Insects belong to a large division of the Invertebrate animals called the Arthropoda and in which are also included such animals as Crabs, Lobsters, Water Fleas, Centipedes, Spiders and Scorpions. The body of an Arthropod resembles that of a worm in being divided into a number of similar parts or segments, but rarely has more than twenty segments whereas the worm has about 150. Frequently several of the segments of an Arthropod are fused together. The skin is covered by a cuticle consisting of the horny substance chitin, which forms a protective armour for the animal and also provides a basis to which the muscles are attached. In crabs and lobsters the cuticle is particularly thick and is impregnated with chalk so as to make a very tough shell. The Arthropoda are also characterized by the possession of jointed legs, which are arranged in pairs and of which at least one pair are modified to form jaws. At the joints of the body and limbs the cuticle is soft and flexible, so that movement is possible. The following are the four chief classes of the Arthropoda :—
Crabs, lobsters, shrimps, prawns, water fleas, barnacles, wood lice.
Centipedes and millipedes.
Spiders, scorpions, harvestmen, mites.
The Chief Characteristics of Insects.
The body is divided into three parts, the head, thorax and abdomen.
The head bears a pair of antennae, a pair of compound eyes and three pairs of jaws
The thorax consists of three segments, each of which bears a pair of many-jointed legs. The 2nd and 3rd segments each bears a pair of wings.
The breathing organs are narrow branching air tubes or trachea;.
The heart is a long tubular structure, and the blood runs in large spaces or sinuses, I.e. the blood system is open.
Most insects pass through a complex metamorphosis in the course of their life history.
The Insecta are classified into a large number of orders, e.g. Coleoptera, Lepidoptera, etc.