Fig. 88.—Abdominal Spiracle (left side) in side view, show­ing the bow: × 70; p, later­al pouch of spir­acle, seen from within. The tes­sel­ated struc­ture of the spir­acle and tra­chea is shown at A (× 230), and the mar­gin of the exter­nal aper­ture at B (× 230). (P. ameri­cana.)

Each of the eight abdominal spiracles is constructed on this plan; the first merely differs from the others in its larger size and dorsal position, being carried upon the lateral margin of the first abdominal tergum, whereas the others are placed on the side of the body, each occupying an interspace between two terga and two sterna. The bow is of about the same length in all; hence the apparent disproportion in the figures of different spiracles. The external aperture of the abdominal spiracles is oval or elliptical, placed vertically and directed backwards.

We have already pointed out that the wall of the air-tube, for a short distance from the spiracular orifice, has a tesselated instead of a spiral marking. In the thoracic spiracles the tesselated cells are grouped round regularly placed setæ (fig. [85] I). The chitinous cuticle within the opening is crowded with fine setæ, which are often arranged so as to form a fringe on one or both sides of the internal aperture. (Supra, p. [152].)

Mechanism of Respiration.

In animals with a complete circulation, aërated blood is diffused throughout the body by means of arteries and capillaries, which deliver it under pressure at all points. Such animals usually possess a special aërating chamber (lung or gill), where oxygen is made to combine with the hæmoglobin of the blood. It is otherwise with Insects. Their blood escapes into great lacunæ, where it stagnates, or flows and ebbs sluggishly, and a diffuse form of the internal organs becomes necessary for their free exposure to the nutritive fluid. The blood is not injected into the tissues, but they are bathed by it, and the compact kidney or salivary gland is represented in Insects by tubules, or a thin sheet of finely divided lobules. By a separate mechanism, air is carried along ramified passages to all the tissues. Every organ is its own lung.

We must now consider in more detail how air is made to enter and leave the body of an Insect. The spiracles and the air-tubes have been described, but these are not furnished with any means of creating suction or pressure; and the tubes themselves, though highly elastic, are non-contractile, and must be distended or emptied by some external force. Many Insects, especially such as fly rapidly, exhibit rhythmical movements of the abdomen. There is an alternate contraction and dilatation, which may be supposed to be as capable of setting up expirations and inspirations as the rise and fall of the diaphragm of a Mammal. In many Insects, two sets of muscles serve to contract the abdomen—viz., muscles which compress or flatten, and muscles which approximate or telescope the segments.[152] In the Cockroach the second set is feebly developed, but the first is more powerful, and causes the terga and sterna alternately to approach and separate with a slow, rhythmical movement; in a Dragon-fly or Humble-bee the action is much more conspicuous, and it is easy to see that the abdomen is bent as well as depressed at each contraction. No special muscles exist for dilating the abdomen, and this seems to depend entirely upon the elasticity of the parts. It was long supposed that, when the abdomen contracted, air was expelled from the body, and the air passages emptied; that when the abdomen expanded again by its own elasticity, the air passages were refilled, and that no other mechanism was needed. Landois pointed out, however, that this was not enough. Air must be forced into the furthest recesses of the tracheal system, where the exchange of oxygen and carbonic acid is effected more readily than in tubes lined by a dense intima. But in these fine and intricate passages the resistance to the passage of air is considerable, and the renewal of the air could, to all appearance, hardly be effected at all if the inlets remained open. Landois accordingly searched for some means of closing the outlets, and found an elastic ring or spiral, which surrounds the tracheal tube within the spiracle. By means of a special muscle, this can be made to compress the tube, like a spring clip upon a flexible gas pipe. When the muscle contracts, the passage is closed, and the abdominal muscles can then, it is supposed, bring any needful pressure to bear upon the tracheal tubes, much in the same way as with ourselves, when we close the mouth and nostrils, and then, by forcible contraction of the diaphragm and abdominal walls, distend the cheeks or pharynx. Landois describes the occluding apparatus of the Cockroach as completely united with the spiracle. It consists, according to him, of two curved rods, the “bow” and the “band,” one of which forms each lip of the orifice. From the middle of the band projects a blunt process for the attachment of the occlusor muscle, which passes thence to the extremity of the bow. The concave side of each rod is fringed with setæ, and turned towards the opening, which lies between the two. Upon this description of the spiracles of the Cockroach we have to remark that there is no occluding apparatus at all in the thoracic spiracles, which are provided with external valves. In the abdominal spiracles the bow is perfectly distinct, but the “band” of Landois has no separate existence. Though the actual mechanism in this Insect does not altogether agree with Landois’ description, it is capable of performing the physiological office upon which he justly lays so much stress—viz., the closing of the outlets of the tracheal system, in order that pressure may be brought upon the contained air.

The injection of air by muscular pressure into a system of very fine tubes may, however, appear to the reader, as it formerly did to ourselves, extremely difficult or even impossible. Can any pressure be applied to tubes within the body of an Insect which will force air along the passages of (say) ·0001 in. diameter? It may well seem that no pressure would suffice to distend these minute tubules, in which the actual replacement of carbonic acid by oxygen takes place, but that the air would either contract to a smaller volume or burst the tissues.