THE THORACIC EXO-SKELETON. i6i 



the dorsal gills of the annelids, so far as their position is 

 concerned. It appears probable, therefore, that the wings are 

 a modification of the respiratory organs of some ancestral 

 form. 



Although our knowledge of the more generalised Insecta 

 goes far to render it certain that the earliest forms of insect 

 life were entirely terrestrial, and that the aquatic habits, like 

 the vermiform condition of the larvse, are acquired, or inter- 

 calated modifications, the conditions observed in the Termites, 

 and the existence of temporary tracheal brushes in the rudi- 

 ments of the wings of the fly-nymph, show that the terrestrial 

 habits of the earliest and most generalised insects cannot be 

 used as arguments against the view first enunciated by Gegen- 

 baur,* that the wings of insects are modified gills which have 

 entirely lost their respiratory, and have assumed a new function, 

 that of aerial locomotion. 



Development of the Wings. — The wings first appear as papillae 

 of the epiblast, which soon exhibits a cavity filled with stellate 

 mesoblast and blood. This sac-like wing becomes broad and 

 flattened, so that it presents an upper and a lower wall, and a 

 tuft of nearly parallel and very numerous tracheal vessels is 

 developed in the wing cavity. The walls of the wing sac 

 become plicated in fan-like folds radiating from its attachment 

 to the thorax. The angles of the folds become thickened, and 

 form the primary nervures. 



Achitinous epidermis is deposited on the surface of the wing 

 sac. In many nymphs this attains considerable thickness, and 

 appears externally when the larval integument, under which 

 the wing is formed, is shed. 



The first cuticular layer does not persist in the imago, but is 

 shed after a second cuticular layer — the permanent cuticle 

 of the wing which becomes the wing membrane — is deposited. 

 The nervures are developed on the convexities of the primary 

 folds, and in the corresponding grooves of the opposite surface 

 of the wing sac. 



The tracheal vessels of the wing remain only in the young 



* ' Grundziige der Vergl. Anat.' 



