552 EXTERNAL ANATOMY OF INSECTS. 



Libellulina. These are generally admitted to have the 

 former of these parts*, but besides this they have also 

 the latter, which is the most ample and conspicuous 

 piece in the whole trunk"; intervening, 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 lateral 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 Tipida oleracea, proving that this piece is the real re- 

 presentative of the collar. Enough, I think, has been 

 said to satisfy you that I have not changed my senti- 

 ments 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 c ; but having had no oppor- 



a Plate IX. Fig. 6. a. b Ibid. Fig. 7- g . 



c 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 Melolontha he thinks they are in the metathorax un- 

 der the wings (c. i. 457 — . Plate XXII. Fig. 13. c. f . 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 metathorax, near the 

 poiser (c. i 457)- I observed myself lately, that Elophilus 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 j when the 

 wing was moved towards the head, a different sound was emitted from 

 that produced when it merely vibrated. 



