INTRODUCTORY. 41 



butterflies is related to ordinary products of the metabolism of 

 the body.* 



Mr. Poulton has recently found that the red fluid excreted 

 by the Tortoiseshell and other Vanessida?, immediately after 

 they escape from the chrysalis, contains uric acid ; the fact 

 that this substance is evacuated in these butterflies may 

 perhaps have something to do with the absence, or at least 

 the presence of only traces, of a yellow colouring in these 

 butterflies ; metabolism must be nearly at a standstill in the 

 butterfly. 



Dr. Urech has also pointed out that there is a close con- 

 nection between the pigments found in the urine of butterflies 

 and the colour of their wings. 



We might expect that caterpillars, which lead such extra- 

 ordinarily active lives, as far as devouring food is concerned, 

 would normally store up a quantity of integumental pigment. 



* Joum. Chem. Soc., 1889, and JaJn-esb.f. Thier. Chemie, 1890. 



