Insects are by far the largest group of organisms (Class Insecta) on earth. Whether measured in terms of numbers of species or numbers of individuals. Insects live in every conceivable habitat on land and fresh water, and a few have even invaded the sea. More than 70% of all named animal species are insects, and the actual proportion is doubtless much higher, because millions of additional forms await detection, classification and naming.
Most insects are relatively small, ranging in size from 0.1mm to head, thorax, and abdomen; three pairs of legs, all attached to the thorax; and one pair of antennae. In addition, they may have one or two pairs of wings. Most insects have compound eyes, and many have ocelli as well. The mouth parts of insects are elaborate. They usually consist of the jaws or mandibles, which are tough and unsegmented; a secondary pair of mouth parts, the maxillae, which are segmented; and the lower lip, or labium, which probably evolved from the fusion of another pair of maxilla-like structures. The upper lip, called the labrium, is of less certain origin. The hypopharynx is short, tongue-like organ (in chewing insects) that lies between the maxillae and above the labium, the salivary glands usually open on or near hypopharynx. Within this basic structural framework. the mouth parts vary widely among groups of insects, mainly in relation to their feeding habits.
Many orders of insects- such as Coleoptera, the beetles; Hymenoptera, grasshoppers, crickets, and their relatives- have chewing, or mandibulae mouth parts. In other orders, the mouth parts may be elongated or style-like. For example, in some flies (order Diptera) such as mosquitoes, black flies, and horse flies. there are six piercing, fused styles: the labrium, the mandibles, the maxillae, and the hypopharynx; the labium sheaths the stylets. In more advanced flies, the labium may be the principal piercing organ or may be expanded into large soft lobes through which liquid food is absorbed. The insect thorax consists of three segments (tagmata), each of which has a pair of legs. Occasionally, one or more of these pairs of legs is absent. Legs are completely absent in the larvae of certain groups- for example, in most members of the order Hymenoptera, the bees, wasps and ants- and among the flies, order Diptera. If two pairs of wings are present, they are attached to the middle and posterior segments of the thorax; if only one pair of wings is present, it is usually attached to the middle segment. The thorax is almost entirely filled with muscles that operate the legs and wings. The wings of insects arise as sack-like outgrowth of the body wall; in adult insects, they are solid, except for the veins. The internal features of insects resemble those of the other arthropods. Insects posses sophisticated means of sensing their environment, including sensory hairs to detect chemical signals called pheromones. Most young insects hatch from fertilized eggs laid outside their mother’s body. The zygote develops within the egg into young insect, which escapes by chewing its way out or by bursting the shell. During the course of their development into adults, young insects undergo ecdysis a number of times before they become adults and stop molting permanently. Most insects molt 4 to 8 times during the course of their development. The stages between the molts are defined as instars.
There are two principal kinds 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 that immediately precedes the final molt. During this stage, the insect is called a pupa or chrysais, depending on the group to which it belongs. A pupa does not normally move around much, although the pupae of mosquitoes do move around freely. A very large amount of internal reorganization of the insect’s body takes place while it is a pupa or chrysalis. In the insects with simple metamorphosis, the immature stages are often called nymphs. They are usually quite similar to the Adults, differing mainly in their smaller size, less well-developed wings, and sometimes in their color. More than 90% of the insects, including the members of all of the largest and most successful orders, display complete metamorphosis, in which the juvenile stages and adults often live in distinct habitats, have different habits, and are usually extremely different in form. In these insects, development is indirect. Larvae in insects are immature stages, often worm-like, which differ greatly in appearance from the adults of the same species. Larvae do not have compound eyes. They may be legless or have legs as well as sometimes having leg like appendages on the abdomen. Pupae do not feed and are usually relatively inactive. As pupae, insects are extremely vulnerable to predators and parasites, they are often covered by a cocoon or some other protective structure. Groups of insects with complete metamorphosis include the moths, and butterflies; beetles; bees, wasp, and ants; flies and fleas.