Insects are by far the largest group of organisms (Class Insecta) on earth. Either measured in terms of the number of species or of individuals. Insects live in every imaginable habitat on land and freshwater, and some have even invaded the sea. More than 70% of all named animal species are insects, and the actual proportion is certainly 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 the thorax; and a pair of antennas. In addition, they may 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 jaws or mandibles, which are hard and not segmented; a secondary pair of mouth parts, the maxillae, which are segmented; and the lower lip, or lip, which probably evolved from the fusion of another pair of maxillary-like structures. The upper lip, called the labrium, has a less certain origin. The hypopharynx is a short tongue-like organ (in chewing insects) that lies between the jaws and above the lip, the salivary glands generally opening at or near the hypopharynx. Within this basic structural framework. The parts of the mouth vary widely between insect groups, mainly in relation to their feeding habits.

Many orders of insects, like the coleoptera, the beetles; Hymenopterans, grasshoppers, crickets, and their relatives have chewing mouthparts or jaws. In other orders, the parts of the mouth can be elongated or stylish. For example, in some flies (Diptera order) such as mosquitoes, black flies and horseflies. there are six fused and perforating styles: the labrium, mandibles, maxillae, and hypopharynx; the lip wraps around the stilettos. In more advanced flies, the lip 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 pairs of legs are absent. The 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 flies, in 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 midsegment. The thorax is almost completely filled with muscles that operate the legs and wings. Insect wings emerge like a sac 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 detecting 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 inside the egg and becomes a young insect, which escapes by chewing or breaking the shell. During the course of their development to adulthood, young insects undergo ecdysis several times before becoming adults and permanently stopping molting. Most insects molt 4 to 8 times during the course of their development. The stages between molts are defined as instars.

Metamorphosis

There are two main types of insect metamorphoses: simple and complete. In simple metamorphosis, the wings develop internally during the juvenile stages and appear externally only during the resting stages that immediately precede the final molt. During this stage, the insect is called a pupa or a chrysisais, depending on the group to which it belongs. A pupa normally does not 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 tend to be quite similar to adults, differing mainly in their smaller size, less developed wings, and sometimes in 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 generally 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 have no legs or have legs and sometimes have leg-shaped appendages on the abdomen. The pupae do not feed and are usually relatively inactive. As pupae, insects are extremely vulnerable to predators and parasites, often covered by a cocoon or some other protective structure. Groups of insects with complete metamorphosis include moths and butterflies; beetles, bees, wasps and ants; flies and fleas.