HYBERNATION OF INSECTS. 533 
Europe would be leading them into perpetual and fatal errors—which in 
spring would be inducing them to quit their ordinary occupations, and 
prepare retreats and habitations for winter, to be quitted again as soon as 
a few fine days had dispelled the frosty feel of a May week ; and in a mild 
winter’s day, when the thermometer, as is often the case, rises to 50° or 
55°, would lure them to an exposure that must destroy them. It is not, 
we may rest assured, to such a deceptious guide that the Creator has 
intrusted the safety of so important a part of his creatures ; their destinies 
are regulated by feelings far less liable to err. 
What, you will ask, is this regulator? IT answer, Instinct —that faculty 
to which so many other of the equally surprising actions of insects are to 
be referred ; and which alone can adequately account for the phenomena 
to be explained. Why, indeed, should we think it necessary to go further? 
We are content to refer to instinct the retirement of insects into the earth 
previously to becoming pupz, and the cocoons which they then fabricate ; 
and why should we not attribute to the same energy their retreat into 
appropriate hybernacula, and the construction by many species of habita- 
tions expressly destined for their winter residence? The cases are exactly 
analogous ; and the insect knows no more that its hybernaculum is to 
protect it from too severe a degree of cold during winter than does the 
full-fed caterpillar when it enters the earth that it shall emerge a beau- 
teous moth,! 
Tam, &c. 
1 ‘The reasoning in the preceding pages, as to cold not being the sole and direct 
cause of hybernation in insects, is strongly confirmed by the facts observed with re- 
gard to the hybernation of snails by M. Gaspard, who found that he could not bring 
on this state of existence out of its proper season by submitting them to artificial 
cold nearly to the freezing point, while he ascertained that at the proper period they 
prepare for hybernating at very different degrees of temperature, varying from 37° 
to 77° Fahr. (Zoological Journ. i. 93.). If it be said that some change in the sensa- 
tions of insects, either from an internal or external cause, must probably exist, in 
order to lead them to adopt a state so different from that of their usual habits as 
hybernation, this is readily admitted ; but what is contended in the preceding letter 
is, that these causes are not simply cold, and that we are as yet ignorant of their 
nature. Dr. Jenner has argued (Phil. Trans, 1823) that it is not cold, but the tumid 
state of the ¢estes and ovaria in swallows, and other migratory birds, which is the 
proximate cause of their leaving us at the approach of winter; and some analogous, 
though different, internal change may haye a share in causing insects to exercise 
their hybernating instinct; but this change remains to be ascertained. Mr. New~- 
port’s idea that it is caused by an accumulation of fat pressing upon the trachew, and 
thus inducing a plethoric condition of body, and consequent inclination to sleep, 
might explain why insects become torpid after entering their winter quarters; but 
not distinguishing, as it appears to me, the two very distinct actions of seeking out 
for and preparing hybernacula, and becoming torpid after entering them, it leaves, 
as the theories of other physiologists have done, the former, which is so essential a 
peculiarity of hybernation, wholly unexplained: just as Dr. Jenner’s hypothesis, 
though it may explain why swallows should be uneasy and desirous of changing 
their abode, throws no light on that mysterious faculty by which they are directed, 
with unerring certainty, through the trackless air to the very spots, perhaps a thou- 
sand miles distant, that suit their new corporeal sensations. An accumulation of 
fat, supposing it to exist, may induce drowsiness and torpor, whether in cold 
climates like ours, in winter, or in tropical regions, where insects, as well as lizards, 
and even crocodiles, &c., retire under ground, and sleep during the excessive heat; 
but there is obviously no natural connection between this plethoric state and the act 
of seeking out and preparing and retiring to a suitable dormitory. If fat and 
plethora are sufficient to induce this propensity, why do not these conditions, which 
uu 3 
