The Cockroach breathes like other adult Insects, and the spiracles are ten in number—two on the thorax and eight on the abdomen. The thoracic spiracles may be pretty readily seen, but those on the abdomen are not so easy to make out. But by cutting away, with a fine pair of scissors, the edges of the plates that cover the upper and under surfaces of the abdomen and the membrane that unites them (Fig. 34), we may discover them as the open ends of small tubes. While dealing with the insect in this fashion, it will be easy to take out a piece of the tracheal tube, which may be compared with Fig. 28.
Fig. 34.—Cockroach, showing Spiracles.
The abdomen consists of a series of rings or segments, the exact number of which is rather difficult to decide, from the fact that some are concealed and others altered in form. Dr. Sharp[19] says that ‘it is considered that ten dorsal and ten ventral plates exist, though the latter are not so easily demonstrated as the former.’ In the male, ten above (dorsal) and nine below (ventral), and in the female two less in each case, may be made out without dissection.
From the sides of the tenth segment two organs, the cerci (Fig. 35, a), are given off, one on each side. These may be distinguished from the styles of the males by their presence in both sexes. Our inch lens will show that each cercus consists of sixteen rings. If we use the half-inch, we shall see that each ring is set with hairs of different lengths.
When we have got so far it may be well to compare the structure of a cercus with that of an antenna (p. 67). In each we have a succession of jointed rings giving flexibility to the organ, and the rings in each case are studded with hairs. It has been shown pretty conclusively—and we may verify the experiments—that the antennae are sense-organs. Are we not justified in coming to the conclusion that, since the antennae and the cerci resemble each other in structure, they also resemble each other in function? If the Cockroach receives sensations by means of the antennae, is it not probable that it also receives sensations by means of the cerci?
Having worked over the Cockroach from the outside, it will be advantageous to get some acquaintance with its internal anatomy. This is not a difficult matter. The specimen is to be pinned down, under water, with its back uppermost. The wings having been removed, a longitudinal cut is to be made down the centre from the posterior part of the abdomen to the back of the head, and the two sides of the integument turned back. Or the junction between the upper and lower plates on each side may be cut through with a cutting needle, and the whole integument removed.
The first task is to clear away the fat-body, a whitish substance which overlies the chief organs of the body. When this is picked to pieces and floated off the digestive system will be exposed. After this has been worked over a few times there should be no difficulty in dealing with similar parts in other Insects. At the back of the head lies the gullet or oesophagus leading into the crop (c), at the base of which lies the gizzard (g). The interior of this organ is furnished with six strong chitinous teeth, with small ridges of the same substance between them. Towards the posterior end are six cushions, all set with fine bristles. Behind this comes the stomach (v), into which open seven or eight tubes, closed at one end, and between it are the Malpighian tubes, which are concerned in the process of excretion. The small intestine (co) succeeds, and behind this is the rectum (r).
Fig. 35.—Alimentary Canal of Cockroach.