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? I 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 pupae, 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. 1 



I am, &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 testes 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 have 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 tracheae, 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 



MM 3 



