HYBERNATION OF INSECTS. 567 



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.' I am, he. 



' 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 regard to the hyber- 

 nation of snails by M. Gaspard, who found that he couM not bring on this stale 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 difierent, inter- 

 nal change may have a share in causing insects to exercise their hybernaiing instinct; but 

 this change remains to be ascertained. Mr. Newport's idea that it is caused by an accu- 

 mulation 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 thousand miles distant, that 

 suit iheir 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 ; and 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 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 maij 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 

 kiads observed in the insect tribesT 



