Norman Cunliffe 113 



more active in warm, sunny and calm weather, but under adverse 

 conditions they seek shelter in the areas protected by the plant 

 foUage, in which areas their activity would presumably be influenced 

 only by the maximum shade temperature. When sweepings are taken 

 on successive days at a definite hour, it is considered that variations in 

 the records, due to fluctuations in the maximum temperatures in the 

 sun, are to a great extent eliminated, especially when the results are 

 meaned as explained above. 



Charts I and II show a general prevalence of the fly in the field 

 throughout the warm season together with certain periods of high pre- 

 valence^. The intervening minima will be real if it can be shown that 

 the absence of flies from the field is not falsely represented owing to 

 the weather conditions being unfavourable for collection. 



Considering the minima in succession, first on Chart I and then on 

 Chart II, it will be seen that the weather conditions in the first week of 

 August, 1919, were favourable for collection, the velocity of the wind 

 being about normal {vide Table I), rain absent and temperature on the 

 up-grade. One cannot determine the reality of the first two minima in 

 Chart II, as the crops, at this time of the year, show very little growth 

 and collection by sweeping may not be giving very accurate data. About 

 June 8th, however, conditions were good although few flies were caught. 

 On July 7th to 12th conditions were bad, the weather generally being 

 Avet, cold, and windy. Yet on July 14th, under conditions much the 

 same as those of July 16th, only one- third of the number of flies was 

 collected. The weather on July 27th is recorded as being sunny and 

 calm, yet the number of flies caught was distinctly low and a similar result 

 was obtained under similar conditions on August 7th. Immediately after- 

 wards, the prevalence curve tends to rise but at this time the crop was cut. 



Thus the minima appear to be caused by a real absence of the flies 

 from the field and therefore the frit-fly has certain periods of high 

 prevalence. 



Interpretation of the Prevalence Curves. 



The tendency in the past has been to conclude that periods of high 

 prevalence coincided with the appearance of successive generations. The 

 abundance of flies has been roughly correlated with the condition of the 

 crop and at the same time associated with the three generations at 

 present recognised. 



* As the oat crop is nearly alwaj's removed before the life-history for the year is com- 

 pleted, one or more maxima will not appear on the Charts. 



Ann. Biol, vm 8 



