72 INTERNAL ANATOMY OF INSECTS. 
M. Chabrier’s opinion that inspiration is ordinarily by 
the abdominal spiracles, and expiration by those of the 
trunk of insects*.. He seems to have been led to the adop- 
tion of this opinion, not so much by experiments similar 
to that of Reaumur just stated, but by observing that in 
many instances these two sets of spiracles differ from each 
other, the latter having a convex and the former a con- 
cave mouth or bed®. In some cases, however,—for in- 
stance during flight,—-he supposes the spiracles of the trunk 
may receive as well as emit the air®: he likewise is of opi- 
nion, and it seems not improbable, that by means of these 
openings in the trunk, from the rush of the superfluous 
air through them, insects produce those sounds for which 
they are remarkable,—as the humming of bees and flies. 
In the former he thinks the sound is produced by the 
pneumatic apparatus covered by the ends of the collar ; 
while in the latter he attributes it to the spiracles in the 
metathorax behind the wings attended by a poiser’. I 
incline, however, to M. Dufour’s opinion ¢,—that the vocal 
spiracles in the Hymenoptera, as well as in the Diptera, 
are those behind the wings. Perhaps both theories may 
be right; for if you take any common humble-bee, you 
will find that, in the hand, it produces one kind of sound 
when its wings are motionless, and another more com- 
plex and intense when they vibrate. In numerous in- 
stances, however, there is no very striking external dif: 
ference between the spiracles of the trunk and those of 
the abdomen : this observation applies more particularly 
to the caterpillars of Lepidoptera ; but whether these re- 
ceive the air by those of the abdomen, and return it by 
* Sur le Vol des Ins. c. 1. 423. 
» Tbid. 454. and c. iv. 66. note 1. © Ibid. e. 1. 453. 
4 Tbid. 459, 456. © Tbid, 459. 
