158 
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 is 
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. 
