March, 1905.J Powell : Wings of Certain Beetles. 9 



ties worked on, but it seems certain that they do so at an early stage 

 in the development of the wing, probably at the moult preceding the 

 evagination of the wing, and their main purpose is to supply the disc 

 with an abundance of air at this important stage of its development. 



(b) The Trachece of the Wing. — Shortly before the beginning of 

 the prepupal period, several branches arise from the main trachea and 

 push into the wing, extending nearly to its margin. These are the 

 permanent wing tracheae, and they are accompanied into the wing by 

 the tracheoles, which begin to uncoil at this time. These tracheae 

 correspond in number and general position to the veins of the pupal 

 and adult wing, and their early development, taking place soon after 

 the evagination of the wing begins, is similar to what occurs in the 

 wings of Heterometabolous nymphs. According to Comstock and 

 Needham, ; ' in wings developing externally like those of a dragon-fly 

 one sees the principal tracheae passing very early out into the wing 

 bud, branching freely and forming by multitudinous terminal anasto- 

 moses a network of capillary tracheoles. In the beetle wing these fine 

 tracheae and tracheoles follow rather closely the course of the vein 

 cavities, and are most readily seen by an external examination of the 

 wing just after pupation. 



III. The Origin of the Wings. 



The question of the origin of the wings of insects is one over 

 which there has been much controversy. In all insects the wings first 

 become recognizable as slight thickenings of the hypodermis in the 

 pleural region near the place where the suture arises between the 

 dorsum and the pleurum, but whether they have been modified from 

 some other structure or have been developed as entirely new structures 

 is uncertain. 



There have been three theories advanced to account for the origin 

 of the wings of insects : 



1. That they have been developed from tracheal gills. 



2. That they have arisen as lateral outgrowths of the tergum or 

 pleurum of their respective segments. 



3. That they arise from degenerated spiracle discs of the meso- 

 and metathorax. 



The theory that the wings of insects have arisen from tracheal 

 gills was first advanced by Gegenbauer and adopted by Lubbock and 

 has since been advocated by Pratt (1897). This theory, of course, 



