14 THE BOOK OF THE FLY 



i.ivcn-fly, M. corvina. The back of the thorax of the 

 house-fly is marked sometimes distinctly, sometimes 

 indistinctly, with four dark lines on an ash-grey back- 

 ground ; the lesser house-fly has three faintly darkish 

 lines only. Quite a number of outdoor flies have 

 similar markings, but these often look like closely 

 adjacent or indistinct spots. The wing pattern is the 

 readiest guide for distinguishing the lesser house-fly, 

 both male and female. The males of the hairy (almost 

 bristly) raven-fly also indulge in the dancing habit, but 

 still more so do those of the latrine-fly, Fannia scalaris, 

 which may be distinguished by its uniformly ashy-grey 

 abdomen. 



These common co-inhabitants of our dwellings vary 

 in size according to their nourishment when in the larval 

 stage (maggots) ; after the perfect insect emerges from 

 the ptiparium, it swells out and fattens, but does not 

 grow in the real sense of the word. If 1000 house- 

 flies will weigh an ounce, then it may be calculated that 

 1600 average specimens of the other kind will likewise 

 weigh an ounce. 



In representing that the house-fly exceeds the lesser 

 house-fly in numbers in the proportion of twenty or 

 thirty to one, it must be borne in mind that the occur- 

 rence of the latter varies widely casually according to 

 the locality, and temporarily according to the time of 

 the year, being more commonly observed when and 

 where the other kind is scarce. 



The lesser house-fly has summer broods at longer 

 intervals than has the common house-fly. Towards 

 the end of the summer its last brood hibernates in the 

 puparium, and emerges as early as the end of March 



