52 ONERA A. M. HAWKES 



and reared the offspring, to find that they produced spotless 

 larvae and white cocoons. One seemed, therefore, justified in 

 presuming that the parent P. ricini came from a stock which 

 produced only white cocoons. 



P. cynthia differs markedly from P. ricini in that it has, typi- 

 cally, a red-brown cocoon. Owing to some unknown cause, the 

 intensity of colour varies considerably. I have not up to the 

 present been able to rear this species in order to study the varia- 

 tions of intensity and the possible causes which may produce 

 that variation. A related species, P. canningi, also has the 

 cocoon colour varying from a rich red-brown to a pale fawn : this 

 species has been reared, but only for one generation. Experi- 

 ments were to be made on the second generation to test if mois- 

 ture had any effect on the intensity of colour, but the moths un- 

 fortunately did not pair. It is important to notice that neither 

 P. cynthia nor P. canninghi, is known ever to have produced a 

 white cocoon. 



As the result of the crossing, the female, P. cynthia laid about 

 180 eggs, a large proportion of which were fertile. At the end 

 of three weeks there were 149 larvae, which produced 135 cocoons 

 and from them there emerged 111 moths, only three being crip- 

 ples. The family was exceptionally healthy in spite of the 

 exotic conditions under which it was grown. The larvae grew 

 quickly and reached a length of 2| to 3^ inches — as large as any 

 grown at Pusa. There was very little disease. 



These cocoons exhibited every variety of fawn and pale brown — 

 none were white, none were the deep red-brown characteristic 

 of P. cynthia. One was therefore, inclined to conclude that the 

 brown colour acted as a weak dominant. 



The cocoons (Fi generation), were spun in the privet leaves 

 on which the worms had been reared or on the nets which cov- 

 ered their cages. The nets were either black or white. The 

 colour of the nets had no influence upon the intensity of the 

 colour of the cocoons, as both darker and lighter cocoons occurred 

 side by side on both black and white nets. One colour phenom- 

 enon, however, constantly occurred; when a cocoon was spun in 



leaf, it was always a darker colour on the side towards the leaf, 



