II 



FLIES WITH AQUATIC LARVAE 



117 



FIG. 33. One element 

 of fin of larva of 

 Corethra. 



but are prehensile, and play a great part in the 

 capture of the prey. Each consists of a simple basal 

 joint, which ends in four or five 

 long curved bristles. When at 

 rest, these bristles are directed 

 downwards and backwards, and are 

 flexed upon the basal joint. A 

 special muscle serves for the ex- 

 tension of the antenna, which is 

 brought back to its ordinary rest- 

 ing position by the elasticity of the 

 parts. The labrum is long and 

 narrow, and assists in the capture 

 of the prey. The mandibles re- 

 semble hands in shape, and bear 

 four or five long curved finger-like 

 teeth. The maxillae are mere stumps, and the labium 

 is reduced to a small triangular plate. Any small 

 animals captured by the antennae are crushed by the 

 mandibles and pressed into the mouth, but they are 

 not swallowed, as we might naturally expect. The 

 back of the mouth is closed by a circular fringe of 

 stout bristles arranged like a lobster-pot, or what 

 is in some parts of the country called a weel for 

 catching fish. The body of the victim is thus im- 

 prisoned and acted upon by the salivary secretion, 

 so that only the fluid products of digestion enter 

 the stomach. If the larva is gently pressed between 

 two glass slips, the pharyngeal tube can be everted, 

 and by a more natural process of eversion the in- 

 digestible parts of the food are voluntarily expelled 

 from the mouth of the living Insect. A somewhat 



