HOUSE-FLY AFT) OTHER TWO-WINGED INSECTS. 115 



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. 125, 

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

 pnparium) 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 become dry, when it takes to flight. It thus lives a 

 fortnight before acquiring wings, and may live a few weeks, 

 perhaps until frost; and in a few cases may pass the winter 

 within the house, and appear out of doors in the spring. 



There are probably 10,000 species of the order of Dip- 

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

 of them the preceding description will apply. Hence, by 

 studying thoroughly one fly, we can obtain a good idea of 

 the general structure of all. 



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- 

 mony between the different kinds and their surroundings, 

 and thus the order is rich in species and individuals. 



