VI DIPTERA 253 



run parallel to the tip of the wing, whereas in Musca they converge so as 

 nearly to meet. These flies are both very common, but they are probably 

 of less importance from the health point of view as they are less apt 

 to interfere with human food. The larvae, recognizable by the pointed 

 projections from the sides of the body, are occasionally found as parasites 

 in the intestine of man. 



The Blue-bottles or Blow- flies {Calliphora) and the Green-bottle 

 {Lucilia), which normally deposit their eggs on dead meat and fish, are 

 well-known flies, and both are liable on occasion to lay their eggs in 

 uncovered wounds. 



Among the Muscidae there are also included a number of blood- 

 sucking flies. The common biting Stable-fly, Stomoxys, is usually mis- 

 taken for a House-fly, which it closely resembles in general appearance, 

 apart from the straight pin-like proboscis projecting forwards from its 

 head. Closely allied to Stomoxys is the African genus Glossina (Tsetse), 

 recognizable by the wings when at rest lying flat one over the other and 

 projecting back beyond the tip of the abdomen. As already indicated, 

 the various species of Glossina are of great practical importance from 

 their acting as intermediate hosts for disease-producing trypanosomes. 

 The Glossinas are readily attracted by moving bodies ; they are active 

 during the day ; and like mosquitos they avoid white surfaces — so that 

 white garments are desirable in Tsetse-infested districts. The egg develops 

 within the uterus of the parent and there is eventually brought forth a 

 fully developed larva which is deposited on loose soil into which it at 

 once burrows and pupates. Deep shade and a certain amount of moisture 

 are essential to the well-being of the Glossinas, and consequently something 

 can be done towards keeping down their numbers in the neighbourhood 

 of settlements by clearing the zone along the margins of lakes and rivers 

 of trees and brushwood. 



The Oestridae are remarkable from the fact that the larvae are 

 parasitic on vertebrates : the adult fly as a rule does not feed, the mouth 

 parts being reduced to functionless vestiges. Gastrophilus deposits its 

 eggs on the fore-parts of horses, and when licked off and swallowed they 

 develop into " bots," peculiar larvae surrounded by rings of hooks which 

 hang on to the lining of the stomach, often in great numbers. Hypoderma 

 similarly deposits its eggs on the body of cattle, but in this case the 

 larva (" warble ") takes up its position under the skin of the back. 

 These warbles when numerous do much damage to hides by the perfora- 

 tions through which they make their way out. Occasionally they occur 

 in man, and in South America they are in places regarded popularly as 

 the larvae of a large moth ! 



