ITS OUTER SKELETON. 63 



The rudimentary wing of the female Cockroach illustrates 

 the homologv of the wings of Insects with the free edges of 



C-J / t* 



thoracic terga, and this correspondence is enforced by the study 

 of the development of the more complete wings and wing-covers 

 of the male. The hinder edges of the terga become produced 

 at the later moults preceding the completely winged stage, and 

 may even assume something of the shape and pattern of true 

 wings ; it is not, however, true, though more than once stated, 

 that winged nymphs are common. Adults with imperfectly 

 developed wings have been mistaken for such. 



Origin of Insect Wings. 



The structure of the wing testifies to its origin as a fold of 

 the chitinous integument. It is a double lamina, which often 

 encloses a visible space at its base. The nervures, with their 

 vessels and tracheal tubes, lie between the two layers, which, 

 except at the base, are in close contact. Oken termed the wings 

 of an Insect " aerial gills," and this rather fanciful designation 

 is in some degree justified by their resemblance to the tracheal 

 gills of such aquatic larvae as those of Ephemeridce, Perlidac, 

 Phryganidoc, &c. In the larva of Cldoeon (Ephemera) dipterum 

 (fig. 30), for example, the second thoracic segment carries a pair 

 of large expansions, which ultimately are replaced by organs 

 of aerial flight. The abdominal segments carry similarly 

 placed respiratory leaflets, the tracheal gills, which by their 

 vigorous flapping movements bring a rush of water against 

 their membranous and tracheated surfaces. 



Gegenbaur* has argued from the resemblance of these 

 appendages to wings, that the wing and the tracheal leaflet are 

 homologous parts, and this view has been accepted as probable 

 by so competent an observer as Sir John Lubbock.'f' 



The leaflets placed most advantageously for propulsion seem 

 to have become exclusively adapted to that end, while the 

 abdominal gills have retained their respiratory character. At 

 the time of change from aquatic to terrestrial life, which takes 

 place in many common Insects when the adult condition is 



* Gruudziige der Yergl. Anat. (Arthropoden, Athnmugsorgane. ) 

 f Origin and Metamorphoses of Insects, p. 73. 



