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 [Pg 355] 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.