THE HORNY SKELETON. 227 



thence also that even perfect insects remain paler if they cannot, 

 immediately after quitting the pupa case, get into the light. From the 

 same cause the many pale yellow and particularly red-legged varieties 

 proceeds which we find in vast numbers of truly black or dark brown 

 insects. We must not, however, wholly attribute the darker colouring 

 solely to the effect of light ; the increase of the pigment during the 

 development contributes much to it; indeed in some, namely, such 

 insects whose legs remain of a bright red whilst the remainder of their 

 body is entirely coloured, it may be caused by the original deficiency 

 of the pigment. The effect, nevertheless, of the solar light is incon- 

 testable, particularly in the colouring of larvae, for they are always 

 variegated, when from the very commencement of their life they have 

 been exposed to the influence of light, as is the case, for example, in 

 the caterpillars of the Lepidoptera. Also, from variegated or coloured 

 larvae, beautiful insects appear to proceed, whereas, from dull-coloured 

 ones, or pale or brown, and more or less uniform coloured ones, brown 

 or black insects. But the influence of climate is great upon colour, 

 and, as is the case in birds, we find the most beautiful and gayest 

 colours in tropical climates, whereas, the farther they recede from the 

 equator, the darker or blacker they become. 



160. 



In structure, the horny case displays considerable conformity with 

 the skin in general, as it, like the latter, consists of three layers. 



The exterior and finest layer, the epidermis, is smooth, shining, and 

 without any traces of texture. It admits of being pretty easily 

 separated from the coloured mucous rete lying beneath it, particularly 

 in recently developed insects which have been preserved in spirits of 

 wine, and is, in the majority of cases, colourless, sometimes, too, even 

 brown, and but rarely black, if the mucous layer be black. Uncoloured, 

 as it is in general, it is transparent and perforated all over with small 

 holes, through which hairs rise when the surface is hirsute. 



Beneath this delicate epidermis we find the soft rete mucosum. 

 According to Straus it consists of two layers, of which the superior 

 smooth one is closely attached to the epidermis, and this alone appears 

 coloured. It is here we find the cause of the glittering, brilliant 

 colours with which many insects are so beautifully decorated. In the 

 butterflies and many others, namely, those with membranous wings, 



