XXX vin 



two colours to be referred to hereafter, resulted in the darkening of the pupae, /. e. 

 an increase in black ; yellow or orange surroundings, therefore radiating a very 

 bright light, caused the pupa to become dark green however. In the latter 

 some darkening, therefore, occurred also, but whence this green colour? 



According to W. Petersen the colour of the pupa is due to the pigment 

 in the cuticula and the hypodermis. Now the pigment in the latter is green 

 in the larva and sometimes remains so in the pupa. If, however, an accu- 

 mulation of pigment occurs in the cuticula this green colour becomes darker. 

 Yellow or orange light, however, is said to prevent the formation of dark 

 pigment in the cuticula, causing it to become translucent when the pupa would 

 appear green owing to the pigment of that colour in the hypodermis. This may 

 well be the case but those who do not judge from a chemico-physical point of view 

 will be satisfied neither with this nor, therefore, with the chromo-photographic 

 explanation. It will be asked what, then, is the cause of the appearance of 

 dark pigment in the cuticula, as it appears to occur normally and is prevented 

 abnormally by such a particular light? What natural process is disturbed in 

 its normal course by this illumination? Petersen does not explain this; let 

 us see, therefore, if we may not be able to trace it. 



The pupa of P. Brassicae L. is described by the Dutch Entomologist 

 Snellen as yellowish-green with black dots, that of P. Rapae L., as yellowish- 

 grey or brown with three yellow stripes, and that of P. Napi L., closely allied 

 to the latter, as dull green, finally that of P. Daplidice L, as green. Green, 

 therefore, appears to be the original colour of the Pieris pupae, partly turning 

 to yellow and in which, moreover, black commences to appear. The occurrence 

 of green in the forementioned experiments, therefore, can be nothing but the 

 reappearance of the colour peculiar to the pupae of P. Brassicae L. and 

 P. Rapae L. at an earlier stage of colour development, and is doubtless the 

 return to an earlier colour form similar to that occasioned in the colour develop- 

 ment in the imagines by the experiments with heat and cold on the pupae, 

 referred to on page XII supra, the B. forms of Dr. E. Fischer ; the result of 

 disturbance by abnormal influences in the natural process of colour develop- 

 ment, in the one case caused by heat or cold, in the other by too bright a light. 



We must here draw attention to a series of other experiments. 



In order to find out whether the colour of the environment also influences 

 the colour of the cocoons of Heterocera, Poulton made several trials. Thus 

 he found that larvae of Saturnia Carpini H. (Pavonia L.) formed very dark 

 cocoons in dark surroudings but white ones in bright light; that Eriogaster 

 Lanestris L. produced creamy white cocoons on white paper but dark-brown 

 ones between leaves ; that the cocoon of Halias prasinana L. is likewise brown 



