374 Mr. F. Meriifield and Mr. E. B. Puulton 



on 



compared with the extreme. The two forms may be 

 classified as (1) ydloiv-gvccn, and (2) yrey* The former 

 have a yellow ground-colour with deep green markings, 

 the latter a bone-coloured ground with dark purplish 

 brown, and in places black markings. With few exceptions 

 the dark markings of the grey forms correspond in position 

 with the green markings of the ydloio-grccn forms, the 

 chief exceptions being on the surface of the pupal wings, 

 where the green forms a more continuous area, less 

 interrupted by the yellow ground-colour than the dark 

 marking is by bone-colour. 



This correspondence in position becomes all the more 

 interesting and remarkable when it is remembered that 

 the two markings are entirely different in constitution, 

 origin, and even in the pupal layers in which they are 

 respectively situated, the green pigment being relatively 

 unstable, probably a modified form of chlorophyll derived 

 from the food-plant, and situated in the deeper laminated 

 layers of the pupal cuticle, the dark pigment being very 

 stable (remaining permanently in the empty pupal case), 

 solely due to the metabolic activity of the animal organism, 

 and confined to the thick layer of cuticle which lies above 

 the laminated layer and forms the outermost part of the 

 pupal shell, (See Poulton, Proc. Roy. Soc. 1885, Vol. 

 xxxviii, p. 279, in which however the laminated layer is 

 erroneously distinguished from the " true cuticle.") 



We can again classify the (1) yclloiv-green and (2) grey 

 forms as (a) dark and (/>) light, thus : — 



1. 



-r;r „ { «. Liqlit foriiis. 



Yelmv-gi'een -j j j^ ;; ,. "^ 



2 Grev I ''• ^^^^'^ 



-" ^^^''' \J). Dark 



* I formerly spoke of the darker pupa) as brown (Phil. Trans. 1887, 

 I. c. p. 407), a description which is clearly incorrect. Mr. Merrifield 

 has introduced the appropriate term "bone-coloured," which accurately 

 expresses the appearance of the ground-colour of these pupae as well 

 as those of the cf)rresponding forms of P/em napi and P. rapse. The 

 whole appearance of these pupae is however due to the combined im- 

 pression made by the pale ground-colour and the dark markings, and 

 I think that ";/»t;/" expresses this effect as a whole more truly than 

 any other word ; although there are pupae in which the markings are 

 so inconspicuous that the effect seen is that of the ground-colour 

 alone. 



E, B. P. 



