Norman Cunliffk 117 



broods at E and /'' failed to reproduce themselves, in the field tliey would 

 undoubtedly do so at this time of year. In 1919, when breeding flies on 

 grasses (p. 120), it was found (1) that parent flies at A gave rise to flies 

 at B, and (2) that either the flies at yl or 5 gave rise to another series of 

 flies in the middle of September. In the field flies would be constantly 

 emerging. 



Under natural conditions moisture is abundant, a sufficiency of 

 food always available and suitable breeding places are provided in 

 succession by the growth of the host plant, namely first the tillers, 

 then the young panicles, and lastly the young grain. In addition wild 

 grasses are always present and a few common species can be utilised for 

 breeding purposes. For example, flies bred from oat stems at A would 

 normally have oviposited on the oat panicles, but when compelled, 

 reproduced themselves on the young stems and tillers of grasses^ and 

 similarly the flies at D bred on oat tillers in place of oat grain. All that 

 seems to be required by the larvae is young and rapidly growing tissue 

 and neither the position of the tissue on the plant nor the species of 

 plant {within small limits) is important. Therefore there is no reason to 

 assume that reproduction would be checked by lack of food or host 

 plants. 



The irregularity of the emergence obtained experimentally would be 

 highly magnified in natural conditions and the various maxima which 

 appear on the charts will be composed of flies of different broods and 

 generations, the complexity becoming very great towards the end of the 

 season. Although it is fairly clear that the first generation becomes 

 abundant in the first week of June, after this time it has been found 

 impossible to separate the different broods and generations on the charts 

 or to associate them with the high prevalence periods. As the flies are 

 not restricted to particular breeding places, the proportions of the broods 

 and generations at any one period becomes a matter of minimal im- 

 portance. 



It would seem from the experimental data obtained in 1919-20 that 

 three or even four distinct generations may be produced in a favourable 

 season, 



(&) High Prevalence and Environment. 



Many factors influence the emergence of an insect breeding under 

 normal conditions in the field. With a phytophagous insect, growth of 

 host plant, rainfall and temperature are probably the more important 



^ Vide Section B. 



