246 COLORATION [CH. 



maturation. It is always far more evident in males than in 

 females. The pigment appears to be a product of the process 

 of maturation of the gonads, and is therefore to be considered 

 as of mesodermal origin. It is, however, sooner or later taken 

 up by the hypoderm cells, and excreted through the fine pores 

 of the cuticle, so that it appears as a supracuticular pigment, 

 which can often be removed by rubbing. The most extraordinary 

 case known to me in Odonata is that of Argiolestes griseus albescens, 

 in which the pruinescence of the thorax resembles an abundant 

 growth of white fungus. 



Pruinescence in Odonata is always of a whitish or pale bluish 

 colour. By combination with a subcuticular red pigment it may 

 give a purple appearance to the insect. It is distinguished from 

 all other forms of colouring by its gradual growth or onset, 

 beginning long after metamorphosis. The parts first affected 

 appear to be those associated with special sexual organs, e.g. 

 either the second or the ninth abdominal segment. From one 

 or other of these centres it may spread all over the body. In some 

 cases only the thorax becomes pruinescent. In most Odonata 

 the ventral surface of the abdomen becomes pruinescent after 

 a time, in both sexes, even when the rest of the body is not affected. 

 In the females, there seems to be a factor inhibiting pruinescence 

 until late in imaginal life, after most or all of the eggs are laid. 

 A very old female may become rapidly pruinescent, so as to 

 resemble the male in coloration. 



Body-Coloration (Plates I, III). 



In this section, I propose to give a phylogenetic treatment 

 of the subject of Odonate colour-patterns, based on a careful 

 study of all the forms known to me. 



The most obvious fact is that the bicolorous type of pattern 

 not only predominates, but forms a basis for the production 

 of those unicolorous patterns known to occur. Many species in 

 which the males are unicolorous have bicolorous females. More- 

 over, the newly-emerged males also shew signs of the bicolorous 

 pattern for a greater or less period of time. All this points to 

 the fact that the bicolorous pattern is the most archaic form of 

 pattern in the Odonata. 



