Summary. The food of aphids is the cell sap of plants, which they obtain by penetrating the tissues by means of a flexible, chitinous, piercing organ, which is composed of the maxillary stylets and the mandibles. The maxillary stylets are closely apposed, thus forming two canals, which extend to the extremity of the compound stylet thus formed. The dorsal canal is the suction canal along which the plant juices are conducted into the pharynx. The ventral canal is the ejector salivary canal down which the saliva is pumped into the plant. The penetration of the piercing organ is brought about by the retraction of the labium and the forward prolongation of the “forehead.” The labium at its distal end grips the piercing organ in a pincer‐like manner. With Aphis rumicis the piercing organ passes intercellularly through the cortex, only occasionally passing through individual cells. Eventually it reaches the vascular bundles. The saliva is able to dissolve a passage for the piercing organ through the walls of the cells. It is also able to convert starch into sugar. A well marked sheath, the stylet sheath, is formed round the piercing organ when it is in the plant tissues, its wall being composed of substances produced by the reaction of the saliva on the cell sap. It probably consists of callose and insoluble calcium pectate together with a deposition of tannin. The saliva causes plasmolysis of the cells and disorganisation of the cell contents. With Aphis rumicis on beans, the phloem elements of the vascular bundles are the chief source of the food supply, although other cells of the plant, such as the cortex and mesophyll of the leaf may be tapped for nourishment, especially when the plant is heavily infested. Frequently in Rumex, xylem vessels are tapped for food. The presence of a thick cuticle may prevent young aphids from piercing into the tissues and inhibit a general infestation of the plant. The sucking out process is usually intracellular, although intercellular suction sometimes occurs in the loose cortical tissue. In the case of Macrosiphum on the rose, occasional cells react under the influence of the saliva, resulting in a cellulose thickening of the cell wall of affected cells. The varying constitution and concentration of cell Bap in different plants is an important consideration in respect to the biology and physiology of aphids. The more favourable development and reproduction of aphids on certain plants is probably associated with the nature of the cell sap, and an investigation of the cell sap of plants under varying cultural conditions and manurial treatment appears to be necessary. The composition of the excrement of aphids, or honey dew, depends upon the species of plant and aphid concerned, and is in close relationship with the composition of the cell sap of the plant and the digestive processes of the aphid.