43 



or pigment-factor soon becomes ochre-yellow in tint, and having 

 remained in this stage from twenty-four to thirty hours, the mature 

 colours begin to show themselves. 



The scales of insects, which vary much in shape, are hollow and 

 loosely attached to the surface of the body or wing by a short, slender 

 pedicel which fits into a minute, close-fitting socket that perforates 

 the wing membrane. Their surfaces have frequently an elaborate 

 series of longitudinal ridges, marked with striae, and with transverse 

 striae between them. The primary use of scales is undoubtedly to 

 protect the body, but the secondary uses have become as important 

 as, or perhaps more important than this. The two most important 

 of the secondary uses to which the scales have been put are — (i) the 

 formation of the colour patterns by which the insect is protected, 

 either by the similarity these bear to the environment of the insect 

 or by their startling nature, so that when suddenly exhibited to a 

 would-be enemy the latter is sufficiently taken aback to give the 

 insect a chance of escape ; (2) by their modification into androconia 

 or scent-glands in certain male insects, thus forming an important 

 secondary sexual character, and increasing the chance of procreation 

 in the species. 



We may now consider very briefly the nature and uses of the 

 colours of insects, and may at once state that these arise in one of 

 three ways : (1) by interference, (2) by diffraction, (3) by the presence 

 of pigment within the scales. The physical structure of the body- 

 surface, wing-surface, or scale-surface may give rise to interference 

 or diffraction colours, or both. We have seen that the body- 

 surface, wing-surface, and scale-surface of insects is composed of 

 chitin, and Schneider tells us that the latter consists of a number of 

 irregular laminae of exceeding thinness, and as these layers are super- 

 imposed upon each other, we have at once the layer of thin 

 transparent plates necessary to produce interference colours. We 

 have also seen that the chitinous surface of the body or wing or 

 scales is frequently covered with exceedingly fine ridges and striae. 

 Kellogg says that there are some 500 striations to the millimetre on 

 the scales of the imagines of Alicropteryx (one species of which, of a 

 metallic green colour, is abundant in the month of May in the flowers 

 of buttercups), and as many as 1400 striae to the millimetre on the 

 scales, of the fine tropical butterflies belonging to the genus Morpho. 

 The continually changing colours of many insects, as the position of 

 the insects is made to vary, are due to diffraction. 



The third way in which the colours of insects may be developed is 

 due to pigment deposited in the cells of the cuticula or on the inner 

 surface of the scales. Mayer has shown that when the protoplasm is 

 withdrawn from the scales of Lepidoptera at the time of their forma- 

 tion a secretion of haemolymph enters, and deposits on the inner 

 surface of the scales a material which varies in different species and 

 in different parts of the same wing, and has the power of absorbing 

 certain rays of light and reflecting others, and thus giving us the 



