determine the Cocoon Colour of Plusia moneta. 407 



" The evidence which I brought forward went to show that 

 the statement that there is any relation between the colour 

 of these cocoons (E. lanestris) and that of the substance to 

 which they are attached, was founded on a mistake " ; and 

 " it was to be concluded, that the cause determining the 

 production of light cocoons was removal from the food, 

 or the state of annoyance incidental to such removal, and 

 that in fact the light-coloured cocoon was an abnormal 

 product resulting from unhealthy conditions." This may 

 well be the case in Eriogaster, but it is not the case in one 

 Philosamia hybrid (loc. cit. p. 54), in P. moneta, and in the 

 above two specimens of Halias. 



Again, Bateson experimented with Saturnia carpini, and 

 concluded (p. 207), " that there is no relation between the 

 colour of the cocoons of S. carpini and that of the sub- 

 stances to which they are attached " ; and (p. 209) " it 

 may be safely concluded that the brown colour of the 

 cocoons is derived from the alimentary canal." When 

 Bateson placed pieces of white silk in the fluid ejected 

 from the mouth of the larvae, the silk became brown; 

 but Dewitz, who has also done experiments on S. carpini 

 (= S. pavonia), found that when a piece of white silk was 

 put in water it became brown. 



May makes a report of two broods of larvae of Saturnia 

 carpini. In brood 1, reared by Bell, the larvae were badly 

 fed and kept in a practically air-tight cage with a damp 

 atmosphere, the resulting sixteen cocoons being all dark. 

 Brood 2 was reared by May, the larvae being well fed and 

 kept in a dry, well-lighted cage; nineteen cocoons were 

 produced — eighteen pale, one dark. When six of the pale 

 ones came in contact with damp, they turned dark almost 

 immediately. May says : " With regard to the one dark 

 cocoon, it may have been splashed on pulling old stalks 

 out of the water-bottle." 



Dewitz believes that there is a "chromogene " in the 

 silk of S. carpini, which under the influence of the oxygen 

 of the air and an alkaline fluid becomes brown. 



Comparing these results with my experiments on the 

 Philosamia hybrid (p. 56), I am inclined to think that 

 water per se can produce the change of colour in the 

 completed silk. 



Dewitz states that some hours after the last drop of frass 

 has been expelled, a second, colourless intestinal excretion 

 takes place in S. carpini, and that it is this that produces 



