II 



FLIES WITH AQUATIC LARVAE 



143 



D 2 " 3 very early assume the appearance of undeveloped 

 wings, being broad, thin sheets, inclosed in folds of 

 similar shape. V 1 " 3 soon take the form of long, 

 cylindrical processes, which in like manner, are in- 

 closed in tight-fitting cases. The case or sheath, in 

 both the dorsal and the ventral series, is merely the 

 outer part of the same fold which gives rise to the 

 appendage itself. Suppose the sleeve of a coat to be 



FIG. 42. i, Diagram to illustrate the formation of a new appendage : 2, one of the 

 developing legs of the Crane Fly (Tipula) in section. 



partly telescoped or withdrawn into the upper part. 

 We shall then get an arrangement like that repre- 

 sented in the figure. A will be the shoulder of the 

 sleeve, C the wrist end, and B the part which comes 

 immediately in contact with C, separating it from A. 

 This is not unlike the folded-in and developing 

 thoracic appendage. A is a part of the new cuticle 

 which will help to form the wall of the thorax, C is 



