An insect is a member of the class Insecta, which is in the phylum Arthropoda (see arthropods). This class contains organisms that in the adult normally have six legs, three distinct regions to the body (head, thorax, and abdomen), one pair of antennae, and, often, one or two pairs of wings. Most insects are terrestrial and breathe through a system of tubes called tracheae.

 

Insects include butterflies, beetles, bees, ants, spring-tails, silverfish, cockroaches, earwigs, termites, flies, aphids, lice, and fleas. More than one million different species are known out of a global diversity estimated at 10 million insect species. Of the 32 orders into which insects are classified, the largest is Coleoptera, or the beetles, with 125 different families and around 350,000 known species. Another diverse and large order is Hemiptera, the bugs.

 

Thorax

The thorax is divided into three parts: prothorax, mesothorax, and metathorax. Each part carries a pair of legs. Most insects have wings: one pair or, more usually two. They are attached to the mesothorax and metathorax.

 

Exoskeleton

The hard outer "shell" of an insect, called the exoskeleton, provides an anchor for the muscles. It is made of a tough waterproof substance, chitin, and entirely covers the insect's body.

 

Legs

All insects have three pairs, which are modified according to the use they serve, such as digging, jumping, striking, and swimming. They are also jointed rather like our own. Although their structure is very different, some of the parts have been given the same name.

 

Detailed anatomy of an insect leg 1. Coaxa – the basal segment by which the limb is attached to the thorax; it is usually short. 2. Trochanter – always a short segment. 3. Femur – like the thigh of vertebrates, this is usually the strongest segment of the leg. In such insects as grasshoppers it is thickened and contains powerful jumping muscles. 4. Tibia – this or the femur may be the longest segment; the tibia is usually slender. 5. Tarsus – this is almost always divided into two to five smaller segments. 6. Pretarsus – the insect's foot. It has a complicated structure. There is usually a pair of claws, and under each claw a pad called a pulvillus; between the claws is a bristle. This helps the insect to hold on to both rough and smooth surfaces.

 

Eyes

An insect usually has two kinds of eyes: small ones called ocelli on top of the head, and a large compound eye on each side of it.

 

The compound eyes of insects are very complicated. They consist of a large number of small facets, each of which is itself a tiny eye having a lens and a retina, just as our eyes have, though their structure is very different.

 

The number of these facets varies in different species from less than 10 to 4,000 in the house-fly, while some dragon-flies have up to 30,000.

 

Although they are complicated, these eyes are not as efficient as ours in producing a distinct image, and are not able to focus, but are very sensitive to movement, so that an insect can quickly see the approach of an enemy. This is why a fly is so hard to catch.

 

The ocelli are much simpler; although they are obviously light-sensitive organs of some kind, their real function is not known with certainty.

 

Antennae

In addition to its eyes an insect has another pair of sense organs on the head – the feelers or antennae. The most important function of the antennae is as organs of smell. A male moth finds the female by his sense of smell; if his antennae are cut off he cannot find her.

 

The form of the antennae varies greatly in different kinds of insects. They may be thread-like, thickened like clubs, or elaborately branched.

 

Mouth

Insects eat in one of two ways, either by chewing or by sucking. Their mouthparts are modified according to the way in which they eat. A grasshopper chew up leaves; a butterfly (right) sucks nectar from flowers.

 

How an insect breathes

Although the thorax is the chief center of activity in an insect, it does not contain the main organs of respiration. These are in the abdomen, or hinder part of the body, and consist of fine branching tubes called trachea, which open to the exterior by a row of holes, known as stigmata, on each side. Through these air enters and is carried to all parts of the body.

 

Metamorphosis

Many insects, including some of the most familiar ones, develop in a remarkable way. The illustration below shows the four successive stages in the life if a butterfly – a progression known as metamorphosis. Among other insects with a life-history of this kind are moths, beetles, bees, and ants.

 

Entomology

The scientific study of insects is called entomology. The foundations of entomology were laid by the Dutch naturalist, Jan Swammerdam (1637–1680). His researches and his book General History of Insects provided a system for classifying insects. Swammerdam also discovered (1658) the existence of red blood corpuscles.