Insects are by far the largest group of organisms (Class Insecta) on earth. Whether measured in terms of number of species or number of individuals. Insects live in every conceivable habitat on land and freshwater, and some have even invaded the sea. More than 70% of all named animal species are insects, and the true proportion is undoubtedly much higher, because millions of additional forms are waiting to be detected, classified and named.

Most insects are relatively small, ranging in size from 0.1 mm to the head, thorax, and abdomen; three pairs of legs, all attached to thorax; and a pair of antennae. Also, they can have one or two pairs of wings. Most insects have compound eyes, and many also have ocelli. The mouthparts of insects are elaborate. They usually consist of the mandibles or mandibles, which are hard and unsegmented; a secondary pair of mouthparts, the maxillae, which are segmented; and the lower lip, or labium, which probably evolved from the fusion of another pair of jaw-like structures. The upper lip, called the labrium, is of less certain origin. The hypopharynx is a short tongue-like organ (in chewing insects) that lies between the maxillae and above the lips; salivary glands usually open in or near the hypopharynx. Within this basic structural framework. mouthparts vary widely among insect groups, primarily in relation to their feeding habits.

Many orders of insects, such as Coleoptera, the beetles; Hymenoptera, grasshoppers, crickets, and their relatives have chewing mouthparts, or mandibles. In other orders, the mouthparts may be elongated or style-like. For example, in some flies (order Diptera) such as mosquitoes, black flies and horseflies. there are six fused penetrating styles: the labrium, the mandibles, the maxillae, and the hypopharynx; the lip surrounds the stylets. In more advanced flies, the labium may be the main piercing organ, or it may expand into large soft lobes through which liquid food is absorbed. The thorax of the insect consists of three segments (tagmata), each of which has a pair of legs. Occasionally one or more of these legacy pairs are missing. Legs are completely absent in the larvae of certain groups—for example, in most members of the order Hymenoptera, bees, wasps, and ants—and among flies, the order Diptera. If there are two pairs of wings, they are attached to the middle and posterior segments of the thorax; if there is only one pair of wings, it is usually attached to the middle segment. The thorax is almost completely filled with muscles that operate the legs and wings. Insect wings arise as a sac-like outgrowth from the body wall; in adult insects, they are solid except for the veins. The internal characteristics of insects resemble those of other arthropods. Insects possess sophisticated means of sensing their surroundings, including sensory hairs for detecting chemical signals called pheromones. Most young insects hatch from fertilized eggs that are deposited outside their mother’s body. The zygote develops inside the egg into a young insect, which escapes by chewing or breaking the shell. During the course of their development into adults, young insects ecdysis several times before becoming adults and permanently ceasing to molt. Most insects molt 4-8 times during the course of their development. The stages between molts are defined as instars.

Metamorphosis
There are two main types of metamorphosis in insects: simple and complete. In simple metamorphosis, the wings develop internally during the juvenile stages and appear externally only during the resting stages immediately preceding the final moult. During this stage, the insect receives the name of pupa or crisais, according to the group to which it belongs. A pupa does not normally move much, although mosquito pupae move freely. A great deal of internal reorganization of the insect’s body takes place while it is a pupa or chrysalis. In insects with simple metamorphosis, the immature stages are often called nymphs. They are usually quite similar to adults, differing mainly in their smaller size, less developed wings, and sometimes their color. More than 90% of insects, including members of all the largest and most successful orders, show complete metamorphosis, in which the juvenile and adult stages often live in different habitats, have different habits, and are usually they have an extremely different shape. In these insects, development is indirect. Insect larvae are immature, often worm-like stages that differ greatly in appearance from adults of the same species. The larvae do not have compound eyes. They may be legless or have legs and sometimes have legs as appendages on the abdomen. The pupae do not feed and are usually relatively inactive. As pupae, the insects are extremely vulnerable to predators and parasites, often covered by a cocoon or some other protective structure. Insect groups with complete metamorphosis include moths and butterflies; beetles; bees, wasps and ants; flies and fleas.