THE Insecta is a group so large and diverse and yet so specialised that the study of it requires special methods and knowledge. The study of insects, or entomology, has its roots in zoology, but it now flowers almost independently of the mother science. The blooms, however, must be viewed together to appreciate their mutual contributions to the pattern of life.
The insect body is divided into three well-defined regions : the head, a division called the thorax, and the abdomen. The head carries such organs of sense as the eyes and antenna;; the thorax, the locomotor)’ limbs or appendages—the legs and wings—while the abdomen is completely devoid of such structures, any appendages which it bears being connected with pairing. The cavity of the thorax is almost completely filled by the wing muscles, while that of the abdomen is occupied by the digestive glands, fat body and such parts of the alimentary canal as the crop and gizzard, and the reproductive organs. If the external features of an insect are examined, it will be seen that the rings or segments of the abdomen are much more complete and more easily made out than those composing the head and thorax. In the adult, the rings of the head and thorax have fused in connection with the special functions of eating and flight, etc. As already mentioned, each ring of the primitive arthropod carries paired limbs, so that the presence of paired limbs is evidence of a segmental origin which is othenvise hidden in such regions as the head and thorax.
On this evidence, coupled with other observations relating to the arrangement of the nervous system, we find that the head is composed of six or seven rings or segments, four of which are indicated by the antennas, and three pairs of jaws. The thorax is made up of three rings which are identifiable by the three pairs of legs which they carry, the wings being borne on the middle and last of these segments. The abdomen
is composed of eleven rings, which are much less specialised than those of the head or thorax. Any apparent reduction in the number of rings towards the tip of the abdomen is due to telescoping and specialisation in connection with the organs of reproduction.
THE INSECTS’ SHORT CUT TO DEEP BREATHING INSECTS have a peculiar method of breathing. Quite a lot of the lower animals breathe either entirely, or in part, by the surface of the skin, but the thick horny covering which is present in insects makes this impossible. Unlike higher animals, they have no structures at all comparable with lungs, where the blood of land vertebrates is oxygenated and from which the oxygen is carried round in the blood circulation to the various tissues of the body. No such carrier of oxygen exists in insects.
The blood of insects no doubt serves primarily for food distribution and excretion. The air takes a short cut to the tissues through a series of apertures situated along the sides of the body; in the typical insect there are ten pairs of these apertures (called spiracles). Two are found on either side of the thorax, and the remaining eight pairs are along the sides of the abdomen. These spiracles, which are oval in outline, can be opened and closed, thus enabling the animal to control its respiration in accordance with its needs. The spiracles lead into long, ramifying tubes which come into intimate contact with all the tissues, and it is along these tubes that the air travels. The important thing to remember is the fact that the air is brought directly to the organs of the body and therefore requires no carrier like the blood.
Following on this is the question of circulation, of which one of the main functions in the higher animals is that of respiration. In insects where respiration is carried on independently of the blood, we find that apart from the tubular heart and aorta, there are no structures corresponding to blood-vessels. The blood from the aorta is discharged into the body cavity—in other words, no well-defined blood system exists.