II FLIES WITH AQUATIC LARVAE 109 



fingers would be used to support a needle or other 

 slender instrument which we wished to pass into a firm 

 object, such as a piece of leather. As the piercing 

 instruments penetrate further and further, the labium 

 is bent backwards near the middle of its length, so as 



o 



to shorten it without withdrawing its support from 

 the piercing parts. When these preparations are 

 completed, the Gnat is in a position to draw the 

 blood into its mouth. One part of the oesophagus is 

 dilated, and forms a sucking-bulb ; in front of this is 

 a minute valve which apparently prevents the escape 

 of blood from the mouth. The momentary enlarge- 

 ment of the sucking-bulb reduces the pressure within, 

 and the blood flows up along the front split tube. 

 In the male Gnat these parts are a good deal simpli- 

 fied. The saw-teeth of the maxillae, the unpaired 

 lancet, and the sucking-bulb are all undeveloped, 

 and it is probable that the male Gnat, if it feeds 

 at all, obtains its food in a quite different manner 

 from the female. Some writers, however, maintain 

 that the male Gnat can bite. Why the female Gnat 

 should draw blood at all is a difficult question. It has 

 been surmised that a supply of highly nutritive fluid 

 is necessary for the formation of the numerous eggs, 

 but the force of this, as of every explanation which 

 has hitherto been suggested, is a good deal weakened 

 by the circumstance that Gnats abound in certain 

 regions where no quadrupeds are found, and very 

 few animals worth biting. They have been found in 

 immense numbers in uninhabited parts of Labrador, 

 on the tundras of Siberia and the desolate Kerguelen 

 Island. " Of the millions of mosquitoes which in the 



