12 FLIES FOUND IN HOUSES 



tubular ovipositor, which, in repose, is telescoped within the 

 abdomen." The chitinous portions of the male armature are of 

 great interest to the anatomist, and are of considerable im- 

 portance in the classification of some groups of flies, but are 

 too complex for consideration here. 



Reproduction. 



Most flies undergo a complete metamorphosis in which there 

 are four well marked stages. These stages in the life-cycle are : 



1. The egg. 



2. The larva or maggot stage. 



3. The pupa or chrysalis stage. 



4. The imago or perfect fly. 



"Most flies lay eggs, which are usuall}' deposited, in a manner 

 which simulates conscious foresight, in a medium or in a 

 pabulum suitable to the future larva or maggot. The eggs are 

 large and often sticky so as to adhere in masses. Some flies 

 such as the flesh flies {Sarcophaga) give birth to small living 

 larvae" (Alcock, 191 1, p. 41). 



The larvce of the principal group of flies under discussion, 

 which hatch out from the eggs, are segmented worm-like 

 creatures with small heads and without obvious appendages 

 except chitinous mouth hooks. They move by means of 

 locomotory pads placed beneath the posterior segments. During 

 their growth they shed their skins or moult, like the caterpillars 

 of butterflies, on several (three in house-flies) occasions. When 

 full-fed the mature larva usually crawls to some dry place and 

 rests for a short time preparatory to changing into the pupal or 

 chrysalis state. Most of these larVcC feed on various decaying 

 materials and, probably in consequence of this fact, their stud)^ 

 has been to some extent neglected. Recent observations have 

 shown that some of them are affected by the lieat produced by 

 the fermentation of the substance in which the}' are living, 

 others by light, and others b\' the condition of the food suppl)\ 



