STRUCTURE OF INSECTS. 11 



parts of the trunk or thorax, being the small triangular piece which 

 is seen at the base of the suture of the elytra of beetles. The chief 

 ventral or pectoral piece in each of the three thoracic segments is the 

 Sternum, which varies considerably in its size and form : thus in the 

 Elateridae the jtwosternum is elongated into a point extending between 

 the middle legs, whilst in the large Hydrous piceus the JMesosternum 

 and Metasternum are soldered together between the middle legs, the 

 former projecting in a point between the anterior legs, and the latter 

 extending far beyond the base of the hind legs. The modifications of 

 form in the various thoracic segments result from the complicated 

 machinery requisite for the due performance of the two chief kinds 

 of insect locomotion, namely, leg-movements of various kinds, and 

 wing-movements ; but more especially from the great diversity of the 

 wings and the occasional transfer of wing-motion to a single pair of 

 wings, this pair being either the anterior, as in Diptera, or the pos- 

 terior, as in Coleoptera : thus^ whilst in msects which have the four 

 wings of nearly equal size, the two aliferous segments of the thorax 

 ■fee+wg also of nearly equal size, and their subsegments smiilarly 

 developed (ex. gr. Hemerobius) ; those tribes of insects which haye 



one or Th" - ft^^"'' pnir "^ ^nng a *i nni- n mgpppi-illy floirnlnpnrl ; — hav e 



the segment to which such pair of wings is attached consequently 

 increased in size : thus in Diptera the mesothorax nearly occupies the 

 entire thorax, there being only mesothoracic organs of flight. In the 

 Hymenoptera, the posterior wings exist, but of a small size ; the me- 

 tathorax is therefore much larger than in the Diptera, but much 

 smaller than the mesothorax. In like manner, in those orders which 

 have the posterior pair of wings enlarged, the mesothorax is di- 

 minished, and the metathorax equally enlarged : this is especially the 

 case in the Beetles, but still more strikingly so in the Strepsiptera, 

 are as destitute of fore wings as the Diptera are of hind ones. 



The Wings, or organs of aerial progression, (upon which the Linnaean 

 arrangement is principally founded,) bear no resemblance to those of 

 birds, being, on the contrary, more analogous to the wings of bats, 

 consisting ordinarily of a delicate double membranous plate, traversed 

 by more or less numerous tubes, which Dr. I>each regarded as repre- 

 senting a system of bony air vessels (Pterygostea), but which the 

 recent microscopical observations of Mr. Bowerbank and others 

 have proved to be veins. These organs of flight, with respect to 

 their consistence, are termed elytra, hemelytra, tegmina, membra- 



