414 



ZOOLOGY. 



ture producing this is very singular ; but the buzzing of flies 

 appears to depend on the rapid escape of the air through the 

 thoracic stigmata during the violent movements of flight. 

 Finally, there are other insects which produce sounds by a 

 mode as yet unknown, such as the nocturnal butterfly or 

 death's-head sphinx. 



520. The nervous system of insects has 

 been already in a great measure described 

 ( 509). It is composed principally of a double 

 series of ganglions, reunited by longitudinal 

 cords (Fig. 432) ; the number of the ganglions 

 corresponds to the number of rings ; sometimes 

 equidistant and extending from one end of the 

 body to the other, at other times they approach 

 each other, and become confused into a nervous 

 mass. The cephalic ganglions are large, and 

 give origin to the nerves of the eyes and 

 antennae, &c. The first pair of ganglions 

 behind the gullet give off* the nerves of the 

 mouth, and the longitudinal filaments con- 

 necting them with the cephalic ganglions 

 from the nervous collar surrounding the gullet. 

 Finally, the brain gives off on each side a 

 nerve, which, ascending on the stomach and 

 uniting with that of the opposite side, con- 

 stitutes a median nerve, situated above the 

 digestive canal, and having in its course two 

 ganglions. The three pairs of ganglions 

 following behind the gullet belong to the 

 three rings of the thorax, and give off the 

 Fig, 432. nerve of the limbs and wings ; generally 

 speaking, they are very near each other, and 

 much larger than the following pairs, which belong to the 

 abdomen. 



521. Insects vary much in their mode of nourishment, 

 some living on the juice of plants, whilst others feed on solid 

 aliments, and are either carnivorous or phytophagous; and 

 these differences correspond to a remarkable modification in 

 the structure of the mouth. In the grinding insects, such as 

 the scarabei, the may-bug, the blatta (Fig. 433), the grass- 

 Jioppers, the opening of the mouth is furnished anteriorly 

 with a median piece called labium, or upper lip (a, Fig. 

 434), and presents on each side a kind of large moveable 



