43 



The mean values of infestation for the varieties ranged from 

 37 to 1,037. 



These values allow of the varieties being- tentatively grouped 

 into classes representing various degrees of susceptibility ranging 

 from 98% to 3%. The results obtained indicate that resistance 

 or susceptibility may be largely determined by genetic (actors in 

 the plant. 



XXXVI. J. Davidson. "Biological Studies of Aphis 



RuMicis Linn. V. The Penetration of Plant 

 Tissues and the Source of the Food Supply of 

 Aphids." Annals of Applied Biologv, 1923. 

 Vol. X. pp. 35-54. 



The food of aphids is the juices of plants which they obtain by 

 penetrating the tissues by means of a delicate piercing organ 

 formed by four chitinous stylets. 



The piercing organ passes between the cortical cells — occa- 

 sionally through intlividual cells — to the vascular bundles. 



The saliva secreted by the aphis acts on the middle lamella of 

 the cell wall. It also causes plasmolysis of the cells; and it is 

 able to convert starch into sugar. 



The phkfim tissue is the chief source of the food supply, but 

 other cells of the plant, such as cortex and mesophyll, may be 

 tapped for nourishment. 



The sucking out jjroiX'ss is usually intracellular, although in- 

 tercellular suction sometimes goes on. 



The varying physiological constitution of different plants or 

 even varieties of the same sjiecies of plant is important in relation 

 to the biology and physiology of aphids. 



The comjK)sition of "honey dew" — the sugary excrement of 

 aphids — is in close relationship with the |);irti<Milar species of plant 

 and aphids concerned. 



XXXVII. H. M. MoKKis. "Tiie Larval and Pupal Stages of 



the BiiiiONiD.i:. Part L" Bull. Entom. Re- 

 search, 1921. Vol. XII. pp. 221-232. 



Deals chiefly with the biology and metamorphosis of Bibio 

 marci whose larvai infest grass-land and have been reported to 

 injure various crops. 



XXXVIII. H. M. Morris. ''On the Larva and Pupa of a 



Parasitic Phorid Fly — Hypocera Incrassata 

 Meig.'^ Parasitology, 1922. Vol. XIV. 

 pp. 70-74. 



Deals with the biology of a species not hitherto investigated, 

 which parasitizes larvae of Bihio ward. 



XXXIX. H. M. Morris. ''The Larval and Pupal Stages 



of the BiBioNiD.E. Part IL" Bull. Entom. 

 Research, 1922. Vol. XIII. pp. 189-195. 



An in\estigation of the biology and metamorphosis of Dilophus 

 kibrilis and D. albipennis, the former species being recorded as 

 injuring the roots of various plants. 



