Mar. I900.] MerCER : DEVELOPMENT OF WinQS IN LfPIDOPTERA. 15 



temporary system extends in straight lines from the axial region and 

 follows the course of the tracheae TFig- 26. //) practically parallel 

 with them and functions only during the larval life. 



The C/nvii^i's in tlie Structure of the Trachecc. — In the very young 

 pupa the tracheae (Fig. 30, /) retain the characteristics of the tracheae 

 of the ])receding stage except that the cells of the epithelial layer are 

 not so distinct. This indistinctness increases with the age of the 

 pupa until in the old pupa (Fig. 35, T) the walls between the cells 

 have disappeared. At this time the epithelium appears under low 

 powers to be thrown into folds ; under high powers its degeneration 

 is revealed. The nuclei are still (piite distinct ; but the cell body is 

 becoming semi-transparent except in a branched central area where 

 the i)rotoplasm is of greater relative density. In this stage of the de- 

 generation of the trachea the intima appears as a thin membranous 

 structure in the lumen. Sometimes it appears merely as a line, at 

 other times as a loop in a great variety of forms (PI. ^', P'ig. 38, ///). 



In a later stage the inner boundary of the cells is broken down and 

 the cell l)ody is evidently disappearing. The central branched part 

 of the cell body is the last to go (PI. V, Fig. 39) and projects for a 

 time into the lumen of the trachea. 



A degenerating trachea in this stage is figured by Schiiffer (his 

 figure 37) and described as Semper's Fliigelrippen (p. 646). He re- 

 gards the branched remains of the cell body described above as tree- 

 like processes of an intima, and the remains of the intima he believes 

 to be a secretioai from the walls of the tube. 



Semper's original description of the so-called wing-ribs ('57, Taf. 

 XV, Fig. 6) was evidently based on an examination of a trachea 

 which had just begun to degenerate. The intima had separated from 

 the ei)ithelium and was mistaken for a central nerve, but the epithelial 

 cells were still distinct and of the usual form. 



This mistake of Semper has found its way into a large part of the 

 literature on this subject and has been the cause of much confusion. 



It is evident that this degeneration of the wing tracheae finally be- 

 comes complete ; for in the wing-veins of the adult the trachea; are 

 lacking. 



The Ch(i/i}(es in the Hypoderniis and tlie Basement Alcmbrane. — Our 

 knowledge of the changes that take place in the course of the develop- 

 ment of a wing has been greatly confused by what are evidently mis- 

 conceptions regarding the nature of the membrane limiting the inner 



