The internal organs.—Figs. [ 163] and [ 223], which represent a general dissection of a frog and of a cockroach respectively, exhibit in the most striking manner the essential differences in the structure of vertebrates and arthropods. The fact that the skeleton of the frog is wholly internal and that of the cockroach wholly external, has already been mentioned. It will be seen, further, that while in the frog the central nervous system (the brain and spinal cord) lies entirely dorsal to the digestive canal, in the cockroach the great nerve chain ([Fig. 223], n, n,) is mainly ventral—the only dorsal part of the central nervous system being the so called brain (brn.), which is connected with the ventral chain by a ring surrounding the gullet (gul.). The heart of the frog is ventral to the digestive canal; that of the cockroach is the most dorsally placed organ of the body.
Fig. 223.—Cockroach; general dissection of female from the left side. abd. 1, first, and abd. 5, fifth abdominal segments; brn., “brain”; cer., cercus; c.gl., glands which form the egg-case; cr., crop; f., feeler; giz., gizzard; gul., gullet; int., intestine; lab. pa., labial palp; mx. pa., maxillary palp; n, n, nerve chain; r. ov., right, and l. ov., left ovary; sal. du., salivary duct; sal. gl., salivary gland; spir., spiracles; st. 7, brood-chamber; th. 1, first, th. 2, second, and th. 3, third thoracic segments. (× 2.)
Habits and life-history.—Cockroaches infest kitchens and pantries; they are of social habits, and hide together in crevices during the day, but come forth at night to feed. They are not at all fastidious as to diet, but are especially fond of starchy foods, which they are able to digest ([p. 234]) by means of the fluid formed in their large salivary glands ([Fig. 223], sal. gl.). The eggs are laid sixteen at a time in a little oblong case, which the female carries about in a boat-shaped receptacle (st. 7, [Fig. 223]) at the end of her abdomen, until she finds a suitable place in which to deposit it. When the little cockroaches hatch, they are quite white, but except for the absence of wings they closely resemble the parents in form, and run about and feed freely from the first. The chitinous exoskeleton is shed from time to time as the animal increases in size, a new coat, at first soft and wrinkled, but rapidly stretching and hardening, having previously formed beneath the old one. Shortly before the last moult, the wing-covers and wings begin to grow out from the angles of the second and third thoracic segments. In this manner the young animal gradually takes on the form and dimensions of the adult.
The position of the cockroach in the insect class.—The cockroach is familiarly known as the “black-beetle,” but the name is a very misleading one, because the true beetles differ from cockroaches not only in structure, but also in life history. Cockroaches are grouped with earwigs, grasshoppers, crickets, and locusts in the order Orthoptera,[28] a term alluding to the fanlike folding of the hind-wings.
63. A WATER BEETLE.[29]
1. The habits of Dytiscus.—Search a pond for water beetles. Put them in a wide-mouthed bottle with some of the weeds to be found in the pond, and take them home and observe their habits. Among the larger beetles—especially from ponds with a clear surface (not covered with duckweed, etc.)—will probably be seen some with a yellow band round the edge of the upper surface. These are Dytisci. Notice the ovoid, smooth body, the breadth of the hind legs, and—in the male—the cup on each fore leg. Try to see how the legs of each pair are used. On what does the animal feed? Take a small piece of meat in a pair of forceps and hold it near the beetle’s head. If the animal does not notice it, stroke one of the feelers with the meat; how does the beetle behave? What is the use of the feelers? Notice how the beetle rises to the surface immediately it stops paddling. Which end of the body sticks out of the water? Why does the beetle need to come to the surface? In April look for larvae ([Fig. 224], A), and describe their appearance and habits.
2. External characters.—Kill a Dytiscus by dropping it for a moment into boiling water. Examine it first from the dorsal surface, noting the almost unbroken oval outline, and the firmness of the armour. Notice that the inner edges of the two wing-covers fit closely together in the middle line. How does the beetle compare in this respect with the cockroach? Observe the small triangular plate between the anterior ends of the wing-covers; open out the covers and wings to find of which segment of the thorax it is a part. Examine the wing-covers and wings closely; notice that the wings are folded transversely as well as longitudinally. Spread and pin out one wing-cover and wing of one side, and draw a dorsal view of the animal. Dissect out the jaws and compare them with the jaws of the cockroach ([Fig. 222]). Examine and draw one leg of each pair.
The bordered little diver.—In nearly every English pond there at times occurs a beetle—an inch or more in length—which is known to naturalists as Dytiscus marginalis. The name is somewhat cumbrous, but it would be difficult to find a more appropriate one; for “bordered little diver”—the plain English of the scientific term—indicates at once the peculiarities which soonest strike the observer: the creature’s skill in diving, and the yellow band which runs round the edge of the olive-green body. The outline of the beetle is an almost unbroken oval, and it is worth noticing that either such a boat-shape ([Fig. 224], B), or the cigar-shape of the torpedo is adopted by almost all actively swimming aquatic animals. The resistance of the water is further lessened by the smoothness of the beetle’s armour, which forms a hard shell enclosing the body. As in all insects, the body is divided into head, thorax, and abdomen. The eyes are large, and are so arranged that the animal can at the same time see objects both above and below—a great advantage to a creature living so active a life. The feelers are very sensitive organs of touch, and possibly of smell also. The jaws consist of one pair of mandibles and two pairs of maxillae, and are shaped much like those of the cockroach; they are powerful, and render Dytiscus a very formidable enemy to the more peaceable inhabitants of the pond.