EXTERNAL ANATOMY OF INSECTS. 533 
rior section of the trunk, which bears the arms and con- 
tains the muscles that move them. ‘This part has free 
motion, or a motion independent of that of the rest of 
the trunk. This indeed seems a necessary result of the 
direction and uses of the arms. It consists of an upper 
and lower part—the prothorax and antepectus. 
i. Prothorax*.—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 
Orders it is less so. With respect to it, insects may be 
divided into two classes—those that have and those that 
have not a 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 the Blattzna. In 
the Homopterous section of the Hemiptera, in many ge- 
nera, it has become extremely short; while in the Hefe- 
ropterous section its dimensions are not much reduced. 
In the majority of the Newroptera, likewise, it is compa- 
ratively large; in the Lzbellulina much shorter, and in 
the Trichoptera and Lepidoptera nearly evanescent®.— 
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 ex- 
plain, 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 prothoraz, the 
manitrunk still remains under the form of an antepectus, 
* Paate VIII... 
> 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 re- 
presenting the prothoraz, and quite unlike the collar of Hymenoptera. 
