XHI] COLORATION 251 



C. The Thoracic Pattern (fig. 115). 



That the basic difference between marginal and central types 

 is not wholly lost on the thorax can be seen by comparing the 

 thoraces of an Aeschnine and a Gomphine Dragonfly. In both, 

 pale bands alternate with dark areas of ground-colour. But, on 

 the whole, we may say that the pale and dark portions are reversed 

 in the two cases. In the typical Aeschnine thorax, the two pairs 

 of broad bands correspond with the two pale central areas of two 

 abdominal segments. In the Gomphine thorax, the pale bands 

 correspond with the middle lines and sides of the abdominal 

 segments, so that an opportunity arises for the production of the 

 many-banded pattern seen so often in this subfamily. Likewise, 

 the marginal type is preserved in the formation of an anterior 

 transverse annulus, not complete, it is true, but of great interest, 

 in that, by fusion with the dorsal bands, it produces the peculiar 

 "seven-marks" so distinctive of this subfamily. In connection 

 with this, a study of the thorax of Petalura is of interest 

 (fig. 115). 



a b. c 



Fig. 115. Dorsal thoracic colour patterns, marginal type, a, Petalura pulcherrima 

 Tillyard. 6, c, formation of the Gomphine " seven-marks." Original. 



In the Zygoptera, the sides of the thorax tend to become 

 uniformly pale, by loss of the metathoracic ground-colour bands. 

 Thus only two isolated pale bands are left, one on either side of 

 the mid-dorsal carina. A very interesting correlation can be 

 established between the thoracic colour-pattern and the nature 

 of the habitat in certain genera, e.g. Argiolestes (PL III, figs. 2-8), 

 where rock-dwellers tend to become dark, while orange or red are 

 developed in the dense jungle, and blue or grey in the open sunlight. 



In many Aeschninae the thoracic bands become narrowed, 

 divided up into spots, or even abolished, particularly the dorsal 

 bands. In the Libellulidae, when a heliochrome or metallic pattern 

 is produced on the abdomen, the thoracic bands also become 

 masked or abolished. The thorax, however, assumes the helio- 

 chrome colouring more slowly than the abdomen, so that many 

 species with bright red or blue abdomens have only brown thoraces. 



