538 EXTERNAL ANATOMY OF INSECTS. 
abdomen, which re-enters the body when the pressure is 
removed *. M. Latreille seems to think that these vesi- 
cles have some analogy with the poisers of Diptera and 
the pectens of scorpions; and that they are connected 
with the respiration . 
4. We are next to say something upon the shape of 
the prothorax. 'The forms of the thoracic shield, espe- 
cially in the Coleoptera, are so various, that it would be 
endless to aim at particularizing all; but it may be use- 
ful to notice a few of the most remarkable. The pro- 
thorax of Moluris, a darkling-beetle, approaches the 
nearest of that of any insect to a spherical form, from its 
remarkable convexity ; in the wheel-bug (Reduvius ser- 
ratus) it is compressed, and longitudinally elevated into 
a semicircular serrated crest: it is crested, also, in many 
Locuste and Acride, in some having two parallel ridges ; 
but, generally speaking, its surface is more depressed. 
In Necrodes it is nearly circular, in Blatta petiveriana 
semicircular, in Nzlzo and some Coccinellide crescent- 
shaped, in Carabus obcordate, in Cantharis and Sagra 
approaching to a square, in Languria to a parallelogram ; 
in many Cimicide, Belostoma, &c., it is triangular, with 
the vertex truncated ; it is trapezoidal in ZLlater, in Ateu- 
chus rather pentagonal, and exhibiting an approach to 
six angles in some other beetles *: but the prothorax 
most singular in form is that of some species of M. La- 
treille’s genus Heleus *, as H. perforatus, Brownii, &c. : 
in these its anterior angles are producted, and curving 
* De Geer iv. 74. » Organisation Extérieure des Ins. 177. 
* A subgenus, related to Lebia (Hewxagonia K. in Linn. Trans. 
xiv. 563—.) and some Cimicide, are so circumstanced. 
4 Regne Animal iii. t. xiii. f. 6. 
