458 



TEXT-BOOK OF ENTOMOLOGY 



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 rapid- 

 ity of respiration; and the enlarge- 

 ments of the air-tubes, rapidly filled 

 with air at each inspiration, render it 

 possible to supply the demand. 



PIG. 425. Head of Jfetofontfui ,-!- 

 ffaris, showing the numerous air-sacs, rep- 

 resented only on the left side, front view. 



The case is thus seen to be very different 

 from that of those fishes which, having a swim- 

 ming-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 ap- 

 paratus : "a greater 

 amount of air can by their 

 means pass in and out dur- 

 ing inspiration and expira- 

 tion, especially through the 

 larger bronchi, and con- 

 sequently 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. Wit>- 

 dersheim's retention of the 

 old idea that the specific 

 gravity of the body is les- 

 sened (p. 262) seems, how- 

 ever, to be incorrect, as 

 the weight of the bird's 

 FIG. 426. Tracheal, nervous, and digestive systems of the hnHv i nnt riimiiiiehpri hv 



honey-bee (the tracheal system on the right side duly partially 0( ^ 1S n0t """""Shed by 



drawn) : tb, the large vesicles in the abdomen : */. '.stigmata; the air contained in the 



hm, honey stomach; cm, chyle stomach: rm, urinary tubes; Rnn(s 



rd, rectal glands; ed, rectum ; a, antenna; an, eye; br-b t , sacs - 



legs. After Leuckart, from Lang. 



_cm 



