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 
a Vol. I. p. 254—. 
b De Geer vi. 67. t. iii./. 10. ss. 14. Mr. W. S. MacLeay (Philos. 
Mag. 2V. 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. 
c De Geer 66. I. iii./. 13. 
VOL. IV. E 
