64 



barley. The summer generation is mainly upon spring barley, 

 but in seasons unfavourable to the fly couch grass may be utilised. 

 Very rarely wheat is a summer host plant. The life-history is 

 described in detail. 



The type of distortion caused to the host plant depends on 

 the stage of growth of the plant when attacked and the degree 

 of distortion of the plants depends upon the rate of growth at 

 the time of attack. 



The relation of the fly to the different kinds of host plants 

 is described, particularly as regards the winter generation, and is 

 shown to vary with such factors as date of emergence of the 

 flies, weather conditions during the oviposition period and amount 

 of growth of the different kinds of host plants. 



In dull and cool weather the flies will lay few eggs but are 

 stimulated to rapid egg laying by bright and sunny weather. A 

 single fly may lay about 150 eggs. More than one act of coitus 

 is necessary to fertilise all the eggs which a female is capable 

 of laying. The length of life of the imagines is probably about 

 a fortnight for flies emerging in spring, but may be over two 

 months for the autumn emerging flies. 



Certain manures (particularly superphosphate) have a marked 

 beneficial effect in reducing the infestation of summer barley by 

 gout fly, owing entirely to their stimulating effect upon the 

 maturing of the ear and the growth of the ear-bearing internode. 



While small dressings of nitrogenous manures may reduce 

 infestation, large dressings will not reduce it and may have a 

 tendency to retard growth of the ear and so increase infestation. 



Early sowing of spring barley is efficacious in preventing 

 infestation by gout fly. 



Preventative measures suggested are early sowing of spring 

 barley, good cultural conditions on the soil, and manuring (e.g., 

 with superphosphate or farmyard manure) to stimulate early 

 growth (see paper LXXII.). 



L. J. Davidson. " The Penetration of Plant Tissues and the 

 Source of the Food Supply of Aphids." Report Inter- 

 national Conference Phytopathology and Economic 

 Entomology, Wageningen (Holland), 1923, pp. 72-74. 



The food of aphids is the cell sap of plants, which they obtain 

 by penetrating the tissues by means of their piercing, suctorial 

 mouth-parts. The mechanism of piercing and suction and the. 

 action of the insects' saliva on the plant tissues is discussed. 

 With Aphis rumicis the phloem is an important source of the 

 food supply but other tissues, including the cortex and mesophyll, 

 may be drawn upon, particularly in the case of heavily infested 

 plants. 



LI. J. Davidson. " Factors which Influence the Appear- 

 ance of the Sexes hi Plant Lice." Science, 1924, p. 

 3G4. 



A short discussion of the observations of Marcovitch on this 



subject, in relation to results obtained in experiments at 

 Kothamsted. 



