{) 'THE ENTOMOLOGIST S EECORD. 



plienomonon of aestivation, occurring in various districts where pro- 

 longed drought occurs, takes place in very highly organised beings 

 with aquatic habits. This would, however, suggest that the resting 

 state is after all more closely related to want of pabulum than to 

 temperature. 



If we grant, then, the existence to insects of a capacity to assimie 

 a resting state, even for a brief period, it is obvious that this will be 

 assumed whenever the conditions necessary to activity, no matter which 

 of them, are wanting. 



At first, any prolongation of the resting stage will be generally 

 fatal ; but, since those individuals that possess the power of resting 

 most fully will more abundantly survive, if the period of resting be 

 slightly prolonged, we see how, by natural selection (if so be that 

 adverse conditions gradually become more severe), the rest period may 

 become prolonged, as in hybernation, and how those that feed well 

 beforehand will have the advantage. If we consider the insects of the 

 British Isles, or of temperate regions generally, we find that with very 

 few exceptions they all hybernate at some stage or other, and since the 

 stages at which this occurs are so various, sometimes, even in the same 

 genus or family, it is clear that the tissues of insects are capable, at 

 any stage, of developing that habit of resting, under the necessities of 

 the struggle for existence, into a prolonged rest, probably with con- 

 siderable rapidity. 



In the Lepidoptera, the earlier forms hybernated as larva?, or we 

 may suppose that the earliest forms were tropical, and did not hyber- 

 nate ; then, as they passed northward (or southward), or as northern 

 (or southern) conditions advanced into their areas, a brief resting period 

 became gradually prolonged into hybernation. 



The earliest forms (Hepialus, Cossus, itc.) being hidden and 

 internal feeders, may have had a rest period in their original (tropical) 

 habitat, but one calculated rather to carry the cycle of metamorphoses, 

 on until suitable weather for emergence occurred, than from any neces- 

 sity for rest on the part of the larva itself. This would, under the 

 necessary change of environment, form a basis for natural selection to 

 build a hybernating habit upon. 



Those families and genera that are exclusively tropical have no 

 doubt been at first purely tropical by accident, and have then had the 

 tendency to hybernate bred out of them, till it became so attenuated 

 that it could not develop rapidly enough when required, and so these 

 forms became permanently confined to tropical regions. 



Suppose a form that in the tropics does not hybernate at all, 

 spreading northward to temperate regions. According to varying 

 habits it finds the colder period meets it at difierent stages — ogg, larva, 

 pupa, imago. Let us suppose that its first experience of colder weather 

 is a few inclement days during the imaginal period. This will kill 

 many or make them infertile ; but we assume the inclemency not to be 

 so great but that a few survive. Selection then begins and continues 

 until the species (more or less modified, no doubt) has pushed north- 

 ward to almost glacial conditions, and the imago hybernates say for 

 six months. It is very possible that species have thus gone to and fro 

 from temperate regions to the tropics (or from temperate to tropical 

 conditions) many times, and we may thus account in some instances 

 for the different stages at which different species hybernate. 



