II FLIES WITH AQUATIC LARV^ 103 



phenomena. The surface-fihn is unable to penetrate 

 the fine spaces between the flaps for precisely the 

 same reason that it is unable to pass through the 

 meshes in a piece of gauze. 



After three or four moults the larva is ready for 

 pupation. By this time the organs of the future fly 



Fig. 24. — Pupa of Gii.at (Culex). Respiratory trutiipet of do. 



are almost completely formed, and the pupa assumes 

 a strange shape very unlike that of the larva. 



At the head-end is a great rounded mass which 

 incloses the wings and legs of the fly, beside the 

 compound eyes, the mouth-parts, and other organs 

 of the head. Each appendage has its own sheath 

 (part of the proper pupal skin), and the appendages 

 are cemented together by some substance which is 

 dissolved or softened by alcohol. At the tail-end is 



