i6o THE INTEGUMENTAL SKELETON OF THE IMAGO. 



characteristic of the imago in the Insecta, but are confined to 

 the meso- and metathoracic somites. 



The wings are sac-Hke prolongations of the syndesmotic 

 integument between the dorsum and the pleuron ; each wing, 

 therefore, exhibits two layers of thin integument one above the 

 other. These are closely united in the mature adult, but in 

 the immature imago or fully - developed nymph they are 

 separated by a layer of spongy cellular tissue, of a reticular 

 character, the spaces of which are blood sinuses continuous 

 with those of the thoracic cavity. In the nymph this tissue is 

 permeated by a brush of dichotomous sub-parallel tracheal 

 capillaries. 



The upper and lower layers of integument are termed the 

 upper and lower wing membranes. Diagrammatic repre- 

 sentations of the wing-bearing somites are given in most works 

 on elementary comparative anatomy. I shall not, therefore, 

 reproduce them here. 



The Homology of the Wings. — The wings are developed from 

 the edges, epipleura, of the dorsal plates of the meso- and meta- 

 thorax. 



The edge of the dorsal plate of the prothorax closely 

 resembles the epipleuron of the Crustacea, but never attains the 

 characters of a wing. In the Cockroach larva both the meso- 

 and metathorax have precisely similar edges to the dorsal 

 plates ; within these the wings are developed. The rudi- 

 mentary elytra of the female Cockroach are manifestly a 

 modification of the edge of the tergum of the mesothorax. 



In many aquatic larvae leaf-like appendages occur on both 

 the thoracic and abdominal dorsal plates, in the position of 

 wings. These leaf-like appendages contain tufts of tracheae, 

 and have the function of gills. Graber says: In the young 

 larvae of the Termites, which live in damp places, similar 

 tracheal gills occur on the thoracic and abdominal dorsal 

 plates, and the development of the wings is effected by the 

 enlargement and modification of those which belong to the 

 meso- and metathorax [10, Bd. i, p. igo] . 



The tracheal gills of insects are, moreover, very similar to 



k 



