A Blue Morpho butterfly (Morpho sp.).
Insects are the most numerous life form on the planet (in terms of number of species). Approximately seven out of every eight living species are insects. Insects show a number of characteristics, the three by which they are most easily recognised are:-
- The body is divided into three distinct regions - head, thorax, and abdomen. Each region is further divided into segments. Typically, there are six in the head, three in the thorax and eleven in the abdomen. In the more advanced insects segments may become fused together, particularly in the abdomen. The structure of a generalised insect is illustrated below, but insects are a very diverse group and have evolved many different forms:-
- There are three pairs of walking legs on the thorax, one pair to each segment. The legs show a very characteristic structure, but this is often modified to fulfil a variety of tasks, e.g. swimming or holding of prey. The generalised structure of an insect leg:-
- The adults of most insects show two pairs of wings, one pair on each of segments two and three. The wings are supported by a series of veins, the pattern of veins being important in classification. The naming of the veins and regions on an insect wing:-
More information on insect wings and flight.
Insects 'breathe' through a system of branching tubes, the trachea. Oxygen and carbon dioxide move along these by a process called diffusion. The trachea open on the surface of the body at special pores, the spiracles. The insect may control the size of these pores. It is this method of 'breathing' which stops insects from getting very large. The insect body cannot get bigger than a diameter of about three centimetres. Above this size diffusion of oxygen into the body tissues becomes too inefficient for the insect to live.
More information on insect respiration.
The head bears a pair of compound eyes. These consist of a number of individual 'eyes', each of which produces a separate image. Hence the overall picture that the insect sees is made up of a series of dots. This is rather like a television picture, but with much poorer sharpness. This type of eye is very good at judging distance and movement. Hence insects which are active predators, such as dragonflies, have very well developed eyes.
The antennae (or 'feelers') are mainly organs of smell and taste. They do have other functions in certain insects, e.g. they may be used to detect air currents. Hence the word 'feeler' does not describe their function and should not be used. They are made up of individual 'segments' that are attached to one another by flexible membranes. There are muscles inside the antennae that allow them to move. Antennae come in a variety of forms, and may be different in male and female, e.g. in chironomid midges. The antennae of male chironomid midges are 'bushy' because smell is very important in finding a female to mate with.
More information on insect antennae.
Related resources on the AES Web site
- Insect fact files
- Information on each group of insects
- Identification advice for non-members
- An identification key to adult insects
- Caresheets for insect pets