EXTERNAL ANATOMY OF INSECTS. 401 



1. With respect to the Jirst of these circumstances, the 

 layers of which the integument consists, it seems to ex- 

 hibit some, although not an exact, analogy with the skin, 

 rather than the skeleton, of the vertebrate animals a . In 

 these last, the skin is stated to consist of four layers. 

 Of these the exterior one is the epidermis, or scarf-skin : 

 under this is the rcte mucosum, or mucous tissue, which 

 gives its colour to the skin ; next follows the papillary 

 tissue formed by the extremities of the nerves, and in 

 which the sense of touch principally resides ; the last and 

 innermost layer is the skin proper, or leather, called Der- 

 mis, Derma, or Corium b . Two of these layers M. Cu- 

 vier assigns to insects. They have, he observes, in every 

 state, a true epidermis* ; and in their state of larva he 

 finds that the infinite variety of colours that so adorn 

 many of them is produced by a mucous substance ob- 

 servable between the epidermis and the muscles d : this 

 seems analogous to the rete mucosum. To this, dried 

 and mixed with their horny substance, he attributes also 

 the colours of the perfect insect : " for," says he, " when 

 the Lepidoptera are in the chrysalis, the little coloured 

 scales which are to ornament their wings, are then in a 

 state of mucosity similar to that which is found under the 

 skin of the caterpillar. The colours of the Arachnida," 

 he goes on, " are also due to this mucosity : it is disco- 

 verable under the skin, and has the appearance of mi- 

 nute glandular points of which the shades vary consider- 

 ably. But in the Coleoptera, and many other Orders, the 



a Anat. Compar. i. 119. b Ibid. ii. 540. 



c Ibid. .547. d Ibid. 553. 



VOL. III. 2d 



