II FLIES WITH AQUATIC LARV^ 209 



" The lower chamber communicates with the 

 mouth-opening by a short passage, which is imper- 

 fectly divided along its length by a fleshy, tongue-like 

 prominence, the tip of which is beset on its upper 

 surface with innumerable spines. Near the openings 

 of the two branches of the passage, the inner wall of 

 the mouth is armed with a pair of shell-like plates, 

 concave towards the mouth, and bearing upon their 

 concave surfaces numerous prominent ridges. 



*' The apparatus is employed in the following way. 

 The food of the larva consists of organic particles, 

 scraped from submerged objects by the booklets 

 around the mouth. When the larva is feeding, its 

 movements resemble those of a pig, working over a 

 heap of refuse with its snout. As soon as a quantity 

 of particles have been detached, the larva dilates the 

 upper chamber of its pharynx by means of the 

 attached muscles. Then the water, bearing the par- 

 ticles with it, rushes into the mouth, and passing 

 through the lower chamber, is sucked into the upper 

 chamber. The current, as it sweeps past, lifts the 

 fringes of barbules. Then the upper chamber con- 

 tracts, and the superfluous water escapes by the 

 mouth. In doing so, it closes the valve-like fringes. 

 The particles are left clinging to the barbules, and 

 only the filtered water passes out. When a sufficient 

 quantity of food has been collected, the passage into 

 the oesophagus is opened, and the whole mass is 

 swallowed. 



" The roughened surfaces of the shell-like plates 

 lie on either side of the tongue-like prominence 

 previously mentioned. A great multitude of cross- 



P. 



