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XXII. On the Factors which determine the Cocoon Colour 

 of Plusia moneta and other Lepidoptera. By Mrs. 

 Onera A. Merritt Hawkes, B.Sc. (Lond.), M.Sc. 

 (B'ham). Communicated by Dr. A. D. Imms, M.A. 



[Read November 15th, 1916.] 



1. The Cocoon Colour of Plusia moneta. 



The cocoons of Plusia moneta are normally bright yellow, 

 but a white one is occasionally found. It is supposed by 

 many entomologists that when such a phenomenon occurs, 

 under natural conditions, in this or other species, that 

 the whiteness is due to " weakness " of the larva. I 

 have shown (Hawkes, 1916) that this is not the case in 

 one Indo-Chinese hybrid moth (Philosamia ricini x P. 

 cynthia), and it may be that further experiment will 

 demonstrate that the lack of colour is usually due to well- 

 defined environmental or physiological causes, rather than 

 to any such indefinite condition as " weakness." Tutt 

 in his " British Noctuae " only mentions yellow cocoons 

 in P. moneta, but Barrett notes the variation from deep 

 yellow to white. 



In June 1916 two larvae of Plusia moneta were found 

 feeding on Monkshood (Delphinium) in Edgbaston, Birming- 

 ham. They were reared in two glass-lidded boxes. When 

 about to spin-up, all the leaves were removed from one 

 box; thus, one box was dry, whilst the other was damp 

 from the water evaporated from and transpired by the 

 leaves. As a result, cocoons of two colours were spun — 

 a yellow one in the damp box, and a white one in the dry 

 box. 



A piece of the white cocoon was then cut off and placed 

 in the damp box, where, after twelve hours, it had become 

 a deeper yellow than the complete yellow cocoon. 



The larva in the yellow cocoon did not pupate, but, owing 

 to some unknown cause, left its cocoon and died. The 

 larva in the white cocoon pupated and produced a perfect 

 imago. The white cocoon was kept in a dark dry box, the 

 box being opened daily to admit fresh air. Towards the 



TRANS. ENT. SOC. LOND. 1916. — PARTS III, IV. (APRIL '17) 



