SUMMARYI. The insects studied are selected as being representative of the group HEMIPTERA. They are: Capsidae:Calocoris bipunctatusFab.,Lygus pabulinusLinn. Typhlocybidae, “Leaf‐hoppers”:Eupteryx auratusLiv.,Zygina pallidifronsEdw. Aleyrodidae, “White Flies”:Asterochiton vaporariorumWestw. Aphididae:Myzus persicaeSulzer,Macrosiphum solanifoliiAshm.,Myzus drcumftexusBuckt. Coccidae:Aspidiotus hederaeVallot.,Dactylopius longispinusTarg.‐Tozz.II. The insects exceptAspidiotus hederae, are studied chiefly in relation to the potato plant.III. (a) Capsidae. These bugs penetrate the tissue by pressure, the passage of the stylets being assisted by the disintegrating effects of the accompanying saliva. The objective of the beak lies in both cortex and vascular bundles. Both leaf surfaces and stem tissue are punctured. The path through the tissue is intra‐cellular. The effect of the saliva upon the cells of the plant host is described in detail.(b) Typhlocybidae. The leaf‐hoppers penetrate the plant by pressure. Both leaf surfaces are attacked, the larvae being usually confined to the lower surface. The stem is not punctured as a rule. The objectives of the stylets are the palisade cells and the vascular bundles. The track of the piercing organ is intra‐cellular. The effect of the saliva is described, the chlorophyll and cell contents are destroyed over a wide area while the cell walls are left intact. There are some indications of a stylet track.(c) Aleyrodes (White Fly) penetrates inter‐cellularly, entrance being made between two epidermal cells, in the case of young larvae often through a leaf stoma. The under surface is always the one attacked. The objective is usually the phloem elements of the vascular bundles, in young larvae the stylets are often confined to the cortex. Damage to the cells of the host was not very evident microscopically, though a few cells showed plasmolysed contents. Very slight indications of a stylet track were visible.(d) Aphides. These insects usually penetrate between two epidermal cells, the track on the whole being inter‐cellular.M. solanifoliihowever, a large aphis, often has an intra‐cellular stylet track. In each case the phloem was the objective. A well‐defined stylet track is formed, most evident in the case ofM. persicae. Damage by the saliva is largely confined to the actual track of the stylets. Some wound reaction is noted in the phloem caused byM. persicae. The under surface of the leaf is usually selected for penetration, the stem is also attacked.(e) Coccidae. Of this groupAspidiotusexhibits an intra‐cellular path through the parenchyma of the host, in this case a Cordyline. The stylets enter the tissue through an epidermal cell. The phloem is not the objective. There is a well‐marked stylet track. Cells in the path of penetration have their contents destroyed. The saliva appears to have the property of dissolving the cell walls.Dactylopius, a mealy bug, also penetrates intra‐cellularly; in this case the phloem and xylem elements are always the objective. On the leaf the insects are situated near or alongside a vein. There is a well‐marked stylet track, the saliva being secreted in large irregular drops. Both leaf and stem tissue are attacked. Some disorganisation of the vascular bundles results from the feeding of this insect. That the saliva ofDactylopiusalso dissolves the cell walls is shown by the ease with which it can penetrate large xylem vessels.A number of photomicrographs illustrating these points are given.IV. A discussion is appended of the relation of the facts elucidated in the study to the question of the transmission of virus disea