings to unfold. The elytra are held in this raised position during flight. Some true bugs have partially sclerotized forewings, known as hemelytra.
The elytra do generate some lift in flight, but their primary purpose is protection when the beetle is on the ground—in the air, beetles are much clumsier than most four-winged insects, and their flight is less energy-efficient. However, the elytra’s sturdiness enables beetles to dig tunnels, scramble through thorny vegetation, and generally survive conditions that would quickly tear the exposed wings of other insects to shreds. The elytra are also a canvas for color—dull tones that blend in, bright warning patterns, or vivid iridescence—which has various possible functions including both camouflage and signaling.
unusual bodies
Although the adult forms in all insect groups have three-sectioned, six-legged, exoskeleton-supported bodies, within these constraints, there are some remarkable variations in body form.
Some of the most bizarre-looking insects in the world are the rhinoceros beetles (family Scarabaeidae), males of which have large, rigid sclerotized horns on their heads and thoraxes, which they use in battle when competing to access females. In the enormous Hercules Beetle (Dynastes hercules), found in South and Central America, the thoracic horn may be longer than the rest of the body and extends even beyond the insect’s sizable mandibles. Males of the Giraffe Weevil (Trachelophorus giraffa) of Madagascar also have exaggerated anatomy for fighting purposes—in this case, a long, slim, giraffe-like “neck” formed by an elongated head and thorax.
Butterflies of the genus Kallima replicate the appearance of dead leaves when resting with their wings closed.
Camouflage is another driver for unusual physical modifications to evolve, such as the flattened extensions of cuticle on the legs and bodies of leaf insects (family Phylliidae) and the Orchid Mantis (Hymenopus coronatus), a southern Asian species and one of several mantises with a petal-like appearance. One of the longest insects in the world, the 22-inch (56cm) phasmid Phobaeticus chani of Borneo is a twig mimic with very slender and elongated body and legs.
The leaf insects (family Phylliidae) have been shaped by evolution into stunning leaf-mimics.
Male rhinoceros beetles (subfamily Dynastinae) use their “horns” to battle with other males and to dig.
The rove beetles (family Staphylinidae) have undersized elytra, leaving a considerable length of segmented abdomen fully exposed. This gives these ground-dwelling beetles more bodily flexibility, enabling them to wiggle into small spaces. Most species have full-size wings and can fly, though in some species the elytra are fused.
eversion therapy
Some insects have body parts that are normally held within the body but can be turned inside out (everted) to protrude outward. These are usually involved in the release of volatile chemicals into the atmosphere—pheromones to attract a mate, or repellent substances to ward off predators. One of the most spectacular examples is provided by an Asian and Australasian moth called Creatonotos gangis. Males of this species have a pair of very large, forked organs called coremata, which are covered with long, fine hairs. When everted from the abdomen’s tip, the coremata may be much longer than the rest of the moth’s body—their size depends on the foods the moth ate as a larva. The hairs of the coremata release a potent pheromone to attract females.
INSECT COLORS
Many insects, from beetles to bees and dragonflies to deerflies, have dazzling coloration. Its purpose may be to entice, disguise, or warn.
The white light of the Sun contains all of the different wavelengths of light that we see as color—we can see the full rainbow of tones when we shine white light through a glass prism, causing refraction (bending and splitting of the light beam). Different shades are produced by the properties of the material that the light strikes.
In nature, color is formed in two different ways. The first is through pigmentation—molecules within the organism’s cells that absorb light of certain wavelengths and reflect back others, which we see as visible color. The second is structural coloration, where the body has some physical feature that bends or scatters white light into different wavelengths. Here, the viewing angle can affect the color tone that we see, and the color is often brightly shining as well as constantly changing—this is known as iridescence, and most commonly produces blue, green, and violet tones.
In many insects, the colors we see are a mixture of pigmentation and structural color. Iridescent butterflies produce their coloration through the shapes of the scales on their wings, and iridescent beetles have ridges on their epicuticles that bend light, but both have underlying layers of pigment cells that enhance the effect. Dragonflies of some species have dark-pigmented bodies but develop a thick waxy bloom (pruinescence) on the cuticle, which creates pale blue tones through light-scattering.
Pigmentations found in insects include melanins (responsible for dark browns and grays), carotenoids (giving red, orange, and yellow tones), and many more. Some pigments are synthesized in the body, while others are sequestered directly from food the insect eats—foods eaten as a larva may provide pigmentation for the adult body.
Many insects’ colors are drab, for camouflage, but bright and even iridescent coloration can be effective camouflage in the right sort of habitat. Butterflies, with the ability to close their wings and thus completely hide the upper sides, can have the best of both worlds—a camouflaged underside for hiding and a colorful upper side to appeal to mating partners.
Ultraviolet and red
The ultraviolet part of the light spectrum is invisible to human eyes, but many insects can see ultraviolet light. This means that some of their colors are not visible to us, but are to each other. By contrast, most if not all insects cannot see red light. Red, carotenoid-based colors are present in many insects nonetheless, often as warning coloration, for example, on the elytra of ladybugs. It is there for the benefit of predatory birds that can see red light—these insects are toxic or they taste bad, and if a bird eats one it will remember that similarly bright red insects are to be avoided in the future.
Natural colors in the insect world Paper Kite Butterfly
Monarch Butterfly
Seven-Spot Ladybug
Pale Tussock Moth Caterpillar
Buff-Tailed Bumble Bee
Rose Chafer Beetle
Banded Jewel Beetle
Scarce Chaser Dragonfly