200 THE FUNDAMENTAL ASPECT OF COLORATION 



to the differential effects of light rays. This sensibility is influenced 

 by certain constituents of diffused light reflected from surfaces in 

 the immediate environment, which affects the colour assumed by 

 the pupae. On black, deep red, blue and certain green backgrounds, 

 for example, darkly coloured pupse resulted. On the other hand, 

 orange, yellow or white backgrounds gave rise to green, or pale, 

 bright pupse, and the same effects were produced by surroundings 

 of green leaves and shoots. Two pigments, a green and a black, 

 bccur in the pupae, and it appears that since rays from the orange 

 and yellow zone of the spectrum are inimical to the superficial 

 black pigment, they conduce to the production of green or pale 

 pupae. It seems tolerably certain, therefore, that those same rays, 

 reflected from green surroundings in Nature, stimulate the pupae to 

 assume a green appearance. The blue rays, on the other hand, 

 tend to produce dark pupae. The idea that the light may act 

 through the larval eyes led Poulton to cover the latter with black 

 varnish, but no resultant influence upon pupal coloration was 

 observed, and it would appear that light produces its effects via 

 the general integument. These extensive researches, so briefly 

 outlined, have stimulated others to experiment along simiilar lines, 

 and their results support the general conclusion that a power of 

 colour adjustment, in relation to their immediate surroundings, 

 exists in certain Lepidoptera. 



While it is generally agreed that the black superficial pigment 

 is melanin, the nature of the green colouring matter is prob- 

 lematical. Whether an enzyme promotes its formation as 

 suggested by Fraulein Brecher, or it is a modified derivative of 

 chlorophyll, is still unsettled. Experiments conducted by 

 Brecher (1921) with blood of larvae of Pieris hrassicce, in vitro, 

 appear to show that, during the sensitive period, yellow rays 

 increased the acidity of the blood and reduced the activity of 

 tyrosinase, while ultra-violet rays had the opposite effect. Her 

 more recent researches (1925) do not confirm this conclusion when 

 applied to an examination of the blood of both larvae and pupae 

 living under the influence of different light rays. When the 

 blood was drawn directly, its ^H reaction was found to be 

 remarkably constant (6-50 to 6-77). On the other hand, Fraulein 



