INTERNAL ANATOMY OF INSECTS. 199 
to its head : and if after having apparently ceased to 
move for three or four houi's it be merely touched, it 
will again begin to move and resume the same action. 
This surprising irritability and contractility of muscle 
doubtless depends upon the peculiar structure of the 
antlia, which is composed of an infinite number of horny 
rings, acted upon by muscles, more numerous probably 
than those which move the trunk of the elephant. The 
motion only ceases when the muscles become dry and 
rigid. 
1 have already, under another head 3 , considered the 
annual sleep, or winter state of torpidity of insects, during 
which an intermission for the most part of muscular mo- 
tion and action takes place. I shall now make a few 
observations with respect to their diurnal sleep, which 
may very properly have its place in the present letter. 
That insects, usually so incessantly busy and moving in 
every direction, require their intervals of repose, seems 
to call for no proof. We see some that appear only in 
the day, and others only in the night, others again only 
at certain hours ; which leads to the conclusion, that 
when they withdraw from action and observation, it is 
to devote themselves to rest and sleep. The cockchafer 
flies only in the evening ; but if you chance to meet with 
it roosting in a tree in the earlier part of the day, you 
will find it perfectly still and motionless, with its antennas 
folded and applied to the breast: — we cannot indeed say 
that its eyes are shut; for as insects have no eyelids, that 
sign of sleep can never be found in them. Again, if a 
Lepidopterist goes into the wood to capture moths in 
■ Vol. II. Let*bb XXVI. 
