EXTERNAL ANATOMY OF INSECTS. 535 



trunk. This indeed seems a necessary result of the di- 

 rection and uses of the arms. It consists of an upper 

 and lower part — the prothorax and antepectus. 



i. Prothorax a . — The upper part of the manitrunk in 

 the Coleoptera, Orthoptera, and Hemiptera, is by far the 

 most conspicuous part of the trunk, but in the other Or- 

 ders it is less so. With respect to it, insects may be di- 

 vided into two classes — those that have and those that 

 have not & prothorax. In the Coleoptera Order it is re- 

 markable both for size and variations in its shape and 

 sculpture. In the Orthoptera, though less various, it 

 is almost equally conspicuous, especially in Blatta. In 

 the Hdmopterous section of the Hemiptera, in many ge- 

 nera it has become extremely short ; while in the Hete- 

 ropterous section its dimensions are not much reduced. 

 In the majority of the Neuroptera, likewise, it is compa- 

 ratively large ; in the Libellulina much shorter, and in 

 the Trichoptera and Lepidoptera nearly evanescent b . — 

 In the Hymenoptera and Diptera, with very few excep- 

 tions, the thoracic shield altogether disappears, at least 

 if I am correct in an idea, which I shall hereafter explain, 

 that the collar usually regarded as the analogue of the 

 prothorax, is really a part of the alitrunk. In these last 

 Orders, though there is no true prothorax, the mani- 

 trunk still remains under the form of an antepectus, 

 bearing the fore-legs, and containing the muscles that 

 move them. 



The prothorax of insects may in general be considered 



a Plate VIII. a. 



h If the head of any individual of these two Orders be carefully 

 taken off', it will be found that above there is a very short piece repre- 

 senting the prothorax, and quite unlike the collar of Hymenojifcra. 



