1NSECTA. 249 



epidermis is smooth, horny, and generally colourless, so that it 

 forms a dense inorganic film spread over the whole surface of the 

 body. Immediately beneath the epidermis is a soft and delicate 

 film, the rete mucosum, which is frequently painted with the most 

 lively hues, and gives the characteristic colouring to the species. The 

 third and principal layer is the true skin or cutis, which is gener- 

 ally of a leathery texture, and, especially in the elytra of beetles, 

 of considerable thickness : this layer is abundantly supplied with 

 nutritive juices, and in its substance the bulbs of hairs, scales, and 

 similar appendages, to be described hereafter, are embedded and 

 nourished. 



(290.) The wings are mere derivations from this common cover- 

 ing, and are composed of two delicate films of the epidennis, stretch- 

 ed upon a strong and net-like framework. Every membranous wing 

 is in fact a delicate bag formed by the epidermic layer of the in- 

 tegument, and in the recently developed insect can be distinctly 

 proved to be such, by simply immersing the newly escaped imago 

 in spirit of wine, which gradually insinuates itself between the 

 still fresh and soft membranes ; and, filling the cavity enclosed be- 

 tween them, distends the organ until it represents a transparent 

 sacculus in which the ribs or nervures of the wing are enclosed.* 

 This structure, however, is only to be displayed while the wings, 

 after being withdrawn from the pupa-case, are still soft and moist, 

 for they soon become so intimately united with the horny frame- 

 work upon which they are extended, that they seem to form a 

 single membranous expansion. 



The, ribs or nervures, whereby the two plates of the wing are 

 thus supported, are slender hollow tubes, filled with a soft paren- 

 chyma, in the interior of some Burmeister detected an air-vessel 

 recognisable by the texture of its walls, and a minute nervous 

 filament. 



(291.) We have still, in order to complete our descriptioirof the 

 external anatomy of an insect, to describe certain appendages which 

 not unfrequently clothe the exterior of the skeleton, and exhibit 

 great diversity of appearance in different tribes. These may be 

 divided into spines, hairs, and scales ; and, however much they 

 may appear to be distinct structures, all these are essentially very 

 nearly related to each other. 



The spines are horny processes developed from the epidermis ; 

 and sometimes, especially in the Coleopterous order, as in some 



* Heusingcr, System der Hystologie, 2 Heft. Burraeister, op. cit. p. 224. 



