HOUSE FLIES AND BLUEBOTTLES 217 



flesh-fly, was abundant in spring; but Calliphora vomi- 

 toria, a bluebottle, did not begin to appear till about 

 the beginning of June, when the former species had 

 begun to decline. Later on, the proportions were re- 

 versed, the bluebottle being in excess, and the flesh-fly 

 scarce. Thus everything favours the fecundity of the 

 carrion-feeders. 



But with the dung-feeders the case is different. Here 

 there is much competition, there being large numbers of 

 dung-feeding flies, as well as beetles, which latter do not 

 feed on the former. The supply 

 of food, too, Portchinski re- 

 gards, curiously enough, as less 

 abundant. These are circum- 

 stances which place fecundity 

 at a disadvantage, and hence 

 have sprung the more moderate 

 powers of multiplication pos- 

 sessed by the dung-feeding flies. 

 But here, again, a very curious FIG. 7 i. Distribution of Colour 



e * _ on Abdomen of Muscat, cor- 



circumstance has arisen, which vina. The clear parts are 



yellow, the shaded parts black. 



gives an extra advantage to the 



less prolific species. There is a fly which is structurally 

 very like M. domestica, differing chiefly in the brighter 

 colour of its abdomen (Fig. 71), which is of some tint 

 of yellow or brown. Its name is Musca corvina, and 

 it frequently hibernates in houses, and may therefore 

 be reckoned amongst the household species. Notwith- 

 standing the close resemblance between these two 

 species in their perfect condition, as well as in that 

 of the full-grown larvae, there is a most extraordinary 

 difference in the circumstances of their development. 

 While the house fly lays 120 eggs, M. corvina lays only 

 24 ; but they are much larger, and hence the larval life 



