240 THE ENTOMOLOGIST. 



NEW EXPERIMENTS ON THE SEASONAL DIMORPHISM 

 OF LEPIDOPTERA. 



By De. August Weismann. 



(Translated from the German by W. E. Nicholson, F.E.S.) 



(Concluded from p. 208.) 



Furnished with this better information, let us turn to the 

 observation of some other cases, which, I beheve, we must con- 

 sider as adaptive seasonal dimorphism. 



Among these I reckon before all our Pieridse, which are 

 seasonally dimorphic, at least partially so. Pieris napi, the 

 green-veined white, exhibits in its spring form the well-known 

 broad blackish green powdering of the under side of the hind 

 wings, which is obviously a protective colouring, and which, in 

 fact, conceals the butterfly sitting asleep upon the leaves of 

 plants just as well as the chequered green and white under side 

 of the species of Anthocharis. But it is just this green protective 

 colouring which is absent in the summer form, and the thought 

 arises that the drier and less brilliantly green surroundings of 

 the summer brood have made this alteration necessary. I well 

 know that experienced entomologists, such as Seitz, have ex- 

 pressed the opinion that our whites enjoy immunity from birds, 

 and would not be eaten by them. But in the first place the very 

 accurate observations of Poulton and others are opposed to this 

 view ; and then with a butterfly asleep it is not a question of 

 enemies among the birds, but of lizards, frogs, toads, and other 

 enemies, whose importance in reference to butterfly-life is only 

 very imperfectly understood. But this inadequate insight into 

 the biology of the insects is just as true of the spring form, and 

 with this the protective significance of the under side cannot be 

 doubted. It is therefore certainly explained by the enemies of 

 the butterfly at rest, whether we know them or not, and whether 

 they still form a danger to the species or only did so in earlier 

 times ; as an old acquisition is only slowly lost, if it becomes 

 unimportant. 



Let us now see whether the results of the experiments agree 

 with this view. 



Besides my old experiments there are those of Merrifield on 

 Pieris napi, and my own, which are here published, on najn and 

 its var. hryonice. Our results appear to be contradictory on one 

 important point, since Merrifield believes he has found that the 

 critical time for the determining influence of temperature is the 

 last days of the pupal period, while it is obvious from my new 

 Experiments I. and II. that the time immediately after pupation 

 is the critical one. The majority of the pupae of the first experi- 

 ment, which were subjected to the summer temperature at this 



