192 INTERNAL ANATOMY OF INSECTS. 
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. 
I have already, under another head *, considered the an- 
nual 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 obser- 
vations 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 nzght, others again only at cer- 
tain hours; which leads to the conclusion, that when 
“they withdraw from action and observation, it is to de- 
vote 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 antennz 
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 
the day-time, he finds them often perched on the lichens 
that cover the north side of the trunk of a tree, with 
their wings and antenne folded, and themselves without 
* Vou, Il. Lerrer XXVI, 
