80 ZOOLOGY 



the group can usually be distinguished by having only two 

 wings, the posterior pair being rudimentarjr and transformed 

 into " balancers " that end in knolls. An account of the prin- 

 cipal families follows. 



The short-horned Diptera (Brachycera i) is the division to 

 which the house-fly, as well as various other common flies, 

 belong. House-flies " (Muscidae) are of several species. In 

 the commonest species the female lays one or two hundred eggs. 

 These, in warm weather, hatch in a few hours into larvae, 

 commonly called "maggots." The larvje are whoUy foot- 

 less, and even the head is only a slightly developed structure. 

 The larvae acquire full size in about a week ; pupate and hatch 

 about a week later. The process of pupation is a complicated 

 one, for all the larval organs, excepting certain ]3atches of 

 tissue, are destroyed. By the growth of these patches the 

 individual is formed anew. These changes are all deep Ipng, 

 and nothing seems more passive than the brown pupal case. 

 Finally the case breaks at one end, and the fully formed fly 

 emerges. The metamorphosis which the fly has just under- 

 gone is a complete one. 



The larvae as well as the adult flies breathe, like other insects, 

 by means of a system of air-tubes (traclieae) ^diich can be 

 seen liest in some aquatic larvae with the aid of a strong mag- 

 nifying-glass. The virtue of kerosene oil in Idlling larval flies 

 is due to the fact that larvie cannot breathe through the film 

 of oil. 



One of the most destructive members of the family Muscidae 

 is the tsetse-fly of tropical Africa (Fig. 84). The bite of this 



^ hrachijs, .short; kcra.^, a horn. 



2 It has been suKKcstcd that these should be called "typhoid flies" or 

 "septic flies," to make familiar the danger that lurks in them. 



