EXTERNAL ANATOMY OF INSECTS. 549 



medipectus. The first piece in the former that requires 

 notice is the collar. I formerly regarded this piece, 

 which is peculiar to the Hymenoptera, Diptera, and one 

 tribe of the Neuroptera, as the representative of the pro- 

 thorax in the other Orders, and this opinion seems at this 

 time very generally adopted, but subsequent observa- 

 tions have caused me to entertain considerable doubts 

 of its correctness. Many other Entomologists have 

 thought it improper to distinguish these parts by the 

 same name a . Much, however, may be said on both 

 sides of this question, and I shall now lay before you the 

 principal arguments that may be adduced in defence of 

 each opinion, beginning with those that seem to prove 

 that the collar is the analogue of the prothorax. First, 

 then, the collar, like the prothorax, is placed precisely 

 over the antepectus, and being placed in the same situa- 

 tion, on that account seems entitled to the same denomina- 

 tion ; especially as in some genera, for instance Chlorion 

 F., it assumes the very semblance and magnitude of a 

 thoracic shield, and is separated from the mesoihorax by 

 a considerable incisure. Again, in some cases that have 

 fallen under my own observation, the collar is endued 

 with some degree of motion distinct from that of the 

 alitrunk, since in Pompilus and Chrysis the animal can 

 make the former slide over the latter in a small degree. 

 A third and last argument is, that no prophragm is 

 formed from the collar : insects that have a thoracic shield 

 are generally distinguished by having the anterior margin 

 of the dor solum deflexed so as to form a septum, called in 



a Latreille Organization &c, 199. Chabrier Sur le Vol des Ins. 

 c. i. 412. c. iv. 54, &c. 



