72 INTERNAL ANATOMY OF INSECTS. 
insects in general, are destined for the escape of the air, 
after it has parted with its oxygen, received by the anal 
ones a . So that there seems very good ground for 
M. Chabrier's opinion that inspiration is ordinarily by 
the abdominal spiracles, and expiration by those of the 
trunk of insects b . He seems to have been led to the 
adoption 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 for- 
mer a concave mouth or bed c . In some cases, however, 
— for instance during flight, — he supposes the spiracles 
of the trunk may receive as well as emit the air d : he like- 
wise is of opinion, 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 opi- 
nion f , — that the vocal spiracles in the Hymenoptera, as 
well as in the Diptera, are those behind the wings. Per- 
haps 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 motion- 
a Mr. B. Clark thinks that he has discovered spiracles in this 
larva in the usual situation, (Essay on the Bots, Sfc. 48. t. \i.f. 3.) 
but they are probably analogous to the spiraculiform tubercles of 
CE. Ovis. Reauni. iv. 566. t. xxxv. 17 — 19. 1. Vallisnieri (Espcricnz. 
8fc. 136) notices them. 5k;- lc Vol des Ins. c. i. 423. 
c Ibid. 454. and c. iv. 66. note 1. A Ibid. c. i. 453. 
s Ibid. 459, 456. f Ibid. 459. 
