570 EXTERNAL ANATOMY OF INSECTS. 



in its termination. In the remaining Orders, as far as I 

 have had an opportunity to examine them, it can 

 scarcely be said to exist separately from the medipectus, 

 except that in Tipula Latr. a bipartite subtriangular 

 membranous piece seems to be its analogue. 



We are now to consider the last segment of the ali- 

 trunk, which, as a whole, may be denominated the po- 

 trwicus ; it bears the second pair of the organs of flight, 

 and the last pair of legs. The upper side of this is the 

 metathorax, and its lower side the postpect/us. 



9. Postdorsolum a . The first external piece of the 

 metathorax is the postdorsolum, which presents itself 

 under very different forms and circumstances in the dif- 

 ferent Orders. In the Coleoptera it is intirely covered 

 by the dorsolum and scutellimi j it is generally more or less 

 of a membranous substance, or partly membranous and 

 partly corneous, which enables it to yield more to the 

 action of the wings in flight; it is usually an ample 

 transverse piece with tumid sides b ; but in the Scarabceidce 

 MacLeay, it is short though very wide; and in Cychrus, 

 and probably other apterous beetles, it is extremely mi- 

 nute and almost obsolete. In the Orthoptera Order, I 

 observe once for all, the part in question, as well as 

 the postscutellum and postfrcenum are mere counterparts 

 of the dorsolum, scutellum, and framum, except that in 

 some cases they are larger c . In the Heteropterous He- 

 miptera at first sight it would appear that all the parts of 

 the metathorax were altogether wanting or absorbed 



a Plate VIII. IX. t' '. Linn, Trans, xi. t. ix. /. 16. c. 



b Plate VIII. Fig. 3. t . 



c Ibid. Fig. 12. Comp. i',k',l', with t', u' , v. 



