INTERNAL ANATOMY OF INSECTS. 73 



less, and another more complex and intense when they 

 vibrate. In numerous instances, however, there is no 

 very striking external difference between the spiracles of 

 the trunk and those of the abdomen : this observation 

 applies more particularly to the caterpillars of Lepido- 

 ptera ; but whether these receive the air by those of the 

 abdomen, and return it by those of the trunk, has not 

 yet been ascertained ; and indeed, too little is at present 

 known upon the subject, and too few facts have been col- 

 lected, to admit of dogmatizing. 



The external signs of respiration in insects are not uni- 

 versally to be discovered. The alternate contraction and 

 expansion of the abdomen is, however, very visible in 

 some beetles, bees, the larger dragon-flies, and grass- 

 hoppers. In one of the latter, Acrida viridissima, Vau- 

 quelin observed that the inspirations were from fifty to 

 fifty-five times in a minute in atmospheric air, and from 

 sixty to sixty-five when in oxygen gas a . But M.Chabrier 

 has given the most satisfactory account of these signs : 

 The abdomen, says he, is the principal organ of inspi- 

 ration ; it can dilate and contract, lengthen and shorten, 

 elevate and depress itself. In flight, in elevating its ex- 

 tremity at the same time with the wings, it contracts it- 

 self, pushes the air into the trunk, and diminishes the 

 weight of the body by the centrifugal ascending force b . 

 In the majority of insects perhaps the dilatation of the 

 abdomen takes place by the recession of the segments 

 from each other by means of the elastic ligaments that 

 connect them ; in others, as the Dynastidce, Galeodes, &c. 

 by the longitudinal folded membrane that unites the dor- 



a Annal. de Chim. xii. 



b Sur le Vol des Ins, c. i. 423, 454. c. iii. 344. c. iv. 66. 



