The head ([Fig. 221]) hangs nearly vertically from the neck. The large, compound eyes (ey.) are somewhat kidney-shaped; the long, jointed feelers are set in sockets just below the eyes. The front of the head is smooth and rounded; hinged on its lower edge is a flap which hangs down in front of the mouth, and is called the labrum or upper lip. Just behind the labrum, and attached to the side and front plates of the head, is the first pair of jaws, the mandibles (man., [Fig. 221]; Mn., [Fig. 222]); they work from side to side, and their inner edges, which bite against each other, are strongly toothed. The second pair of jaws, called the first pair of maxillae (max.¹, [Fig. 221]; Mx. 1, [Fig. 222]) is situated behind the mandibles. Each first maxilla consists of a two-jointed base and two branches; the inner branch bites against the inner branch of the first maxilla of the other side, while the outer and longer branch forms the maxillary palp, which acts as a small feeler. The two second maxillae (the third pair of jaws) are broadly like the first maxillae, but are partially fused to form the labium or lower lip ([Fig. 222], Mx. 2), which hangs down behind the mouth. The outer branches of the second maxillae are called the labial palps (Lab. Pa.). It is well to acquire clear notions of the arrangement of the jaws in the cockroach, in order to understand the great modification which the mouth-parts of many other insects have undergone. It is important to notice that the jaws of all arthropods work from side to side, not vertically as do the jaws of vertebrates.
Fig. 221.—Cockroach. The head and its appendages seen from the left side. cerv., one of the neck-plates; ey., eye; gen., side plate of head; man., mandible; max.¹, first pair of maxillae; max.², second pair of maxillae (labium). (× 5.)
The thorax consists of three segments, each of which bears a pair of legs, upon which the weight of the resting or walking insect is supported. It has been found, by instantaneous photography, that in a walking insect the weight is carried at any instant by the first and third legs of one side and the second leg of the other side. At the next step the body is carried by the remaining three legs. In the American ([Fig. 220]) and German cockroaches both sexes possess two pairs of wings, which are fixed at the front angles of the second and third segments of the thorax. The hind-wings, which alone are used in flight, are folded up fanwise when not in use, and are covered by the smaller fore-wings, which are generally called the wing-covers. In the common cockroach of this country, only the male has well developed wing-covers and wings. In the female the wing-covers remain small, while the wings themselves have disappeared.
Fig. 222.—Jaws of the Cockroach. Mn., mandibles; Mx. 1, first pair of maxillae; Mx. Pa., maxillary palp (outer branch); La., Ga., inner branch; Mx. 2, second pair of maxillae (labium); Lab. Pa., labial palp (outer branch); La., Ga., inner branch. (× 8.)
The abdomen consists of ten segments, although only eight can be clearly seen without dissection. The “telescopic” arrangement of the segments is well shown in [Fig. 223]. Where one segment joins the next, the chitin remains thin and flexible. In each segment the chitin forms a dorsal ([p. 217]) and a ventral plate, which are joined together at the sides by a flexible membrane. The chitinous covering of the upper surface of the abdomen is so transparent that the heart, a median tube, may be seen through it.
How an insect breathes.—If the sides of the abdominal segments be carefully examined, a small aperture will be seen perforating the thin layer of chitin between the dorsal and ventral plates, at the point where each segment joins the next; and, in the thorax, larger but similar openings will be found on each side between the first and second and the second and third legs. These holes are called spiracles ([Fig. 223], spir.); they lead into a complex system of air-tubes which ramify throughout the whole system and supply the organs with oxygen. The tubes are prevented from collapsing by a spiral lining thread of chitin. This peculiar method of respiration, which is characteristic of insects, should be carefully contrasted with the manner of breathing in a rabbit or a man ([p. 242]) and in a tadpole ([p. 344]). It ought to be borne in mind, however, that the essence of respiration is the same in all living things, and consists in a replacement of excess carbon dioxide by fresh oxygen ([p. 242]); the difference lies merely in the manner of effecting the exchange. In the rabbit, the blood, carrying the excess of carbon dioxide, is brought to the air (in the lungs); in the tadpole it is exposed, in the gills, to the dissolved air of the surrounding water; while in the cockroach the air is carried directly to the tissues needing fresh oxygen. The air of the tubes is renewed by a rhythmic action of the abdomen, which can readily be observed in the living insect.