300 INSECTA. 



force of the muscles which move them, that they seem never to be 

 wearied with flying. I have observed one of them (Anax imperator, 

 Leach) sailing for hours over a piece of water sometimes to and fro, 

 and sometimes wheeling from side to side, and all the while chasing, 

 capturing, and devouring the various insects that came athwart its 

 course, or driving away its competitors without ever seeming tired or 

 inclined to alight." 



(798.) In Hymenopterous insects (figs. 171 and 173) the wings are 

 much more feebly organized, but their structure is similar. The nervures, 

 or horny ribs supporting the membranous expansion, are comparatively 

 few ; and in the Diptera they are still less numerous. 



(799.) In several orders the anterior pair of wings are converted into 

 shields for the protection of the posterior ; such is the case in the Ortho- 

 ptera, many of the Hemiptera, and more especially in the Coleopterous 

 genera. In the latter, indeed, they are very dense and hard ; and, 

 being nearly unserviceable in flight, the hinder pair are necessarily de- 

 veloped to such a size as to present a very extensive surface (fig. 149, A), 

 and when in repose are closely folded up beneath the elytra, and thus 

 carefully preserved from injuries, to which they would be constantly 

 exposed without such provision for their security. 



(800.) The above observations relate only to the general disposition 

 and connexion of the different parts of the skeleton, and locomotive 

 appendages connected with it ; it remains for us now to speak more 

 fully of the texture of the external integument, and those modifications 

 which it presents, adapting it to various purposes. 



(801.) The hard covering of an insect, like the skin of vertebrate 

 animals, consists of three distinct layers. The outer stratum, or epi- 

 dermis, is smooth, horny, and generally colourless, so that it forms a 

 dense inorganic film spread over the whole surface of the body. Imme- 

 diately beneath the epidermis is a soft and delicate film, the rete mu- 

 cosum, 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 generally 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 here- 

 after, are imbedded and nourished. 



(802.) The wings are mere derivations from this common covering, 

 and are composed of two delicate films of the epidermis, stretched upon 

 a strong and netlike framework. Every membranous wing is, in fact, 

 a delicate bag formed by the epidermic layer of the integument, 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 between them, distends the organ until 



