DISTEIBUTION OF LEPIDOPTERA. 33 



further south in continental Europe, a winter temperature corres- 

 ponding to 6deg. further south, and a summer temperature cor- 

 responding to a good deal further north, perhaps as much as 8deg. of 

 latitude. 



To return, however, to the action of the milder winter on insects 

 hibernating. Hibernating larva), and m\itatls imitaudis, similar con- 

 siderations apply to other stages, ditter immensely in what one may 

 call the intensity or earnestness of their hibernation. This is so even 

 within the limits of one species, as I found in the case of Arctia caia, 

 where one form of hibernating larva adhered to its hibernation, against 

 the stimulus of a high temperature, for twice as long as the normal 

 form did. Some species go into hibernation at a fixed date and 

 remain for a certain period, and no increase of temperature will alter 

 their programme, they will die at their posts first. Others, again, go 

 into hibernation Avith very honest intentions, but any very considerable 

 temperature yv\\\ make them move on. Others, again, are easily dis- 

 turbed, and one might almost say hibernate because they must, but 

 would rather not. A very large proportion of our hibernating Noctuid 

 larvffi belong to this section. They hide beneath a little soil or rub- 

 bish, sufficient to protect them from' a slight frost, and arc, therefore, 

 able in any mild weather to come to the surface and do a little feeding, 

 and, except in very severe weather, are subject to no cold enough to 

 make them really torpid. It is well-known that there is no difficulty 

 in making these larvae feed up continuously and the moths emerge in 

 January or February. At inland continental stations such larva?, 

 owing to the greater severity of the weather, are obliged really to hibernate, 

 and from the beginning of winter, which there sets in as early as here, 

 are rendered quite torpid, and even on a warm day are too chilled to 

 be able to make any use of it. They consequently are, in spring, less 

 forward than here, but easily make up lost time by the greater warmth 

 of the summer. With us, by aid of the milder winter, they keep 

 slightly active all the time, and so make our mild winter balance our 

 cool summer in getting the cycle of their metamorphoses into the 

 year. 



We may now turn to actual species and see how far we can apply 

 these various circumstances affecting the coast to explain their distri- 

 bution. I may take first AporojiJii/la amtralis, since it is one of which 

 I am quite sure as to my facts both as to life-history and distribution, 

 and it illustrates the principal method of action of our mild south 

 coast winter. A. amtralis occurs on our south coast, and again in 

 quite southern France, and in the Mediterranean region ; I should 

 expect it to occur also in western France, but I am not aware of any 

 records. It is quite absent in central and northern Europe. Its life- 

 history, and it is the same in its southern as in its English habitats, 

 is briefly— that the eggs are laid in September, the larvae feed all winter 

 and pupate at latest in May, remaining all through the summer in the 

 pupal state. The question of a hot or cold summer is indifferent to it, 

 but what is absolutely essential is a winter suflticiently mild to enable 

 the larva to have little or no feeding up to do in spring. This is 

 possible on the warm side of the winter isotherm of 42deg. or there- 

 abouts. My map is a rough one, but the blue line I have drawn is not 

 far out as the northern limit of A. austmlh, though it professes to be 

 the winter isotherm of 42deg. Fahr. Another species that is probably 



