386 PHYSIOLOGY. 



tracheae we have other indirect proofs derived from their structure. 

 These are their anatomical conformity with the tracheae of the higher 

 animals, their distension into bags and bladders, which correspond with 

 the cells of the lungs and its bags; and, lastly, the deficiency of a 

 peculiar respiratory organ, which would be the more necessary in insects, 

 from their being covered with a hard integument, which could not 

 exercise that function. All these facts confirm the tracheae to be the 

 true and sole respiratory organs of insects, and that air containing oxy- 

 gen is received into them through the spiracles, air tubes, or branchiae. 



228. 



If we now return to the mechanism of respiration, we shall find that 

 it presents itself throughout the animal world as a rhythmical motion 

 of the body, whereby the medium containing the oxygen is brought into 

 incessant contact with the respiratory organs. This motion in insects 

 is consequently for the purpose of introducing atmospheric air within 

 the tracheae, which object is attained by the opening of the spiracles 

 which close the apertures of the tracheae. If the abdomen of the insect 

 distends at the same time as the spiracles open, the air must necessarily 

 pass into the tubes which are now opened, and when the abdomen 

 contracts, the just inspired air will consequently be forced out again. 

 Thus all respiratory motion presents itself as a rhythmical compression, 

 and expansion of the cavities of the body, and especially of the abdomen. 

 The muscles which produce this motion are the same as those described 

 above as connecting the several parts of the skeleton together, namely, 

 the straight dorsal and ventral muscles of the abdomen. The thorax 

 appears to participate less in the contraction of the cavities of the body, 

 at least no contraction or dilatation of it is to be detected in insects 

 quietly breathing; and also the intimate and firm connexion of the 

 several parts of it together prevents such an alteration of its compass 

 in repose. But whether the cavities of the tracheae are also contracted 

 upon the considerable compression of the abdomen, is uncertain. 

 Nitzsch * has in many instances observed that there was no alteration 

 during respiration, whereas he detected in the large air bladders of the 

 Diptera and of the Hymenoptcra a distinct compression upon the con- 

 traction of the abdomen, but which evidently appeared to proceed from 

 the latter, and not from a contraction of the air bladder itself f . Hence, 



* Comment, de Respirat. Animal, p. 38. -f- Ibid. p. 39. 



