20 ME. A. HAMMOND ON THE 



adduced, and that we shall find the metathorax of this insect 

 to be as obsolete as are the alary appendages it carries. 



Let us see now how the view of the thoracic structure of the Hy- 

 menoptera thus advocated bears upon the position of the spiracles. 

 Does it introduce an element of harmony into the study of this 

 order as compared with other insects, or one of additional per- 

 plexity ? and, finally, what is its effect on the location we may give 

 to these organs in the Diptera ? I gather from a passage in 

 Westwood* that Latreille has made the observation that the 

 metathorax in insects is never provided with spiracles. The 

 observation is a good one, though not free from error, I venture 

 to think, in the induction he draws therefrom, that they (and the 

 halteres in consequence) are abdominal appendages. Of course, 

 on his view of the Hymenopterous structure, they are excluded 

 from the metathorax of that order inasmuch as, in his opinion also 

 (as I have just mentioned), they occur on that portion of the body 

 which belongs to the fifth or atrophied segment ; and so far as I 

 am acquainted, with the exception of the Diptera, there is no 

 other order of insects in which a metathoracic spiracle may even 

 be thought to be observable in the imago. By regarding, there- 

 fore, the posterior spiracle of the Diptera as mesothoracic, we 

 shall introduce this element of agreement into the structure of 

 the class — not indeed by thrusting it, as Latreille did, into the 

 abdomen, that is, by removing it backward from the metathorax, 

 but by the converse process of removing it forward to the meso- 

 thorax, We shall then have the metathorax in every order of 

 insects devoid of a spiracle. That the posterior spiracle should 

 be mesothoracic is absolutely essential to my argument, since it 

 is surrounded by plates which I propose to show also belong to 

 that segment. 



But again, so far as I am acquainted, in every case where the 

 limits of the thoracic segments are not subject of discussion, the 

 position of the thoracic spiracles is, roughly speaking, between 

 the segments, one pair between the pro- and mesothorax, and 

 another pair between the meso- and the metathorax, though in some 

 orders the latter are suppressed. Both pairs occur, for example, 

 in the Coleoptera and Lepidoptera ; one only in the Hymen o- 

 ptera, viz. the anterior. I say roughly, because I think there is 

 really no debatable ground between the segments, and that any 



* Westwood's ' Introduction,' p. 500. 



