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 Dynastida?, Galcodes, &c. 
by the longitudinal folded membrane that unites the dor- 
a Annal. dc C/iim. xii. 
b Sur le Vol iks Ins. c. i. 423, 454. c. iii. 344. c. iv. 66. 
