THE THORAX. 85 



sternum closely approximates to the above description of that of the 

 beetles ; it is likewise seated between the acetabula of the posterior 

 legs, and appears as a distinct, but undivided plate, as in Scotia (PI. X. 

 No. 2, f. 2 and 3, G). In the Lepidoptera it takes the figure of a 

 semicircle, which lies in front of the coxse of the posterior legs, separates 

 them from those of the intermediate legs, and between them it projects, 

 with its obtuse ends, at the sides of the thorax (PL XIII. No. 4, f. 2, G). 

 It appears indicated in the same situation in the Diptera, but is not 

 separated, for in them all the parts of the thoracic segments are firmly 

 united. In the Hymenoptera, the metasternum merits particular atten- 

 tion, from its deviating from the structure of the other orders by pos- 

 sessing a spiracle peculiar to it, which is placed anteriorly upon its supe- 

 rior lateral margin (see PI. XII. No. 1 and 2, f. 1 and 2, ft). In the 

 Lepidoptera and Diptera, it is placed as in the other orders, between 

 the meso- and meta-thorax. Latreille, therefore, considers this portion 

 of the thorax as belonging to the abdomen, maintaining that no spira- 

 cles are to be found upon those segments of the thorax which are 

 provided with wings ; which assertion is, however, unfounded, as we 

 have seen. He thence concludes that the halteres (see the end of this 

 section) of the Diptera cannot represent the posterior wings of the 

 other orders, because a spiracle is found upon the segment where they 

 are placed. But that this circumstance proves nothing will have 

 become self-evident. 



Between the metanotum and the metasternum, two other horny 

 pieces are found on each side, which we, with Kirby and Spence, 

 distinguish as the PLEURA and PARAPLEURA. Straus calls them 

 ISCHIA, and distinguishes the former as the ischium primum ; the latter 

 as ischium secundum. 



The PLEURA (PI. IX. No. 2, J, j) is contiguous to the metanotum, 

 and is united to it by a delicate membrane ; the membrane of the wing 

 proceeds from it, and this is attached in the same manner to the pleura 

 beneath, as it is affixed above to the metanotum. It is a small, longi- 

 tudinal, scarcely observable plate, which, in repose, is covered by the 

 elytra, and is not perceptible until they are removed. In the Orthop- 

 tera (for example, Gryllotalpa, PI. XI. No. 1, f. 8, j), the pleura is 

 much extended, and posteriorly it is drawn somewhat downwards, so 

 that it extends to the acetabula of the posterior coxae. In the Libellu- 

 lina, it is almost supplanted by the very large parapleurse, and in these 



