Читать книгу A Text-book of Entomology - A. S. Packard - Страница 12

The insects differ from other arthropods in that the body is divided into three distinct regions,—the head, thorax, and abdomen, the latter regions in certain generalized forms not always very distinctly differentiated. The body behind the head may also conveniently be called the trunk, and the segments composing it the trunk-segments.

In insects the head is larger in proportion to the trunk than in other classes, notably the Crustacea; the thorax is usually slightly or somewhat larger than the head, while the hind-body or abdomen is much the larger region, as it consists of ten to eleven, and perhaps in the Dermaptera and Orthoptera twelve, segments, and contains the mid- and hind-intestine, as well as the reproductive organs.

When we compare the body of an insect with that of a worm, in which the rings are distinctly developed, we see that in insects ring distinctions have given way to regional distinctions. The segments lose their individuality. It is comparatively easy to trace the segments in the hind-body of an insect, as in this region they are least modified; so with the thorax; but in the head of the adult insect it is impossible to discover the primitive segments, as they are fused together into a sort of capsule, and have almost entirely lost their individuality.

In general it may be said that the head contains or bears the organs of sense and of prehension and mastication of the food; the thorax the organs of locomotion; and the abdomen those of reproduction.

When we compare the body of a wasp or bee with that of a worm, we see that there is a decided transfer of parts headward; this process of cephalization so marked in the Crustacea likewise obtains in insects. Also the two hinder regions of the body are, in a much greater degree than in worms, governed by the brain, the principal seat of the intelligence, which, so to speak, dominates and unifies the functions of the body, both digestive, locomotive, and reproductive, as also those of the muscles moving the different segments and regions of the body. To a large extent arthropodan morphology and class distinctions are based on the regional arrangement of the somites themselves. Thus in the process of grouping of the segments into the three regions, some increase in size, while others undergo a greater or less degree of reduction; one segment being developed at the expense of one or more adjoining ones. This principle was first pointed out by Audouin, and is called Audouin’s law. It is owing to the greater development of certain segments and the reduction of others, both of the body-segments and of the segments of the limbs, that we have the wonderful diversity of form in the species and genera, and higher groups of insects, as well as those of other arthropods.