Class of Subphylum Uniramia - Insecta

Class Insecta consists of nearly a million described species. There are more species of insects as compared to all the other species of animals combined. The most significant characteristic features of insects are the division of body into three tagmata that is, head, thorax and abdomen; presence of three pairs of legs and two pairs of wings on the thoracic region of the body hence some insects may have no wings. Insects have conquered the terrestrial environment so totally that they occupy every possible niche. They have' also invaded the aquatic habitats hence there are only a few marine species. Insects affect the ecology and human life in a number of ways. They have been both friends and foes of man. Evolution of flight, an impermeable cuticle and tracheal respiration are some of the issues which have contributed to their success on land. The head of insects is a composite structure. It bears a pair of antennae and a pair of compound eyes. There are also generally three ocelli. Three pairs of appendages comprise the mouth parts.

They are a pair of mandibles and two pairs of maxillae, of which the second pair of maxillae are fused to make the labium. The mandibles are covered in front by an upper lip or labrum. Into the anterior region of buccal cavity projects a median lobe such as process, the epipharynx. This arises from the base of the labium. The head is followed through a three segmented thorax - prothorax, mesothorax and metathorax. The tergum of the thoracic segments of insects is known as notum. The two pairs of wings articulate with meso and metathorax. While some primitive insects do not have wings (apterygotes), some higher forms have lost their wings secondarily. A pair of legs articulate with each of the three thoracic segments. Each leg is a jointed structure formed by coxa, trochanter, femur, tibia, and tarsus. The legs of different insects are variously modified to suit several functions such as walking, food collecting etc. The abdomen is composed of 9 to 11 segments. Abdomen bears a pair of sensory structures called anal cerci on the 11^th segment. An intromittent organ in males for the transfer of spermatozoa and an ovipositor in females for the deposition of eggs are as well present in the genital segments of insects.