HOUSE FLIES AND BLUEBOTTLES 215 



matters, and dung as well. Even if half-starved, it will 

 still undergo its metamorphoses, though, of course, the 

 perfect insects will be dwarfed. Like several others, 

 it can even withstand the action of the digestive fluids 

 of the stomach and intestine of living vertebrate animals. 

 Bernard introduced it artificially into the stomach of a 

 dog, but it passed along the intestine and was voided 

 in the usual way alive. Portchinski's similar experiment 

 with a frog had the same result. In the case of a little 

 song-bird, however, the larva was dead when voided, 

 but still undigested. That so common and so hardy a 

 European fly should be one of the most likely to follow 

 man's lead and migrate with him to other parts of the 

 world, would be only what was to have been expected ; 

 and yet, though the five flies mentioned at the com- 

 mencement of the last chapter, together with Cyrtoneura 

 stabulans, are as common in the United States as in 

 Europe, though not indigenous there, S. carnaria has, 

 according to Osten-Sacken, not yet been introduced into 

 America, so that four centuries of European communi- 

 cation with the New World have not sufficed to import 

 this abundant but independent species. 



The curious observations of Portchinski have an im- 

 portant bearing on the subject, though perhaps they 

 will hardly justify the conclusions he has drawn from 

 them. He finds that carrion-feeding flies are, as a 

 group, enormously prolific, while dung-feeding species 

 are much less so : for example, Calliphora, a carrion- 

 feeder, lays from 300 to 600 eggs; while Musca domestica, 

 a dung-feeder, lays only about 120 (Fig. 70). These 

 differences, he argues, are connected with the different 

 conditions, as regards the struggle for existence, under 

 which the contrasted species live. There are, accord- 

 ing to him, comparatively few species of carrion -feeding 



