LARV« OF WATEE-FLIES. 263 



That which is taken for the head is really the head, thorax, 

 wings, and legs of the future Grnat, enclosed in a kind of 

 envelope. The creature undergoes another great change when 

 it leaves the state of the larva and enters that of the pupa, 

 besides the apparent enlargement of the head, namely, the 

 alteration of the position of the respiratory organs. The 

 larva, when it needed to take in oxygen, floated head down- 

 wards, with the top of its breathing-tube just above the sur- 

 face of the water; but as the pupa, on account of its large 

 head, would have no little difficulty in taking up such an 

 attitude, it is supplied with two hom-Uke members, which are 

 affixed to the hinder portion of that part which is called 

 ■the head, and which it can protrude above the surface of 

 the water. The pupa, therefore, when it breathes, floats 

 head uppermost. At the extremity of its abdomen there 

 are two leaf -like appendages. Its motion when swimming is 

 somewhat similar to that of the larva. It takes no food. As 

 the time for the final change draws near, it comes to the sur- 

 face of the water, and remains there perfectly still. Gradually 

 its body seems to get lighter and lighter, and to rise higher 

 and higher out of the water, until the skin between the respira- 

 tory organ splits ; and soon the perfect Gnat is free. The 

 imago uses its late pupal skin as a kind of raft during the 

 time it is drying its wings and preparing for flight. 



The aquatic life of the Gnat under favourable circumstances 

 lasts for about a month. The "egg-boats" of these insects 

 should be procured and placed in a suitable and portable 

 vessel, and then their very interesting life-history may be 

 observed from almost the beginning to nearly, if not quite, 

 the end. The Gnat, I think, will not lay her eggs and 

 build her boat while in confinement, but she may sometimes 

 be seen going through the curious operations upon the 

 surface of the water of some pond or tub. 



In England there are nine different species of Gnats, the 

 commonest of which are the House Gnat (Culex ciliaris) 

 (Fig. 161), the Wood Gnat (C. neTrwrosus), and the Ring-footed 

 Gnat {C. annulatus). The first and the last are those which 

 generally enter dwelling-houses and torment their inmates. 



