HYBERNATION OF INSECTS. 



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observed with regard 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 sensations 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. Newport's idea that it 

 is caused by an accumulation of fat pressing upon the trachea3, 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 distin- 

 guishing, 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 pe- 

 culiarity 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 

 thousand 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 ex- 

 cessive 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 dor- 

 mitory. If fat and plethora are sufficient to induce this propensity, why do not 

 these conditions, which are constantly taking place in many European carnivorous 

 perfect insects in summer, when their food is abundant, lead them then, in Europe 

 as in tropical countries, to seek out or prepare a suitable retreat ? Yet, however 

 full fed insects in temperate climes may be in summer, we know that they do 

 not retire to become torpid at that period. All, therefore, that the present state 

 of our knowledge seems to entitle us to say, is, as expressed in the close of the 

 above letter, written thirty years ago, that the act of hybernation is dependant 

 on the instinct of the insect, and that though this instinct may be, and probably is, 

 excited by some bodily sensation, we as yet know no more of the precise nature 

 of this than of that of a thousand other sensations which may give rise to the 

 endless instincts of different kinds observed in the insect tribes. 



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