In the adult stage, an insect has three pairs of legs (total = 6) and three distinct body parts. An insect also normally has a pair of antennae, two pairs of wings, and eyes and mouthparts adapted especially for its specific lifestyle.

An insect exoskeleton is composed of a hardened material called chitin that is similar to human fingernails. This gives the insect the structure to which muscles can attach and operate, allowing movement. The exoskeleton also protects the insect from desiccation, physical injury, and allows for the myriad of colors, shapes, and sizes that make insects so diverse and interesting.

The three main insect body parts are head, thorax, and abdomen. The head contains the antennae, eyes, and mouthparts. The thorax is the middle body part to which the legs and wings are attached. The abdomen contains digestive and reproductive organs internally and often reproductive structures externally. The sides of both the thorax and the abdomen are lined with tiny openings called spiracles, through which an insect obtains oxygen.

Insect mouthparts differ in appearance due to the fact that the diets of insects vary widely. One of the evolutionary marvels in the study of insects concerns the ability of these animals to feed on such a wide assortment of foods. Nearly all organic materials may be consumed by one or another insect. It is no wonder, then, that insect mouthparts are so different. Mouthparts are often used as a basis for separating insects into their respective orders or families. The four most common mouthparts are illustrated below.

Solid foods are consumed by insects with biting-chewing mouthparts. Beetles, caterpillars, and grasshoppers all have biting-chewing mouthparts. These insects leave behind tell-tale signs of feeding, holes in leaves, trunks of trees, or they simply consume the whole plant or animal.

Several different insects, such as mosquitoes, fleas, assassin bugs, and aphids have evolved a piercing-sucking mouthpart in which stylets actually pierce into plant or animal tissue allowing the fluids there to be sucked into the insects. Insects with this type of mouthpart are commonly associated with disease transmission in both plants and animals.

Specialized flies such as, the house fly, exhibit sponging mouthparts. In this group, saliva and regurgitated foods are pumped externally onto the food source to begin the digestion process. The dissolved or suspended food is then sucked up into the alimentary canal of the insect.

Butterflies and moths uncoil their long tube-like proboscis and insert it into the nectaries of a flower to siphon out the fluids found there. These specialized mouthparts are referred to as siphoning mouthparts.

Insect legs can be as different in appearance as the insects themselves and are often referred to in identification keys. They may be modified according to the specific behavior of the insect, whether it is for jumping, digging, swimming, hopping, grasping, or running. All legs typically consist of the segments shown in the diagram.

The essential make-up of all functional insect wings is the same: a thin membrane, which is supported by veins both around and within the margin. Stark contrasts between the wings distinguishes the four largest orders of insects. Minor differences in wing venation or shape further differentiate insect species. The major veins in an insect wing are illustrated below.