184 THE FUNDAMENTAL ASPECT OF COLORATION 



to become incorporated with the protein constituents of the blood, 

 the closeness of the spectral resemblances is specially remarkable. 

 It is noteworthy that certain green caterpillars become colourless 

 when fed upon parts of plants devoid of chlorophyll, but become 

 green when access to green leaves is established. 



In recent years great advances have been made with regard to 

 our knowledge of the composition of chlorophyll, more especially 

 through the researches of Willstatter and his co-workers. The 

 green pigment or true chlorophyll consists of two components, 

 both of which exhibit in solution a notable red fluorescence. 

 The one, chlorophyll a, forms about 72 per cent, of the mixture, 

 and is blue-green in alcoholic solution when viewed with trans- 

 mitted light. The other, chlorophyll b, forms the remaining 

 28 per cent. ; it is yellow-green when viewed by transmitted light, 

 and differs from chlorophyll a in having two atoms less of 

 hydrogen and an extra atom of oxygen. The yellow components 

 of chlorophyll comprise an orange-yellow pigment known as 

 carotin, which is a highly unsaturated hydrocarbon, and a second 

 yellow pigment, xanthophyll, which is its dioxide. 



Recent researches lend support to Poulton's main conclusions 

 that many caterpillars are coloured by pigments derived from 

 their food, which are absorbed into the blood without undergoing 

 marked changes in their character. Whether the yellow pigment 

 referred to by Poulton under the general term xanthophyll is 

 true xanthophyll, or carotin, appears to be uncertain, and Palmer 

 (1922) advances reasons for suggesting that the chief carotinoid 

 pigment of Lepidopterous larvae may be carotin. This substance 

 is widely distributed among insects and is known to figure in the 

 coloration of beetles of the families Coccinellidae and Chrysomelidse 

 and in red-bugs or Pyrrhocoridae. Recent research has afforded 

 further evidence of its presence in insects. The work of Palmer 

 and Knight (1924a) indicates that the yellow and red coloration of 

 the Pentatomid bug Perillus bioculatus is hypodermal, and largely 

 due to the presence of secondarily absorbed carotin. This insect 

 is predaceous in habit, and the favourite food of the adults and 

 nymphs consists of the eggs and larvae of the potato beetle 

 (Leptinotarsa decemlineata) as well as the beetle itself. They 



