XTTT] COLORATION 247 



The pattern was evidently originally formed segmentally , the 

 pattern of each segment being the same. In the head, owing to 

 fusion of the segments, only certain special features of the pattern 

 can be dealt with satisfactorily. In the thorax, alterations in 

 the original shape and direction of the segments have altered 

 the original plan, which can, however, always be more or less 

 correlated with that of the abdomen. In the latter, certain 

 segments (particularly the second) tend to become specialized 

 beyond the rest. It must be understood, therefore, that the 

 classification of colour-patterns here attempted is based primarily 

 on the study of an unspecialized abdominal segment, such as the 

 fifth or sixth. The colour-schemes of the head and thorax will 

 be correlated with these in subsequent sections. 



The origin of the bicolorous pattern is to be explained on the 

 ground of the bilateral symmetry of the disposition of the internal 

 organs beneath the dorsal integument. The ventral parts do not 

 come into the question. Bright colours are called forth by the 

 influence of bright sunlight. An aerial habit is then, above all, 

 favourable to their formation. Thus the Dragonflies share with 

 their Vertebrate analogues, the Birds, the distinction of possessing 

 the brightest body-colourings in the Phylum. As regards the 

 bilateral symmetry of the pattern, the basis for this is the method 

 of deposition of the ground-colour or cuticular pigment. This 

 may be deposited along the sides and middle line of a segment, 

 leaving two clear areas above the pigmented dorsal tracheae, 

 or, apparently, exactly the reverse may be the case. The former 

 I have termed the central type, the latter the marginal. The terms 

 refer to the position of the chief areas of bright coloration. The 

 marginal type appears to be the older, and probably dates from 

 a period before the pigmentation of the dorsal tracheae set in. 

 We may define the two types as follows: 



A. Marginal Type. On each tergite, the ground colour forms 

 two longitudinal blocks, one on either side of the middle line. The 

 anterior and posterior borders, the middle line, and a large lateral 

 area on each side, have their cuticle unpigmented, so that the 

 brilliant colours of the hypodermal pigment appear in these areas. 



B. Central Type. The pigmentation is exactly opposite to 

 that of A. The ground-colour is only absent in the two longitudinal 



