252 ANIMAL LIFE 



ascent being a passive movement. The significance 

 of this change in posture is seen when the fly emerges. 

 The crack through which this occurs lies on the 

 thorax, and if this were submerged the gnat would 

 drown. Hence the advantage of carrying the arched 

 thorax uppermost. The gnat at once gains the air, 

 straightens and dries its lancets, antenna;, legs, and 

 wings, fills its air-tubes, and flies away. 



The Mosquito. — The mosquito (Anopheles) passes 

 through a life-history similar in all essentials to that 

 of the common gnat. Its larva may, however, be 

 recognised by certain peculiarities, of which its habit 

 of lying horizontally instead of vertically is one of the 

 most obvious. The mature insect is found in most 

 English counties, but is most abundant along the 

 chief river-valleys and over the Fen district. This 

 distribution agrees closely with the occurrence of ague 

 in former times, and the spread of this fever is now 

 generally recognised as the work of the female mos- 

 quito. Allied forms are even more noxious in this 

 respect, and it is certain that malaria is caused by the 

 infection of germs of the disease introduced into the 

 blood during the act of sucking. 



The Harlequin Fly. — Of the vast number and 

 variety of midges and of gnat-like insects that succeed 

 one another with the course of the seasons, but few- 

 are adequately known. In a casual view they are 

 lumped together by their general superficial likeness, 

 and their assumed uniform taste for blood. The 

 professional naturalist splits them into a vast array 



