38 INTERNAL ANATOMY OF INSECTS. 
cally, may be regarded in numerous cases as a mouth 
closed by lips. In caterpillars and many other insects, 
the substance of the crust where it surrounds the spiracle, 
is elevated so as to form a ring- round it. The lips, pro- 
perly speaking, are formed of a single cartilaginous piece 
or platform, with a central longitudinal cleft or opening, 
when closed often extending the whole length of the 
piece a ; bnt in some appearing always open and circu- 
lar : of the former description are those covered by the 
elytra in the common cockchafer; and of the latter, those 
that are not so covered : in some, as in the antepectoral 
pair of the mole-cricket, there appear to be no lips, the 
orifice being merely closed with hairs b . Though the 
aperture is usually in the middle of the platform, in the 
female of Dytiscus marginalise it is nearer the posterior 
side, the anterior or upper lip being the longest. In the 
majority, the mouth or cleft is nearly as long as the spi- 
racle; yet in the puss-moth (Cerura Vinula) it is shorter c . 
Some spiracles, however, are unilabiate, or have only 
one lip. This is the case with Gonyleptes and perhaps 
others d . The lips are usually horizontal, but sometimes 
they dip so as to make the spiracle appear open. 
With regard to the substance of these organs, it is more 
or less cartilaginous, and probably elastic ; the surface 
frequently appears to be corrugate or plaited; this is very 
distinctly seen in the stag-beetle and the cockchafer : in 
the last insect, under a powerful magnifier, we are told 
that the lips appear to consist of parallel cartilaginous 
processes, separated by a cellular web'. In some species 
a Plate XXIII. Fig. 2. b Sprengel, Commcntar. § 7. 
• Ibid. t. iii./. 30. o Plate XXIX. Fig. 23. 
• Ibid. 8. 
