116 Observations on the Habits q/*Oscinella frit 



The environmental conditions, however, may exercise a very decided 

 influence on the prevalence, most probably masking the effect of the 

 emergence of broods or generations. 



For this reason the curves are considered below under two headings, 

 indicating the relationship between («) high prevalence and brood pro- 

 duction, and (6) high prevalence and environment. 



(a) High Prevalence and Brood Production. 



It has been assumed in England that the generation which breeds 

 on grasses over the winter gives rise to another generation which attacks 

 oat stems and tillers, and this, in turn, to a third generation which 

 attacks oat grain. 



Quite recently, it has been suggested by Roebuck (1921), that the 

 life-history is more in accordance with continental observations, which, 

 in general, indicate the occurrence of more than three generations. From 

 field evidence, collected over a period of five years, Roebuck concluded 

 that a "brood" is normally interpolated between Generations I and II 

 as generally recognised at present. It is not clear what is meant exactly 

 by the term " brood," in this paper, but there is no doubt that the presence 

 of only two overlapping broods or generations on the oat crop is con- 

 sidered to be very doubtful^. 



When these prevalence curves in Charts I and II are considered in 

 conjunction with the experimental evidence detailed above, it becomes 

 very difl&culfc to believe that the minima of the curves are, in general, 

 closely associated with the imminence of broods or generations. As 

 indicated on p. 112, and marked on Chart II, parents at C gave rise to 

 a first brood at D which in turn gave a first brood at i''; also the parents 

 at C gave rise to a second brood at E. Although experimentally the 



1 This conclusion is drawn from the following observations (vidt Ann. Apji. Biol, vii, 



p. 181): 



Period of maximum 

 Site of pupa Earliest gathered pupa emergence of flies 



Grass stems . . March 25 May, 1st week 



Base of oat tillers . May 21 June, middle 



Panicle inside leaf June 24 July, 3rd week 



Oat grains . . July 31 September, 1st week 



Aldrich (i) summarises the life-historj^ in North America as consisting of the winter 

 generation and "following the emergence of this brood as adults in the spring, there are 

 four summer broods." But he does not make it clear whether or no he considers these 

 broods to be equivalent to successive generations. In his tabulated results, he onty records 

 that each cage was started " using flies that had emerged a few days before," without stating 

 whether any of the parents were actually bred personally. 



It is probable therefore that his figures have reference to broods only and not generations. 



