1916] Thoracic and Cervical Sclerites of Insects 51 



In homologizing the wings in the different orders of insects, 

 by their venation, Comstock made use of the tracheae, which 

 occupy the position afterwards assumed by the principal veins 

 in the wings of generahzed insects. He termed the veins 

 costa, subcosta, radius, media, cubitus, and anals. This 

 terminology is quite widely accepted and generally used, 

 especially for Lepidoptera. 



Several theories have been proposed to account for the 

 origin of the wings. Gegenbauer, "70, claimed that the wings 

 developed from tracheal gills. Palmen, 77, discredited this 

 theory by demonstrating that tracheal gills occur on the 

 sternum, abdomen, tergum, pleuron and in the anal region; 

 that tracheal gills and the closed tracheal system is a secondary 

 adaptation to the aquatic life of the larva and that aerial res- 

 piration was probably the primitive condition. 



Plateau, 71, thought that the wings developed from hyper- 

 trophied spiracles. Miiller, 75, from a study of the develop- 

 ment of the wings of Calotermes, concluded that they arose as 

 lateral outgrowths of the dorsum. To this theor}^ Pancritius, 

 '84, adds the idea, that the primitive outgrowth of the body wall 

 may have developed into a protective body covering like an 

 elytron, which became modified to form the wings. Packard, 

 '98, accepted and developed Miiller's theory. He apparently 

 thought that primitive winged insects had lateral extensions 

 of the thoracic segments, which acted as a sort of parachute 

 and which later gave rise to true wings. Palaeontological 

 records show that some insects had lateral extensions of the 

 pronotum which may have served as a parachute and that many 

 of the earliest of the Pterygota have well developed wings, 

 which seem to have articulated with the thorax. Packard's 

 theory is plausible. 



Pleuron. The pleuron in a restricted sense consists of 

 the sclerites lying between the dorsum and sternum and forming 

 the lateral wall of any thoracic segment. In nearly all insects 

 it is composed of two sclerites, the episternum or anterior 

 sclerite and the epimeron or posterior sclerite (Fig. 8, esi, emi). 

 In the higher forms, the pleuron is usually connected with the 

 tergum by prolongations of the prescutum and postscutellum, 

 the latter often extending downward for some distance and 

 fusing with the pleuron, in which case it is frequently mistaken 



