8 THE ENTOMOLOGIST S RECORD. 



often, indeed, over a period extending from July to the following 

 February or jNIarch, yet those larvje which feed on the ground on low 

 herbaceous plants or on roots never become thoroughly lethargic, but 

 continue to feed at intervals throughout the whole period of winter in 

 spite of the temperature, whenever, in fact, food is get-at-able and 

 not covered with rime and snow. Usually such larvae go deep down 

 into the ground at the approach of vei-y severe weather, and come up 

 again afterAvards, but larvae have been known to be frozen quite stiff 

 and solid, so much so that they have been brittle, and yet such will, as soon 

 as thaAved, craAvl about, go on eating, and come to maturity. This in 

 itself is almost sufficient to prove that extreme cold, per se, is an in- 

 sufficient external stimulus to produce in an insect the phenomenon of 

 hybernation. 



At the same time, the physical condition of the organism may have 

 been modified to Avithstand the loAV temperature to Avhich the larvae are 

 exposed, but hoAV far this is potent I am not quite able to see, for, 

 although the modification is, say in an insect like Arctia caia, potent 

 up to the fourth skin in Avhich it hybernates, it is impotent beyond, for 

 if the larva change that fourth skin it cannot hybernate but Avill feed 

 up and become an imago, or die in the attempt. 



It Avoiild appear, therefore, that "natural selection " has perfected 

 the insect to undergo hybernation at just one particular epoch of its 

 larval life and at no other. May it not be that this specialisation is in 

 the direction of storing up the necessary energy to Avithstand this long 

 fast, and that Avhen this point of specialisation of the tissues themselves 

 has been reached the larva Avill hybernate quite independently of tem- 

 perature, food supply, or any other external factor ; or, putting it in 

 another form, Ave may suppose that the external factor, indeed, is the 

 quantity of food eaten A\diich has been stored, and that at a certain 

 point of development the adaptation of the insect is so perfect that it 

 responds AA'ithout further stimulus ? 



With regard to tlie hybernating butterflies, the phenomenon appears 

 to be explicable upon much the same grounds, but may be here more 

 especially related to the reproductive system. Vanessas, Avhich pair 

 the season they emerge, invariably, I believe, attempt to produce 

 another brood of young, and then perish ; Avhilst those AA'hich feed 

 without doing so usually pass into hybernation, and go through the 

 duties of paternity and maternity after the Avinter lethargy. 



It appears to me that the influence of temperature, so far as it 

 directly acts on insect life, is relative rather than absolute ; for if an 

 insect Avhich has begun to hybernate be artificially exposed to a Ioav 

 temperature for a short time, and then changed to a comparatively high 

 one, the lethargy ceases — the larA'a seeks for food, and dies if it not be 

 supplied ; the pupa becomes an imago ; the perfect insect flutters about 

 and is ready to lay her eggs. Nor, once the lethargic tendency be dis- 

 turbed, Avill a loAV temperature reinduce it. The larva and perfect 

 insect, then, Avhen re-aAvakened, set about the biisiness of their lives, 

 and failing, OAving to circumstances, die ; and this, in spite of the fact 

 that under natural conditions they Avould go on hybernating, and that 

 their lethargy Avould have been prolonged far beyond the time at Avhich, 

 under these artificial conditions, it ceased. 



We may assume, of course, as 1 haA-e before suggested, that cold 

 was the external stimulus Avhich originally developed the resting habit 



