2i8 OUR HOUSEHOLD INSECTS 



is able to be shortened. This is done at the expense 

 of one of the two transformations. M. domestica, as 

 before mentioned, passes through three stages in its 

 larval life, while M. corvina has only two, the second 

 of the three being, in its case, omitted altogether. This 

 enables it to come to maturity sooner than its relative, 

 and hence gives it an advantage which counterbalances 

 its low degree of fecundity. In some such way as this, 

 Portchinski considers that more prolific flies have been 

 weeded out by less prolific ones from amongst the dung- 

 feeders, so that the majority are now of the less prolific 

 type. But M. domestica, with a degree of fecundity 

 which, though low as compared with the carrion-feeders, 

 is yet high for a dung-feeder, is apparently an exception 

 amongst the latter, and herein M. Portchinski finds the 

 explanation of its close association with mankind, the 

 bond of union being, in fact, in this particular species 

 probably closer than in any other, for the house fly 

 is said to be rarely found far from human dwellings. 

 According to the above theory, the house fly has 

 sought the protection and additional resources of man's 

 society to aid it in its struggles with less prolific insects, 

 which, by their shorter larval life, would otherwise have 

 hurried it out of existence. Whatever may be thought 

 of these speculations, and it would obviously not be 

 difficult to raise objections to them, still the observations 

 on which they are based have revealed some very curious 

 facts which require to be accounted for in some way or 

 other, and which invest with special interest the history 

 of the relations between insects and man. Farther 

 researches by the same investigator show that the 

 developmental history of an insect may depend very 

 much upon climate, the same kind of fly developing in 

 a different way in northern and in southern latitudes. 



