INTERNAL ANATOMY OF INSECTS. 49 
proach to spiracles is made by those remarkable plates 
that are found in such larvee 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* 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 (Musca carnaria), at the 
junction of the first segment of the body with the second, 
two of these plates are planted, which are concave 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 sub- 
stance’. ‘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*. In some cases, several stiff rays or 
spines replace the prominences*. In Echinomyia grossa 
and others the anal plates appear not to be perforated, 
being surmounted only by a central boss‘; but this, 
® Vor. I. p. 251—. b De Geer vi. 67. ¢. iii. f. 10. ss. 14. 
© Ibid. 66. t. iii. f. 13. @PLace XIX: Fie. 11... 
* Reaum. iv. 375—. ¢. xxvi. f. 7, 8. 
VOL. IV. E 
