HOUSE-FLY AND OTHER TWO-WINOEB 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. 135, 

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

 puparmm) within which the larva changes to a chrysalis. 

 Remaining in this stage for about a week (five to seven 

 days), the fly is formed, and, pushing ofE 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 wiU apply. Hence, by 

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

 the general structure of aU. 



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 consequentiy 

 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. 



LlTBEATUKE. 



Loew, H, and Osten Sacken, C. R. Monograph of the Diptera of 

 No. America, 1863-73. Smithsonian Inst. Also other essays by Loew. 



Osten Sacken, 0. R. von. Catalogue of the Described Diptera of 

 North America. Second edition. Washington, Smithsonian Inst., 

 1878. Also numerous papers and monographs published by Smith- 

 sonian Inst., Am. Ent. Soc, etc. 



WiUiston, S. W. Synopsis of the North American Syrphidse. Bull. 

 31, U. S. Nat. Mus.. Washington, 1886.— On the N. Am. Asilidse. 

 Trans. Am. Ent. Soc, xi, xii. 1884-5. 



Also articles by Bergenstanim and Loew, Bigot, Brauer, Burgess, 

 Pitch, Gerstaeker, Leach, McCloskie, Riley, Say, Shimer. Walsh, 

 Westwood, and Williston. 



