Unlike vertebrates, insects are built with their skeleton on the outside of the body. This is known as the exoskeleton. It is made up of a system of sheets and veins interconnected by thin, elastic skin. The result is that the sheets and veins are moveable in relation to each other, which is the same principle as in early armour. The exoskeleton consists among other things of chitin and proteins. Chitin is a highly elastic matter that is chemically similar to cellulose. Chitin forms the basis of the exoskeleton in the form of microscopic threads. The second essential ingredient, protein, is disposed between the threads of chitin.
The insect body consists of three parts: head, thorax and abdomen. They have two large compound eyes, each made up of a large number of small facets. Many insects also have some small eyes, 1-3 of them, in the middle of the head. The head carries a few feelers or antennae. They are articulated and have various looks depending on the individual species. Some are crested, other club-shaped or brush-shaped. Cockroaches and crickets have elegant, whip-shaped antennae.
The mouth is surrounded by several pairs of mouthparts which are actually a kind of remade limbs. They can also have quite different characteristics depending on the various insect groups. The majority of food pests have mandibles, which are external mouthparts that are used to crush food.
In most insects, the thorax is composed of three segments; the prothorax, the mesothorax, and the metathorax. They may be more or less fused. Each segment of the thorax includes a pair of legs and since the abdomen in theory is missing limbs, adult insects typically have six legs. The typical segments of the insect leg are divided into the coxa, the trochanter, the femur, the tibia, the tarsus, and the pretarsus. Most insect feet have five segments and the outer link ends with two movable claws. In addition to the legs, most insects have two pairs of wings attached to the thorax. One pair is on the mesothorax while the other is on the metathorax. The wings are generally thin, transparent, non-coloured membranes supported by a system of veins. The wings can be hairy and butterflies have a dense coat of flat, scale-like hairs. The job of the front pair of beetle wings, the so-called elytrons, is mainly to protect the rear wings and abdomen when the insect does not fly.
In some insect groups, for example as it is the case with ants, there is a constriction between the chest and abdomen. The segments of the abdomen are covered by sheets, one on the back, and one on the front. These are connected by membranes. Originally the abdomen is formed by 12 segments. However, with the majority of insects the number is reduced. Some of the rear segments are designed for reproduction and the outermost part is the anus. In general, insects are missing legs on the abdomen, but silverfish have transmuted limbs of some of the abdomen segments. Caterpillars have abdomen segments called prolegs.