THE DIPTERA. 41 



its ceaseless attacks ; it occurs almost everywhere, but is 

 most abundant in marshy situations. The larva (Fig. ia) 

 inhabits all stagnant waters, where it may be found very 

 abundantly throughout the summer, and when disturbed it 

 plunges about with great agility. Its food consists of the 

 numerous animalcule swarming in all still waters during 

 the greater portion of the year. These are captured by 

 means of two curious anterior appendages, which are 

 fringed with long hair, and pulled through the water like a 

 fisherman's net ; they are then withdrawn into the mouth 

 and the contents devoured, the hungry insect again ex- 

 tending them for a fresh supply. These larvae are generally 

 seen suspended from the surface of the water by the 

 curious air-tube which takes its rise from the penultimate 

 segment of the abdomen, which is of considerable length. 

 Its apex is armed with a row of stiff bristles, which 

 effectually prevent the water from entering the spiracle 

 there situated, so that the insect is enabled to respire when 

 hanging from the surface, independently of any muscular 

 action. It is also worthy of note that the intestine dis- 

 charges itself into this tube, an arrangement which does 

 not exist among the British species. After several moultings 

 the transformation to the pupa state takes place. At this 

 stage the insect(Fig. lb) becomes much thickened anteriorly, 

 this being the region of the head and thorax of the future 

 gnat ; all the limbs are easily detected on a close examina- 

 tion, as with lepidopterous pupae. The upper portion is 

 provided with two short appendages, fulfilling the same 

 function as the air-tube of the larva, and which constantly 

 support the pupa at the surface of the water. The terminal 

 fins enable it to dash through the water with great rapidity 

 when pursued by enemies ; at other times it remains per- 

 fectly motionless, suspended from the surface of the water. 

 It should be mentioned that none of these aquatic pupae 

 take any nourishment, neither have they any limbs properly 



