XIII] 



COLORATION 



249 



We cannot here follow out in detail the wonderful specializations 

 of the marginal pattern to be found in the Agrionidae. A study 

 of the second segment alone will shew the lengths to which 

 pattern-forming can be carried. Perhaps the highest development 

 of colour-pattern to be found in the Zygoptera is the tricolorous 

 pattern, e.g. in Ischnura aurora or Xanthagrion erythroneurum 

 (Plate III) in which bright blue and red occur in the same insect. 



Metallic coloration may be formed on the two blocks of 

 ground-colour, usually giving rise to a dark bronze-black or 

 bronze-green effect. If the annulo-marginal stage is thus affected, 

 the abdomen appears definitely metallic. Finally, the annuli may 

 be suppressed, and a unicolorous metallic pattern produced, either 

 bronze, green or blue. These stages can be followed out in the 

 Lestidae and Calopterygidae. 



B. Central Types (fig. 114). 



The original central type of colour-pattern is preserved for us 

 with very little modification in the long 

 hylochrome series of the Aeschninae, in 

 which there is very little tendency to- 

 wards annulation. The paired longitudinal 

 blocks, which in this type carry the bright 

 hypoderm-colouring, early tend to become 

 oval. By various changes in shape, and 

 by splitting-up, the wonderful patterns in 

 this subfamily have all been formed from 

 the original central type. Fig. 114 shews 

 a series of these. The hypoderm-pigments 

 are usually green or yellow, sometimes 

 bright blue, more rarely brown, grey or 

 cream. The most primitive type is found 

 in the Petaliini, where the blocks are still 

 much elongated. 



One sub-division of the Brachytronini has branched off along 

 a colour-line of its own. The bright hylochrome pattern be- 

 comes gradually suppressed, and a uniform dark brown pattern 

 supervenes. Traces of the original pattern are always to be found 

 on the thorax (remnants of bands), on the head, and often on 



d. 



Fig. 114. Central types 

 of colour-pattern, a, ar- 

 chaic type ; 6, c, stages in 

 the development of the 

 spotted Aeschnine type, 

 by interpolation of the 

 supplementary transverse 

 carina. d, e, /, stages 

 leading to the annulo-cen- 

 tral type (/). Original. 



