Despite the great variety within this class, three states can always be recognized in insects. Generally, they have a tracheal respiratory system and a single pair of antenna, and the body somites are grouped into three functional tagmata: the head, thorax, and abdomen (and generally one or two pairs of wings). This class, the largest group of animals, contains over 750.000 described species. The great success of this group is reflected in the huge amount of species and individuals and their great adaptive radiation. They occupy virtually every niche on land. For most insects, the main factor contributing to their success is their ability to fly.
The insects have an enormous ecological significance. They are the main pollinators for two-thirds off all plants. Some insects are pest species, causing misery, and many diseases are transmitted by insect vectors.
The head, being the first tagma, consist of an acron fused with a number of somites. To the acron belong the compound, faceted eyes, and two or three ocelli. The labrum, derived from a pair of appendages, belongs to the first somite. The second somite bears the pair of antennae, which are multisegmented and lack intrinsic muscles beyond the scape. An intercalary segment is formed from the third segment, which has lost its appendages. The paired mandibles (the main functioning jaws) belong to the fourth somite, the paired maxillae (often having a jaw-like function) to the fifth somite, and the single, but once paired, labium (usually for processing food as a tongue) to the sixth somite.
The tagma responsible for locomotory functions is the thorax, divided into three somites: the prothorax, mesothorax, and metathorax. The terga of the thorax are named nota and the two pairs of wings, if present, articulate on the notal and pleural processes of the mesothorax and metathorax. The wings are composed of two sheets of membranous cuticle supported by a system of sclerotized veins. The wings may be lost, or one or both pairs may be modified for protection or some other function. The wings of primitive insects are netlike, but in the evolution of wings there is a tendency toward reduction to a few cross veins and longitudinal veins. On each of the three thorax segments, a pair of legs articulates with the pleura. The most proximal part of the leg is the basal coxa articulating with the sclerites of the pleural area. It is followed by: the trochanter; sometimes a trochantellus; a large femur; a tibia; a tarsus, which in primitive conditions still consists of five tarsomeres; and a terminal pretarsus with paired claws. The thorax is filled mainly with muscles for locomotion.
A plesiomorphic abdomen consists of 11 somites, which in apomorph conditions can be fewer due to either reduction or fusion. The internal organs of the abdomen regulate the functions for digestion, reproduction and excretion. The pore of the genitals is terminal or subterminal. In males, they are often accompanied by intromittant and clasping organs and with female by ovipository structures. In many families, paired cerci are present. The fertilization of insects is internal, either by transfer of spermatophores or by a male intromittant organ. Normally the female lays eggs, and rarely deposits immatures of larval or later stages.
The metamorphosis of insects is divided into two groups, depending on the changes from hatchling to adult. When the hatchlings resemble the adults, though the wings and reproductive organs are still undeveloped, and when the adult stage is reached gradually with successive moults, the development is called hemimetabolous. The immature stages from hatchling to adult are called nymphs. The higher insects are characterized by holometabolous development from egg to adult and can be divided into three stages. The wing rudiments develop internally. The newly hatched larve is an active feeding stage, without wings. The next stage is a resting and quiet stage, usually spent in a protective location, and is called pupa. During pupation, the adult stage is reached after several moults. One of the factors contributing to the success during evolution of the higher insects is the different food sources, habitats and life styles between larvae and adults.
The class Insecta is divided into two subclasses: the Apterygota (primitively wingless, terrestrial insects) and Ptrerygota (winged or secondarily apterous, terrestrial, and aquatic insects). There are some 25 orders of insects and over 600 families.