DEVELOPMENT OP PIGMENT IN PIERIS RAP^. 243 



brood immacidata of Cockerell, which Tutt (' British Butterflies,' 

 23. 232) describes as " an extreme aberrational form of this brood 

 {metra) without apical tips or spots." But they fall short of this 

 form in that they have faint apical tips, and, though the spot on 

 the upper side of the fore wing appears at first sight to be absent, 

 examination with a lens shows in all of them a few black scales, 

 often not more than one or two, in the position of this spot; also 

 a few scattered scales represent the costal spot on the upper 

 surface of the hind wing. The anterior spot on the under surface 

 of the fore wing is present in all, though it is generally faint. 

 The under side of the hind wing presents the same variation 

 between yellow and dull greenish as is found generally in 

 var. metra as well as in the later brood of rapce. 



I give sixteen as the number of these specimens, but they are 

 only the extreme forms of a series which passes insensibly into 

 typical metra. I have altogether 102 examples of metra taken in 

 May and June. All those which come nearest to the sixteen were 

 taken before May 20th. 



Two questions appear to present themselves for consideration 

 from this experience : (1) Why, having found the form approaching 

 immacidata common at the beginning of May, did I fail to take 

 any between the 18th and 25th? and (2) why was the appearance 

 of this form confined to the early days of May ? 



(1) Although it is common knowledge that the colours of 

 butterflies fade as they grow older, I am aware of no evidence 

 that they ever become more intense after the imago has fully 

 emerged, and I suppose that it is unnecessary to consider the 

 idea that the immaculate became maculate. I also suppose that 

 the average life of members of this species is more than three 

 weeks. We must, I think, conclude then that, the emergence of 

 the immacidata form being confined to the earlier days of May, 

 the maculate form emerged so abundantly from the middle of 

 May onwards, and so far outnumbered the earlier immacidata form 

 still on the wing, that I was able to take fifty from the 18th to 

 the 25th without including a single example of the latter form. 



(2) The second question is of wider interest. I would submit 

 that the facts seem to show that the difference in pigmentation 

 between the insects emerging in the earlier days of May — 

 " faint and frail and first " — and those appearing later is due 

 to the conditions of temperature to which their pupae were 

 exposed. The pupae of those which emerged on May 3rd were 

 subjected to night frosts for three-quarters of the last two 

 months of their pupation, and often to severe frost ; while those 

 emerging later completed their pupation in the balmy weather 

 of May. The warmth and light of May seem to have " brought 

 out" the pigment, to use a photographic simile, which failed to 

 develop under the more rigorous conditions to which those that 

 w'ere earlier on the wing were exposed. 



