Changes in the Alimentary Canal 147 



bably casting-liairs, and facilitate the separation of the 

 pupa from the larval skin, within which it was formed. 

 "We believe that the median line marks the place where 

 the pupal thorax splits to allow of the escape of the fly, 

 and that the lateral lines constitute, so to speak, the 

 hinges on which a pair of flaps bend downwards and 



outwards, to enlarge the 

 opening. These lines of 

 weakness, prepared long 

 in advance, facilitate the 

 escape of the fly, and help 

 to explain the wonderful 

 speed with which it is 

 accomplished (see p. 6). 



The mode of life of the 

 pupa and the extrication 

 of the fly have already 

 been described (p. 8). 



Shortly before pupation changes 

 the alimentary canal un- alimentary 



T 111 canal, 



dergoes marked changes. 

 The epithelium of the 



Fig 112. — Dorsal surface of pupal gtomach bcCOmeS Stripped, 

 thorax and anterior abdominal segments. . , . , 



The tracheal gills and wings are seen on and large maSSCS, lU wllich 

 the sides. Between the tracheal gills a 



median and two curved lateral lines shruuk nuclei are Stlll ap- 

 appear, which in the fresh pupa are ■ t • xi -j. 



white upon a dark ground. The median parent, lie lU tUC CaVlty 

 line indicates the cleft by which the fly r. \ j-T, * A^r^r. ^r^\- 



will escape ; along the lateral lines the (tig. IlSJi tUlS QOCS UOt 



pupal integLiment is thin and flexible, , , -.-.l^^p „„„„„^},p,,p ^^-,A 



and bends downwards and outwards to take piaCC every WnClC ailQ 



enlarge the cleft. ^^ ^^^^^ ^^ ^^ beginning 



and end of the stomach the epithelium persists for 

 a time, but in a shrank and probably functionless con- 

 dition. The stomach of a late pupa or a fly contains 

 hardly a trace of epithelium ; its wall consists merely 

 of muscular tissue and basement-membrane. A similar 

 stripping of the epithelium of the mesenteron has been 



L 2 



