ORDER DIPTERA. 123 



mouth-parts, as seen at F. In a day it grows too big for 

 its skin, which bursts and peels off; this is again repeated 

 a day later. The maggot thus sheds its skin twice, and 

 consequently there are three larval stages; the third stage 

 lasts three or four days. 



When about to transform into a pupa or chrysalis, the 

 body contracts into a barrel-shaped form, as seen in Fig. 

 142, D ; its skin turns brown and hard, forming a case 

 (called piipari'iu)}) within which the larva changes to a 

 chrysalis. Eemaining in this stage for about a week (five 

 to seven days), the fly is formed, and, pushing off one end 

 of its pupa-case, walks nervously about, until its soft, baggy 

 wings expand and become dry, when it takes to flight. It 

 thus lives a fortnight before acquiring wings, and, as a fly, 

 may live a few weeks, perhaps until frost; but in a few 

 cases may pass the winter within the house, or in protected 

 places outside, and appear out of doors in the spring. 



There are probably 10,000 species of the order of Diptera 

 in the United States alone, but to a large proportion of 

 them the preceding description will in general apply. 

 Hence, by studying thoroughly one fly, we can obtain a 

 good idea of the general structure of all. 



In certain flies (Blepharoceridae) there are two kinds of 

 females one kind with mouth-parts adapted for piercing 

 the skin of animals and sucking their blood, and the other 

 with mouth-parts like those of the male, the maxillge being 

 absent. In the females of other flies which suck blood, the 

 males feed on honey. 



In the flies, whose mouth-parts vary astonishingly in 

 structure, so that some are piercers and biters, and others 

 suckers and lappers, there is also a great variety of larval 

 forms, different modes of metamorphosis, and consequently 

 great powers of adaptation to different stations in life. A 

 few species live in the sea, many in fresh water, and many, 

 as the Tachina, are parasites in the bodies of caterpillars 

 and other insects. There is everywhere a wonderful har- 



