no NATURAL HISTORY OF AQUATIC INSECTS CH. 



A. — Pharynx expanding. Hanging valve and fringes open, 

 passing into upper chamber. 



Water and food 



B. — Pharynx contracting. Hanging valve and fringes closed 

 food retained in upper chamber. 



Water driven out, 



C. — Pharynx contracting. Transverse fold closed down upon inferior pharj'ngeal 

 arch (" stirrup"). Food and water forced into oesophagus. 



Fig. 77. — Pharynx of Eristalis larva. Three diagrammatic sections, illustrating the 

 action of the parts (J. J. Wilkinson). In C the " shell-like plates" are seen to 

 the right of the figure. Ketween and behind them is the "tongue-like 

 prominence (triturator)." The muscles which raise the roof of the upper 

 chamber are indicated in all the sections by oblique lines. 





