550 EXTERNAL ANATOMY OF INSECTS. 
as the collar should do, between the prothorax and those 
parts of the trunk to which the wings are attached. 
There is one circumstance connected with the subject 
which should not be overlooked. In the Hymenoptera, 
usually under a laterdl process of the posterior part of 
the collar, is a spiracle or respiratory apparatus ; in the 
Diptera there is also one, though not covered by the part 
in question, in the same situation ; now this you will find 
precisely so situated with respect to the second piece in 
the thorax of Tipula oleracea, proving that this piece is 
the real representative of the collar. Enough, I think, 
has been said to satisfy you that I have not changed my 
sentiments on this subject upon slight grounds. Probably 
traces of the part in question might be detected in the 
thoracic Orders in general, in connexion with some 
vocal or respiratory organ*; but having had no oppor- 
tunity, by an extended examination of living subjects, to 
verify or disprove this suspicion, I shall merely mention 
it, and conclude this head by observing, that the collar 
varies most in the Hymenoptera order, and that its most 
remarkable form is in Vespa, Cimbex, Dorylus, &c., in 
* M. Chabrier (Vol des Ins.) supposes that the humming of insects 
is produced by the exit of the superfluous air from their thoracic 
spiracles, &c.; in MJelolontha he thinks they are in the metathorax un- 
der the wings (c. i. 457—. PLatr XXII. Fic. 13. c. ¢. represents the 
operculum of one of those of Dytiscus marginalis): in the Hymeno- 
ptera, in the mesothorax, near the posterior lobes of the collar (Ibid. 
459. c. iv. 50.); and in the Diptera, in the metathoraxr, near the 
poiser (c. i. 457). I observed myself lately, that Hristalis tenax, 
if held by the anterior part of the body, when it hummed, alter- 
nately opened and shut this spiracle. The wings during the sound 
vibrated intensely. The hum ceased and was renewed, as they were 
restrained from this motion or released from restraint; when the 
wing was moved towards the head, a different sound was emitted 
from that produced when it merely vibrated, 
