210 NATURAL HISTORY OF AQUATIC INSECTS CH. 



A. Pharynx expanding. Hanging valve and fringes open. Water and food 



passing into upper chamber. 



B. Pharynx contracting. Hanging valve and fringes closed Water driven out, 



food retained in upper chamber. 



C. Pharynx contracting. Transverse fold closed down upon inferior pharyngeal 

 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. Between 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. 



