insect

Body plan of an insect. The general features of the insect body include a segmented body divided into head, thorax, and abdomen, jointed legs, feelers or antennae, and usually two pairs of wings. Insects often have compound eyes with a large field of vision.

Any of a vast group of small invertebrate animals with hard, segmented bodies, three pairs of jointed legs, and, usually, two pairs of wings; they belong among the arthropods and are distributed throughout the world. An insect's body is divided into three segments: head, thorax, and abdomen. On the head is a pair of feelers, or antennae. The legs and wings are attached to the thorax, or middle segment of the body. The abdomen, or end segment of the body, is where food is digested and excreted and where the reproductive organs are located. (Class Insecta.)

Insects vary in size from 0.02 cm/0.007 in to 35 cm/13.5 in in length. The world's smallest insect is believed to be a ‘fairy fly’ wasp in the family Mymaridae, with a wingspan of 0.2 mm/0.008 in.

Many insects hatch out of their eggs as larvae (an immature stage, usually in the form of a caterpillar, grub, or maggot) and have to pass through further major physical changes (metamorphosis) before reaching adulthood. An insect about to go through metamorphosis hides itself or makes a cocoon in which to hide, then rests while the changes take place; at this stage the insect is called a pupa, or a chrysalis if it is a butterfly or moth. When the changes are complete, the adult insect emerges.

The classification of insects is largely based upon characteristics of the mouthparts, wings, and metamorphosis. Insects are divided into two subclasses (one with two divisions) and 29 orders. More than 1 million species are known, and several thousand new ones are discovered each year.

The study of insects is called entomology.

Anatomy The skeleton of an insect is external and is composed of chitin. It is membranous at the joints, but elsewhere is hard. The head is the feeding and sensory centre. It bears the antennae, eyes, and mouthparts. By means of the antennae, the insect detects odours and experiences the sense of touch. The eyes include compound eyes and simple eyes (ocelli). Compound eyes are formed of a large number of individual facets or lenses; there are about 4,000 lenses to each compound eye in the housefly. The mouthparts include a labrum, or upper lip; a pair of principal jaws, or mandibles; a pair of accessory jaws, or maxillae; and a labium, or lower lip. These mouthparts are modified in the various insect groups, depending on their diet.

The thorax is the locomotory centre, and is made up of three segments: the pro-, meso-, and metathorax. Each bears a pair of legs, and, in flying insects, the second and third of these segments also each bear a pair of wings. The wings are composed of an upper and a lower membrane, and between these two layers they are strengthened by a framework of chitinous tubes known as veins.

The abdomen is the metabolic and reproductive centre, where digestion, excretion, and the sexual functions take place. In the female, there is very commonly an egg-laying instrument, or ovipositor, and many insects have a pair of tail feelers, or cerci. Most insects breathe by means of fine airtubes called tracheae, which open to the exterior by a pair of breathing pores, or spiracles. Reproduction is by diverse means. In most insects, mating occurs once only, and death soon follows.

Growth and metamorphosis When ready to hatch from the egg, the young insect forces its way through the chorion, or eggshell, and growth takes place in cycles that are interrupted by successive moults. After moulting, the new cuticle is soft and pliable, and is able to adapt itself to increase in size and change of form.

Most of the lower orders of insects pass through a direct or incomplete metamorphosis. The young closely resemble the parents and are known as nymphs. The higher groups of insects undergo indirect or complete metamorphosis, hatching at an earlier stage of growth than nymphs. The life of these insects is later interrupted by the resting pupal stage when no food is taken and the larval organs and tissues are transformed into those of the imago, or adult. When an insect is about to emerge from the pupa, it undergoes its final moult, which consists of shedding the pupal cuticle.

Many insects are seen as pests. They may be controlled by chemical insecticides (these may also kill useful insects), by importation of natural predators (that may themselves become pests), or, more recently, by the use of artificially reared sterile insects, either the males only, or, in ‘population flushing’, both sexes, thus sharply reducing succeeding generations.

Throughout their history insects have proved remarkably resilient. Of the insect families alive 100 million years ago in the Cretaceous period, 84% are still living, compared with only 20% for four-legged vertebrate families.

The world's oldest insect remains yet found are the 400-million-year-old fossil remains of Rhyniognatha hirsti. Originally discovered in Scotland in the 1920s, the remains were only correctly identified in 2004 as belonging to an insect dating 20 million years older than the previously oldest insect fossil. The fossil showed distinctive mandibles only found on winged insects, pushing back the date for the earliest known flying insect by 80 million years.