26 FLIES FOUND IN HOUSES 



The interval between migration and pupation varies from two to four days or over. 

 "With individuals reared indoors this interval varies with the degree of moisture — 



extreme moisture retarding pupation, as also does extreme dryness Temperature 



probably also affects this stage." 



"The actual period of pupation is more constant — about eight days." " In the 

 region studied all the periods are generally quite regular, so that we may consider the 

 period of development from the egg to the imago as covering about fifteen days, varying 

 a day either way." 



" The emergence of the imagines from their pupal cases is interesting. With the 

 great blister-like frontal sac, not unlike a tiny balloon attached to their heads, the 

 case is burst and gradually the body is withdrawn much as a person might extricate 

 himself from a closely fitting tube. All the while the sand particles are thrown aside 

 by the rhythmically inflated sac. Slowly, pull after pull, the imago passes upwards 

 through the sand, and emerges at the surface. After a moment of rest, it starts for 

 the nearest grass stem ; up which it crawls in apparent haste and there it remains to 

 unfold its wings." 



Hermes also made a number of observations on larvae which fed either naturally 

 or under artificial conditions for varying times, and found that there was an optimum 

 period. " From this point either way the chances for pupation and emergence of 

 adults diminish, most rapidl}' of course, at the extremes. The pupa cases of optimum 

 forms and beyond are very chitinous, making a comparatively rigid shell, which affords 

 the optimum of protection. On the other hand, the further below the optimum, the 

 less rigid the case, until at the lowest extreme it is a mere flimsy covering. This 

 shows that here the least possible energy is expended, while the greatest amount 

 possible is stored up for the trying transformation from larva to imago." 



The optimum feeding period is 60 — 72 hours. After 54 hours' feeding "pupation 

 takes place readily and promptly and adults emerge in time, but are short of weight 

 and small in size.'' After 48 or 42 hours' feeding the adults are still smaller. With 

 36 hours' feeding adults were small, and many died before the wings expanded. 

 With' less than 36 hours' feeding adults could not be raised (see Fig. 12, p. 14). 



As yet no comparable observations have been made on the habits of these larvae 

 in places away from shores. 



This fly has been known to lay its eggs in neglected wounds 

 in human beings. Under these conditions extensive sores, with 

 great loss of tissue, may be caused by the larvse (see Chapter xxil). 



"Z. so'icata Mg., is the well-known 'Sheep maggot fly,' 

 which in summer months is often a pest to farmers and flock- 

 masters." _ 



Pollenia rudis Fabr. The cluster fly. 



This is a rather sluggish stoutly built fly of reddish grey 

 colour, a little larger than the house-fly, and often mistaken for 

 it. The male is distinctly smaller than the female. (PI. IV, 

 fig. 2.) 



