F. W. Dry 05 



strength of winds which would tend to blow midges into the sea and also 

 from inland to seaside districts were about equal with the frequency 

 and strength of winds which would tend to blow flies from seaside to 

 inland districts. In 1914 the seaward winds were about twice as 

 frequent and of somewhat greater force than those blowing off the sea. 



It has been objected that even if many flies were drowned in the 

 sea their places would be taken by flies which moved from inland 

 districts; such movement probably does take place. A glance at the 

 five charts will show, however, that since the time of the first brood of 

 1913, when the Midge was reduced almost to a uniform low level in the 

 East Riding and at Garforth, the tendency has been for differences 

 between the indices of the richer and the poorer districts to become 

 greater. This is in spite of whatever migration there may have been 

 from one district to another. 



(d) Seasonal variations in the abundance of the Swede Midge. 



The fluctuations in abundance of the Midge from the first brood of 

 1912 to the third brood of 1914 are shown in Table 1, and for 1913 and 

 1914 have been summarized on p. 88. 



The summer of 1911 was exceptionally hot; that of 1912 excep- 

 tionally wet and cold; that of 1913 was unusually dry with the tem- 

 perature about normal; and in the summer of 1914 the rainfall was 

 about normal and the temperature a little above the average. 



In the summer of 1911 the Midge increased enormously. The first 

 brood of 1912 was a very large one, but at the end of the summer maggots 

 were very scarce, and the first brood of 1913 was a very small one. The 

 Midge made slow progress in 1913, but rather greater progress in 1914. 



Doubtless there is an important relation between the weather and 

 the prosperity of the Midge. In Table 1 it will be seen that the progress 

 made by the Midge in a summer varies roughly with the average mean 

 air temperature or with the number of hours of bright sunshine. With 

 the rainfall figures there is no such clear parallel. Rain probably is 

 harmful to the Midge by washing eggs off the plants and by interfering 

 with flight and egg laying. In hot weather and in the warmer parts of 

 the day the flies are more active than when the temperature is lower. 

 In a hot summer such as 1911 the time taken by the life cycle is shorter 

 than in a cooler summer such as 1913. In some preliminary experiments 

 the time elapsing between the going down into the soil of maggots and 

 the emergence of the flies was found to be shorter at comparatively high 

 than at comparatively low temperature. But I do not think it profitable 

 to discuss further the effect of weather upon the Midge until I have 



