126 INJURIOUS AND USEFUL INSECTS 



head, which is due to the play of light upon a multitude of 

 hairs, all busily sweeping into the mouth the microscopic 

 particles upon which the larva subsists. There are no limbs, 

 but the last segment bears a simple fin formed of a row of 

 bristles, and the last but one sends out a breathing-tube, 

 by which air is taken into the body. In swimming, the 

 regular tufts of bristles which stand out from the sides of 

 the body are, no doubt, of use. The larva propels itself by 

 striking the water suddenly with the hinder part of its body, 

 and can travel pretty fast with a jerking movement. It can 

 remain floating at the surface as long as it pleases, feeding 

 with its fringed mouth, and breathing through its air-tube. 

 This is a little singular, for the body is heavier than water. 

 When a larva, detached from the surface, ceases to struggle, 

 it sinks slowly. It maintains itself at the top of the water 

 by taking advantage of the surface-film which overspreads 

 every free surface of a liquid. By means of this same surface- 

 film a steel needle can be made to swim on water, and the 

 gnat-larva is supported without effort by the pull of the sur- 

 face-film upon the pointed valves which open and close its 

 air-tube. 



The larva, when full-grown, is about half-an-inch long in 

 certain common species. The various parts of the fly are 

 already in an advanced stage of development. As soon as 

 the last larval skin is cast, the new organs rapidly take some- 

 thing like their ultimate form and place, and in a minute 

 or so the creature is transformed into a pupa, strikingly 

 unlike the larva, both in form and habits. As in the harlequin- 

 fly, the pupa is practically a fully formed fly, shrouded for 

 a time in a close-fitting, transparent skin, the pupa-skin. 

 Every limb and appendage of the fly has its own pupal sheath. 

 At the hinder end of the body is a broad tail-fin, formed of 

 a pair of flaps, and this constitutes a very effective paddle. 

 The pupa is buoyant, its air-tubes being distended with air; 

 when at rest it floats at the surface of the water, sinking only 

 when alarmed, and with considerable effort. A pair of 

 trumpets behind the head, shaped like cows' horns, but 

 attached by their points instead of by their broad ends, now 

 serve for taking in air ; their position is so accurately adjusted, 

 that whenever the pupa comes to rest, the broad open ends 

 of the trumpet lie flush with the surface of the water. After 



