The mandible (Fig. 3), with its strongly toothed surface, is capable of biting and grinding into fragments a very varied diet. The food is moistened by the secretion of the salivary glands, which is capable of converting starch into the more soluble sugar. The food is further ground up by a series of hard ridges projecting into the inner face of the gizzard (Fig. 4, 7). The secretion of the so-called hepatic caeca is capable of emulsifying fat and rendering proteins soluble. Thus the ordinary food substances are reduced to a condition in which they are capable of diffusing from the lumen of the alimentary canal into the blood which floods the body cavity.
Fig. 4.—A female cockroach, Periplaneta, with the dorsal exoskeleton removed, dissected to show the viscera. Magnified about 2.1, Head; 2, labrum; 3, antenna, cut short; 4, eye; 5, crop; 6, nervous system of crop; 7, gizzard; 8, hepatic caeca; 9, mid-gut or mesenteron; 10, Malpighian tubules; 11, colon; 12, rectum; 13, salivary glands; 14, salivary receptacle; 15, brain; 16, ventral nerve cord with ganglia; 17, ovary; 18, spermatheca; 19, oviduct; 20, genital pouch, in which the egg-cocoon is found; 21, colleterial glands; 22, anal cercus. (From Latter.)
The external movement—one might almost say ‘the panting’—which is very obvious in the abdomen, the alternate flattening and deepening of this part of the body, is a movement of inspiration and expiration, the air being driven into the stigmata and so into the tracheae or breathing-tubes. There is a considerable variation in the rate of these pulsations, but the cockroach’s heart beats at an average rate of seventy to eighty contractions per minute.
Although cockroaches have fairly developed eyes, they seem to trust very largely to tactile impressions in appreciating their relations to the surrounding world. Their antennae and the palps of their first and second maxillae are constantly touching the surface on which they are resting or moving, and from time to time their antennae wildly wave in the air in a manner which suggests that they are smelling out the external circumstances which environ them. The 39,000 sensory ‘nerve-endings’ which are found in the antennae of the male cockroach are almost certainly olfactory in function. At the posterior end of the body the two ‘cerci’ are also sensitive to tactile impressions, and probably act at the hinder end of the cockroach as the antennae act at the forward end. Cockroaches are certainly keenly sensitive to light, and, as every one knows, they shun the light, and when detected in daylight or candle-light they make as quickly as they can for some dark hole or crevice in which to hide.
Fig. 5.—Egg capsule of P. orientalis (magnified). A, External view; B, opened; C, end view. (From Miall and Denny.)
Cockroaches breed during the summer, and their eggs are laid in packets of sixteen in a capsule or cocoon with rounded ends, and with an upper corrugated edge. These cocoons are very like the little hand-bags ladies have carried since the dressmakers denied them pockets. There are sixteen ovarian tubes in the female, and each of these deposits one egg in each cocoon. The ventral portion of the seventh abdominal segment in the female is shaped like the prow of a boat, and it is in this structure that the cocoon, or egg-case, is built up. Each egg is fertilised by a spermatozoon which has been deposited by the male in the spermatheca of the female. The eggs are placed in a double row, eight in each row, facing each other, and, as they gradually develop, it becomes apparent that the ventral face of one row faces the ventral face of the other row—just as the little choir-boys on the Gospel side of a chancel face the little choir-boys on the Epistle side, but much nearer together—and that their heads are all directed towards the corrugated ridge.
They are at first quite white, but with large black eyes, and it has often struck me how surprised they must be when they awake to consciousness and find themselves staring at a brother or sister cockroach just opposite, of whom they have had hitherto no consciousness. The ripe embryos secrete some fluid, probably saliva, which dissolves the ridge, and it is through this dissolved or softened ridge that they ultimately make their way into the outer world.
Young cockroaches are very active, running about and seeking everywhere for any food of a starchy nature. They are, in fact, miniatures of their parents, for a cockroach, like many of the primitive insects, has a direct development, and there are no such stages as caterpillar and pupa in their life-history.