Fig. 424.—Thorax and abdomen of the cockchafer (Melolontha vulgaris), showing the tracheæ and air-sacs.—This and Fig. 425 after Straus-Dürckheim.
The use of the air-sacs.—It was supposed by Hunter as well as by Newport, and the view has been generally held, that the use of these sacs is to lighten the weight, i.e. lessen the specific gravity of the body during flight. It has, however, been suggested to us by A. A. Packard that this view from the standpoint of physics is incorrect. It is evident that the wings have to support just as much weight when the insect is flying, whether the tracheæ and vesicles are filled with air or not, the body of the insect during flight not being lightened by the air in the sacs. The use of these numerous sacs, some of them very spacious, is to afford a greater supply of air or oxygen than that contained in the air-tubes alone, and thus to afford a greater breathing capacity. The sacs are largest in dragon-flies, moths, flies, and bees, which are swift of flight. When we compare the active movements of these insects on the wing with those of a caterpillar or maggot, it will be seen that the far greater muscular exertions of the volant insect create a demand for a sudden and abundant supply of air to correspond to the increased rapidity of respiration; and the enlargements of the air-tubes, rapidly filled with air at each inspiration, render it possible to supply the demand.
Fig. 425.—Head of Melolontha vulgaris, showing the numerous air-sacs, represented only on the left side, front view.
Fig. 426.—Tracheal, nervous, and digestive systems of the honey-bee (the tracheal system on the right side only partially drawn): tb, the large vesicles in the abdomen; st, stigmata; hm, honey stomach; cm, chyle stomach; vm, urinary tubes; rd, rectal glands; ed, rectum; a, antenna; an, eye; b1-b3, legs.—After Leuckart, from Lang.
The case is thus seen to be very different from that of those fishes which, having a swimming-bladder, can in the water change the specific gravity of their bodies. The case of insects is almost exactly paralleled by that of birds, where, as stated by Wiedersheim, the air-sacs appear to form integral parts of the respiratory apparatus: “a greater amount of air can by their means pass in and out during inspiration and expiration, especially through the larger bronchi, and consequently there is less necessity for the expansion of the lung parenchyma.” In other words, the supply of air in these sacs, as in insects, increases the breathing capacity of the bird during flight. Wiedersheim’s retention of the old idea that the specific gravity of the body is lessened (p. 262) seems, however, to be incorrect, as the weight of the bird’s body is not diminished by the air contained in the sacs.
h. The closed or partly closed tracheal system
Fig. 427.—The lateral and lower series of sacs of Bombus terrestris, ♂: a, c, longitudinal tracheæ;, connected by b, and dilated at f, and again in the succeeding segments; i, k, funnel-shaped dilatations passing over the dorsal surface of the abdomen and anastomosing (g) with their fellows opposite; at l, communicating directly by a large branch.—After Newport.