138 Food Plants of Aphis rumicis 



increased, they distributed themselves beneath the leaves, along the 

 mid-ribs, and secondary veins, and, finally, practically the whole plant 

 in many cases became smothered with the insects. At this stage, both 

 the apterous and viviparous forms became active and restless and ran 

 over the plant as though wishing to escape. Many of them left the 

 plants and died on the soil in the pots. 



From these observations, it would appear that at different periods 

 of the growth of plants, a change in the quality or constitution of the 

 cell sap occurs which renders it unsuitable for the aphids. Further, 

 the pathological condition of the plants, induced by heavy infestation, 

 no doubt causes considerable changes in the constitution of the cell sap. 

 When Broad Beans are infected with Aphis rumicis, the young 

 terminal shoots are first infested, the aphid colonies gradually extending 

 down the stem, and on the veins below the young leaves. 



From an examination of sections through the stems and leaves of 

 some plants, which were infested with aphids, one sees that the stylets 

 are forced through the epidermis and pass intracellularly between the 

 cells of the plant tissues towards the phloem cells of the vascular bundles. 

 The stylets often pass in a very irregular manner between the cells 

 of the cortex, and are not forced into the plant in a direct straight 

 line. 



From a recent study of the mouth parts and mechanism of suction 

 in ScJiizoneura lanigera, which the author has recently carried out, it 

 has been shown that the plant juices pass up to the pharynx through 

 a very minute suction canal formed by the partial fusion of the two 

 internal stylets^. This canal is extremely minute in transverse section, 

 and it is highly probable that the ascent of the sap through it is largely 

 due to capillarity. 



This is an important point for consideration. It is obvious if the 

 surface tension of the cell sap is such that it cannot ascend up the 

 minute capillary tube the aphids would be unable to obtain food. It 

 is probable that in young growing shoots the stylets can be more easily 

 forced into the plant tissues, but there is the question of the difference 

 in the supply and constitution of the cell sap in young, actively growing 

 parts of a plant. It is not improbable that in the case of small plants 

 heavily infested with aphids, the cell sap is rendered toxic, or at any 

 rate distasteful to the insects. 



I have indicated that the individuals living on a plant very heavily 

 infested with aphids become gradually smaller, and apparently derive 

 1 Tliis paper will shortly appear in the Transactions of (he Linncan Society. 



