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 9° 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. 



