18 PROCEEDINGS OF THE ACADEMY OF [Jan.;; 



appear as lateral outgrowths of the caudal margin of the tergum. 

 Thisis well shown in the development of the male Blattid, in which the 

 elytron-like fore wings project from the posterior margin of the tergum, 

 becoming more and more elongate at each moult, and finally develop 

 into^chitinous structures in which the characteristic venation of the 

 wings^is clearly shown. This has led to the theory that the wings 

 arose as lateral outgrowths of the margin of the notum, originally 

 acting as a sort of parachute, but later developing into functional 

 wings. Another theory is that the wings and legs have a similar 

 origin. In a third theory, it is claimed that the wings develop from 

 tracheal gills; and in yet another, it is held that the wings are modi- 

 fied spiracles. It is not proposed to discuss these theories here at 

 length, but, in objection to Gegenbauer's, '78, tracheal-gill theory, 

 it may be remarked that Palmen, '77, has clearly demonstrated that 

 the closed tracheal system is only a secondary adaptation to the aquatic 

 life of the larva, and that aerial respiration was doubtless the primitive 

 one. On this account, it is hardly probable that wings have developed 

 from tracheal gills. 



Walton, '01, believes that the tegulse or pterygoda are rudimentary 

 wings, but, as has been previously discussed, there is absolutely no 

 proof for the statement that these structures are wing fundaments, 

 either from an embryological or a structural point of view. Comstock, 

 '95, suggested that "the wing covers or elytra of earwigs and beetles 



probably correspond to the tegulse that is, they are a pair of 



side pieces of the mesothorax, the parapleura, greatly enlarged." 

 Walton has followed out this suggestion in his theory, and likewise 

 adopts the view that the alulet-like structures under the elytra of 

 Hydrophilus, etc., represent extra wings. Comstock, '98, however, 

 has shown that the elytra are the modified wings, and that the mem- 

 branous structures beneath them are quite comparable to the alulae of 

 Diptera, etc., and are even bordered by the "spring vein" characteristic 

 of the alulae. 3 



In the most generalized insects the tracheation follows the path indi- 

 cated by the chief cuticular thickenings, which later become the veins 

 for stiffening the wings. The tracheation, therefore, is frequently of 

 great value in determining the homology of the principal wing veins, 

 and was much used by Comstock, '98, in the comparison of the 

 venation throughout the orders. The principal veins recognized by 

 him are the costa, subcosta, radius, media, cubitus, and the anals. 



3 See Sharp, '96. 



