INTERNAL ANATOMY OF INSECTS. 73 
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 collected, to admit of dog- 
matizing. 
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 K., 
Vauquelin 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?. 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», 
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, Solpuga, &c. 
by the longitudinal folded membrane that unites the dor- 
sal and ventral segments—in the Libelluline by similar 
ventral folds; and in Cimbex by membranous pieces in 
the first dorsal segment, which De Geer observed was 
elevated and depressed at the will of the animal. 
Air is as essential to insects in their pupa as in-their 
@ Annal. de Chim. xii. 
b Sur le Vol des Ins. c. i, 423, 454. c. ill. 344. c. iv. 66, 
© De Geer ii. 946—. / 
