March, iqos] POWELL : WlNGS OF CERTAIN BEETLES. 11 



later develops. In most insects with an incomplete metamorphosis 

 they are so directly continuous with the tergum and become so solidly 

 chitinized with it that they have generally been interpreted as out- 

 growths from its caudo-lateral margin. ' ' Muller ( 1875 ) has snow tnat 

 in Calotermes the wings arise on the meso- and on the metathorax in 

 same position and cannot be distinguished in their early stages from 

 the prominent lateral fold that develops on the prothorax. 



In every insect, both in the Holometabola and in the Heterome- 

 tabola, in which the early stages of the wing development has been 

 investigated, it has been found that the wing arises in a homologous 

 position on the pleurum and that in the simplest types it develops as 

 a simple outpushing of the hypodermis. In fact, in T. plastographus 

 and in D. valens in their early stages the wing discs are exactly like 

 the leg discs and have a similar mass of tracheoles at their base. 

 While it is not doubted but that the legs have been developed from 

 folds or outpushings of the hypodermis, there is no more reason to 

 believe that the wings have developed from spiracles or in some other 

 circuitous manner than there is in the case of the legs. 



The third theory, which was advanced by Verson (1S90, 1894), 

 is that the wings develop from the discs of the degenerated spiracles 

 of the meso- and metathorax. This theory has been strongly supported 

 by Tower (1903). 



Wheeler (1889), working on a Chrysomelid beetle, Leptinotarsa 

 10-Iineata, observed that during the development of the embryo, 

 every segment of the thorax and abdomen develops a spiracular invag- 

 ination, that these invaginations send off branches some of which unite 

 to form the lateral tracheal trunk, that after the formation of the trunk 

 the prothoracic spiracle closes over and disappears, which is also the 

 case with the metathoracic spiracle, while the mesothoracic spiracle is 

 situated near the suture between the pro- and mesothorax "and in 

 later stages often has the appearance of belonging to the first segment." 

 These observations were confirmed by Tower, who makes the addi- 

 tional statement that in the migration cephalad of the mesothoracic 

 spiracle, "the spiracle alone migrates and the thickened area of the 

 hypodermis remains and probably becomes the fundament of the 

 elytron." He says that after the formation of the longitudinal tra- 

 cheal trunks "the openings in the meso- and metathorax are rapidly 

 cut off, leaving a disc-shaped mass of cells which have a somewhat 

 concentric arrangement. The further stages in the degeneration of 



