234 
COLOUR IN NATURE 
CHAP. 
A careful examination of a series of specimens showed 
that these structures represented the normal branched 
pigment cells, whose processes are here completely- 
contracted. The yellow cells of the dermis were, how¬ 
ever, not contracted, and now displayed themselves 
as a delicate network of branched pigment cells ; it is 
to these that the pale larvae owe their colour. The 
difference in tint between the two sets of larvae is 
thus ultimately due to the differing susceptibilities 
of the pigment cells of the skin. The peritoneum 
exhibited the same variation in colour as did the skin. 
As to the cause of the difference, experiment 
convinced Fischel that it was due to the difference 
in temperature of the water in which the larvae were 
reared ; that of the standing water being g° to 11° 
higher than the running water. This was confirmed 
by removing dark larvae from the cold water and 
placing them in warm water, when it was found that 
in the course of a day or two they had become 
notably lighter, and at the end of at most a fort¬ 
night were completely converted into the pale form. 
Although the pale larvae so produced were quickly 
reconverted into the dark form, it was not found so 
easy to render the original light forms dark. A long 
exposure to the influence of warm water apparently 
renders it difficult for the pigment cells to become 
fully expanded when exposed to cold, no doubt also 
these larvae were actually deficient in dark pigment. 
The colour-change could only be produced in young 
larvae, older ones having apparently lost the power of 
reacting to heat or cold. The effect of prolonged 
exposure to heat or cold is, therefore, to produce a 
stable unalterable type of coloration. 
