SEASONAL DIMORPHISM OF LEPIDOPTERA. 



241 



time, produced the summer form, although cold may have acted 

 upon them towards the end of the pupal period. On the other 

 hand, in the second experiment pupae of the same brood pro- 

 duced the winter form, when they were placed in the refrigerator 

 immediately after pupation, and the result was not materially 

 altered, if they were brought into the incubator three days before 

 emergence. 



I believe, however, that the contradiction between Merrifield's 

 results and my own is only an apparent one, and that the reason 

 is, that with P. napi adaptive and direct seasonal dimorphism are 

 mixed. Theoretically this might be anticipated as not only 

 possible, but probable. A species, which has adapted itself to 

 the seasons of the year by a double protective colouring, can on 

 this account be still directly alterable by heat or cold. And this 

 appears to be the case with P. napi. Only the under side of the 

 wing is really adaptive here, but differences between the winter 

 and the summer form are also exhibited even on the upper side. 

 The winter form possesses the black powdering of the bases of 

 the wings, which is absent in the summer form ; the winter form 

 also exhibits a fine black powdering of the veins of the wings 

 towards the margin of the wings, a character which is most 

 strongly pronounced in the var. bryonies. But while the winter 

 form has the apex of the fore wings only a dull grey, the summer 

 form has here a more sharply defined and larger brownish black 

 colouring. 



A biological value can scarcely be attributed to these charac- 

 ters, and when it is seen that they run parallel to the action 

 of a higher or a lower temperature on the pupae, there is an 

 inclination to regard them as the direct influence of the colour 

 chemistry. 



Should this view be correct, we must expect that circum- 

 stances might arise, which would result in a commingling of the 

 characters of the winter form with those of the summer form ; 

 for example, winter form below and more or less summer form 

 above. This would have to occur, if, for example, cold was 

 prevalent at pupation until the primary constituents of the winter 

 form — so far as they are adaptive and depend on special primary 

 constituents — are rendered active, but then if later on, shortly 

 before emergence, a high temperature was prevalent, and so in- 

 fluenced the chemistry of the formation of colour in the wing, 

 that the upper side had the summer habit. 



It would be very simply explained in this way, how Merri- 

 field came to assign the critical time to the end of the pupal 

 period. He was right in so far as the character directly depen- 

 dent upon temperature is actually first determined at this time, 

 while the beginning of the pupal period gives the decision for 

 the adaptive characters, which are contained in the germ-plasm 

 as duplicated primary constituents. 



ENTOM. — AUGUST, 1896. T 



