iSS COLOUR IN NATURE chap. 



are several points of great interest. Thus he notices 

 that in the case of the larger scales it is not infre- 

 quent for leucocytes to enter the cavity of the scale 

 and there disintegrate. These leucocytes apparently 

 originate from the cells of the fatty body, a structure 

 which has probably much to do with excretion in 

 insects. This passage of " wandering cells " outward 

 to be deposited in the superficial structures is a 

 phenomenon which we shall frequently have to 

 mention in connection with pigmentation. Not 

 infrequently, as in the leech, the cells are themselves 

 pigmented, and deposit their pigment in the skin. 

 It would be a fact of some interest if Urech's 

 hypothesis is correct, and these cells by their 

 disintegration be found to give rise to the peculiar 

 pigments of butterflies. It is of course quite possible 

 that Mayer is also right, and that the haemolymph 

 does give rise to certain of the pigments, especially 

 those of relatively dull tints. As, however, it Ts 

 the bright-coloured waste products which are most 

 important in butterflies, we must return to our 

 consideration of these. Urech's suggestion is of 

 much interest so far as it goes, but we require to 

 know why this decomposition of nuclein should go 

 on in the adult and not in the larva. In attempting 

 to answer this question we must consider the differ- 

 ences between larva and adult, and the physiology 

 of the state which lies between the two. 



