F. W. Dry 07 



away from the fields in which they have spent the pupal stage. So one 

 would naturally expect more flies to move from rich to poor districts 

 than from poor to rich districts, though I have already pointed out 

 that any such migration which may take place does not occur to an 

 extent large enough to prevent there being marked differences between 

 the indices of different districts. A comparison of Charts 1 and 5 will 

 emphasize this point. 



If, however, it is the case that more flies move from the Holderness 

 Inland district to the Holderness Coast district, one conclusion seems to 

 follow. In not a single survey is the average percentage of attacked 

 plants in the Holderness Coast district as high as 1 %. From 1913 

 Brood I to 1914 Brood III the Garforth index has increased from 2 to 

 76, and the Midge has made considerable though less marked progress 

 in other districts, but in the Holderness Coast district it has made 

 absolutely no progress. I therefore suggest that were it not for the 

 continual immigration of flies the Midge would soon cease to exist in 

 this district. This may be the case too in the Flamborough district. 

 And probably the Filey and Bempton figures are kept higher by 

 immigration than they would be if those seaside districts were 

 separated from inland districts by an impassable barrier. And it 

 may be that the reason for the Little Weighton indices being lower 

 than those of the Bainton and Holderness Inland districts is that 

 those two districts receive more support from neighbouring earlier 

 sowing districts. 



One fact, though it scarcely requires census work to demonstrate 

 it, is brought out quite clearly by a comparison of the figures for the 

 different broods. One female fly contains about a hundred eggs, but 

 the charts show that the Midge made but slow progress both in 1913 and 

 1914. There must therefore be a great mortality at some stage or 

 stages of the life-history even in fine weather. This is doubtless due 

 to a variety of causes. 



In seaside districts, I believe, many flies are lost in the sea, but 

 inland probably a large number of flies fail to find swedes — perhaps 

 fail to find a mate — and die without leaving' offspring. 



There is probably a considerable mortality in the soil. In experi- 

 ments I have only reared about 30 % of the maggots put into soil, 

 even with plenty of moisture. And maggots die if they get too dry. 

 In one experiment 100 maggots were put into dry soil from the surface 

 of a garden bed and kept at a temperature at which maggots with 

 plenty of moisture will pupate successfully. In this experiment no flies 



Ann. Biol, n 7 



