INTERNAL ANATOMY OF INSECTS. 49 



proach to spiracles is made by those remarkable plates 

 that are found in such larvae of Diptera, as in that state 

 inhabit substances that might impede or altogether stop 

 the entrance or exit of the air by the ordinary spiracles, 

 such as dead or living flesh, dung, or the like. The 

 CREATOR therefore, as he has seen it good for wise rea- 

 sons a to commission certain insects to feed on unclean 

 food, has fitted them for the offices that devolve upon 

 them, and has placed their orifices for breathing in plates 

 at each extremity of the body. There are usually two 

 of these plates at the head, and two at the tail. In the 

 grub of the common flesh-fly (Sarcophaga carnaria\ at 

 the junction of the first segment of the body with the 

 second, two of these plates are planted, which are con- 

 cave and circular, with a denticulated margin ; in the 

 cavity near the lower side is a round spiracle. These 

 plates the animal can withdraw within the body, so as to 

 prevent this spiracle from being stopped up by any greasy 

 substance b . The posterior extremity of this grub is trun- 

 cated, and has a large and deep cavity surrounded by 

 several fleshy prominences : at the bottom of this are 

 two oval brown plates, in each of which are three oval 

 spiracles, placed obliquely : by the contraction of the 

 fleshy prominences, this cavity also can be closed at the 

 will of the animal c . In some cases, several stiff rays or 



* VOL. I. p. 254. 



b De Geer vi. 67. t. iii./. 10. ss. 14. Mr. W. S. MacLeay (Philos. 

 Mag. N. Ser. No. 9. 178.) says that in this grub the longitudinal 

 trunks of the Tracheae send off at equal distances lateral branches 

 just as if there were spiracula to correspond with them. This 

 is evidently a preparatory step to the formation of those that ulti- 

 mately appear in the perfect insect. 



e De Geer 66. t. iii./. 13. 



VOL. IV. E 



