Mrs. Onera A. Merritt Hawkes on Plusia moneta. 405 



end of the pupal stage one end of the white cocoon had 

 become yellowish, but it did not have even a vestige of 

 the yellow lining described by Winser. 



The experiment with the cut-off piece of silk makes it 

 clear that an environmental cause, and not any excretion 

 or secretion of the larva, was the ultimate producer of the 

 yellow colour ; the conditions under which the two larvae 

 spun-up, and their subsequent history (it was the larva 

 in the white cocoon which became an imago), indicates 

 that environmental moisture and not " weakness " pro- 

 duced the difference in the cocoon colour. This conclusion 

 is supported by the following independent observations. 



1. J. H. Bird writes that empty white cocoons of 

 P. moneta became yellow when immersed in water. R. S. 

 Smallman, in discussing the remarks of Bird, also states 

 that he believes moisture hastens the change from white 

 to yellow, but in that case he would expect to find a larger 

 proportion of white cocoons in the second brood, as that 

 is "produced in the late summer, when presumably there 

 is less moisture than in thu late spring ; but, unfortunately, 

 Smallman makes no statement concerning the relative 

 numbers of the two colours in the two broods. 



2. H. E. Winser states that most of his cocoons were 

 white, but gives no indication as to whether the atmosphere 

 in his breeding-cage was dry or damp; if the former, the 

 white colour is accounted for. 



3. Nicholson remarks that some have thought that light 

 had an effect upon the colour. 



It may here be stated that the two larvae under con- 

 sideration in this paper were bred and spun-up in exactly 

 the same conditions of light and heat, both breeding-boxes 

 having been placed on a shelf in a large light room. The 

 white cocoon which subsequently became yellowish at one 

 end, did so at the same temperature, but in the dark. 



4. Mr. W. H. Edwards of the Natural History Museum, 

 Birmingham, was given, also in June 1916, two white 

 cocoons of P. moneta which had been spun out of doors ; 

 these he put into his greenhouse, where in a couple of 

 days they became yellow. The cocoons were spun dur- 

 ing a period of drought, hence the white colour, and they 

 became yellow when placed in the comparatively damp 

 conservatory. 



5. Sydney Webb and H. W. Andrews mention the variety 

 of colour, and like Nicholson believe it is due to the amount 



