colour of cocooii!^, pupre, and larvce. 209 



cocoons may be akin to that which takes place in the 

 leaves. This suggestion is, of course, merely made 

 for what it is worth. 



If the contents of the gut are dried, the brown sub- 

 stance remains perfectly soluble in water. 



(/). The proof that the green colouring matter from 

 the gut is used to dye the cocoon brown rests on the 

 following observations : — If a larva is irritated it ejects 

 from the mouth a green glairy fluid, which turns red- 

 brown, like the contents of the gut. If a piece of a 

 white cocoon be laid in this fluid for some minutes, it soon 

 acquires the brown colour of a brown cocoon, from which 

 it is indistinguishable. The same is true of a fibre of 

 silk drawn from a spinning animal, which can be dyed 

 in the same way. The colour is then insoluble, and 

 cannot be washed out, having stained the silk like a 

 mordant. In the previous experiments, in 1891, I was 

 puzzled by finding the colour soluble in the evacuations, 

 but insoluble in the cocoons; but this is no doubt the 

 explanation. 



{(j). Lastly, it is to be considered how the colour gets 

 from the animal's gut to the silk. As to this, I have no 

 decisive evidence. I know that a caterpillar may spin 

 brown threads without touching them with the posterior 

 end of the body, and it is therefore clear that the colour 

 may be given out from the mouth, just as it is when the 

 larva is irritated. But the appearance of the interior 

 of a cocoon rather suggests that a large quantity of the 

 size-like matter has been poured out at once. It seems 

 possible, therefore, that there may be a final discharge 

 from the intestine after the cocoon is finished. I am 

 inclined to think that some of the threads are often 

 spun white, and smeared with the colour afterwards, for 

 I have seen threads of a cocoon lately begun, first white 

 and then brown after an absence of an hour, and the 

 animal may often be seen, as it were, " mouthing " over 

 its threads. This is not always the case, for I have seen 

 very dark threads lying adherent to the surface of paper, 

 in such a position "that they could not have been gone 

 over again without staining the paper, but must have 

 been put down brown while still viscous. 



I think, then, it may be safely concluded (1), that 

 the brown colour of the cocoons is derived from_ the 

 alimentary canal; (2), that it is produced in the diges- 



r2 



