244 COLORATION [CH. 



A. Optical or Interference Colours. 



These colours do not really exist as pigments, but are produced 

 by the interference of the light reflected at the two surfaces of 

 the double wing-layer, or by combined refraction and reflection 

 of light which falls on to a body-area pigmented in a special 

 manner. If the wing be unpigmented, the interference coloration 

 takes the form of iridescence, a kind of delicate rainbow or mother- 

 of-pearl effect varying with the angle of vision. This can be seen 

 in the freshly-formed wings of all Odonata. It seems to be due 

 to the presence of a liquid layer (pale greenish blood) between 

 the two wing-membranes. Thus there are, in all, three super- 

 imposed layers with different refractive indices. As soon as the 

 wing is dry, the two wing-membranes fuse, and the iridescence 

 gives place to hyalinity , in which no interference effects are usually 

 visible. In the Calopteryginae, however, the mature wings retain 

 a delicate iridescence of pale green, mauve or purple. This seems 

 to be due to the arrangement of the numerous narrow parallel and 

 unpigmented cells of the wing, which obviously produce an inter- 

 ference effect on the light falling on certain parts of each cell. No 

 cases of permanent iridescence are known in wings in which the 

 cells are large, polygonal, or irregularly arranged. 



The phenomenon of metallic coloration is the effect produced 

 by superimposing an interference effect upon a dull-coloured 

 underlying pigment. We shall therefore deal with this under the 

 next heading. 



B. Pigmentation or Natural Colouring. 



These colours are produced by the deposition of certain coloured 

 substances either in the wing or the body-integument, in the form 

 of fine dust or particles, called pigment-granules. Almost all pig- 

 ments are elaborated by cells of ectodermal origin, though a 

 few appear to originate in certain mesoderm cells, and to migrate 

 later into the ectoderm. We may classify the pigments found 

 in Dragonflies 1 as follows : 



1. Cuticular or Permanent Pigmentation. This occurs in all 

 Odonata, but in very varying amount. The pigment is actually 



1 We are here only concerned with external coloration. Pigments are also 

 elaborated in many internal organs, e.g. the mid-gut and the tracheae. 



