XIII] COLORATION 245 



embedded in the cuticle in the form of fine granules, called 

 "pigment nuclei," deposited in the outer layers of chitin. Since 

 the pigment is protected from decomposition by the presence 

 of the underlying clear layers of the cuticle, it follows that, at 

 death, when the hypoderm cells decay, the cuticular pigments 

 remain unaffected. Hence they are called 'permanent: though, 

 as a matter of fact, continuous exposure to light, for a long time, 

 causes them to fade. These colours form the ground-colours of 

 Dragonfly body-pattern. Blacks' and browns of all shades and 

 degrees on the body, the same colours on the wings, with the 

 addition of the yellow stain called saffroning, and of the rich reds 

 found in some genera, are all cuticular pigments. Orange and 

 fulvous are formed, in part at least, by the same type of pigment. 

 Metallic colours, both on wings and body, are to be classed 

 under this heading. They are formed by an interference effect 

 superimposed on a cuticular pigmentation. The effect is produced 

 by a special arrangement of the pigment-granules, whereby the 

 light received becomes split up in a certain manner. All the 

 refracted rays except those of one colour become absorbed by 

 the pigment, while the excepted colour is reflected back, and 

 produces the well-known metallic effect. All these colours are 

 "permanent," in the sense explained above. The commonest 

 metallic colours in Odonata are emerald green, bronze-green, 

 copper, blue, violet and purple. 



2. Hypodermal or Subcuticular Pigmentation. These are 

 pigments formed either in or above the hypoderm cells themselves. 

 In the Odonata, the pigment is chiefly formed on the outer surface 

 of the cells, and may be extruded to form a definite layer beneath 

 the cuticle. Owing to their position, these pigments are all liable 

 to suffer from decay on the death of the insect. They are, however, 

 always beautifully preserved by immersion in alcohol. Rapid 

 drying may also be employed with more or less success for pre- 

 serving them. 



These colours are nearly all brilliant, viz. sky-blue, bright 

 red, bright orange, lemon yellow, pea-green, French grey, .and 

 other rich shades. 



3. Pruinescence or Exuded Pigmentation. This is a special 

 form of colouring connected, just as in the case of fruits, with 



