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Aphis fabae gynoparae occurred in the same large proportion in simultaneous collections of all aphids alighting and probing on, and taking off from, a host plant (spindle) and a non-host (peach), and behaved similarly when approaching and leaving them in the same conditions. Most alighters took off again from leaves of both kinds within a few minutes, staying longer and probing more times on the host. In atmospheric conditions favouring local 'hovering' instead of dispersal, flying and alighting aphids became concentrated around host plants, not through any specific attraction to them, but apparently because more aphids had accumulated upon them and were now taking off.Brevicoryne brassicae occurred in the same large proportion in simultaneous collections of aphids alighting on a host plant (cabbage) and a non-host (sugar beet). No satisfactory evidence was found of preferential alightment on cabbage and there were indications of preferential alightment on the non-host. A very small minority of the A. fabae and B. brassicae that alighted on their hosts stayed there long enough to larviposit. This minority was rather larger among alighters late in the day, but in the absolute sense, more colonization occurred during earlier periods when more aphids arrived.The intensely dispersive type of host-finding behaviour in Myzus persicae, A. fabae and B. brassicae may be common among Aphididae. It seems ideal for the dissemination of non-persistent plant viruses, more particularly among the lessfavoured host plants of each aphid. The tendency to commensal association between virus and vector provides an ecological framework which may govern the incidence of virus-vector specificity and symbiosis.

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With 7 Text-figures)The feeding preferences and comparative fecundity of laboratory-bred, alienicolae alatae of Aphisfabae were investigated in small leaf cages on spindle and sugar beet leaves, representing a winter and a summer host respectively, both in pots in the greenhouse and while growing naturally outdoors.The aphids' readiness to stay and feed, and their average reproduction rate, both varied between leaves of different ages and kinds, and there was evidence of true fecundity differences among aphids feeding on different leaves. As a general rule, readiness to settle and reproduction rate varied together in the same sense, but they did so significantly less often in comparisons between leaves of different kinds, spindle and beet, than in comparisons between leaves of one kind only. Among the leaves on the same kind of plant, the aphids preferred to feed and reproduced faster, on the whole, on young and early senescent leaves than on mature ones. T o a limited extent, this rule seemed to govern also the aphids' comparative readiness to settle and reproduce, as between leaves of the two different kinds of plant. Allowihg for agedifferences among the leaves, the aphids settled and reproduced better on spindle than on beet leaves. Since the aphids used were alienicolae, this tends to confirm the usual designation of the winter host as primary and the summer hosts as secondary, for a given aphid species.These findings form the basis of a dual discrimination theory of host selection, which assumes that aphids respond behaviourally to at least two main classes of leaf property: one associated with the age of the leaf and the other with the kind of plant. It is suggested that in nature these two sensory requirements of the aphids may be essentially contradictory, and that the shifting patterns of aphid distribution among leaves and plants may depend on the shifting distribution of leaves offering a satisfactory compromise between them. The phenomenon of host alternation is considered in the light of the dual discrimination theory, as a particular instance of a shifting distribution-pattern probably connected with the alternation of the seasons of active growth and senescence in the winter and summer host plants. -Mature w . senescent Young K J Mature K Senescent G, H, 1, J, L N --seven experiments in Table 16.'rABLE I 6 . Comparative reproduction rates, percentages settled and percentages surviving in greenhouse experiments comparing young beet leaves with young and mature spindle leaves Exp.

Water strain in the host plant reduced aphid feeding and larviposition and sometimes increased the proportion of winged offspring. These negative effects on the aphids occurred despite the enrichment of the phloem sap that could be inferred from the senescence and changing composition of the leaves under water strain, tended to obliterate the normal effects of leaf age, and were readily reversible. They are attributed to stimuli associated with a reduction in the quantity of sap obtainable by the aphids owing to reduced turgor pressure or increased sap viscosity. The opposite, positive effects recorded previously with various aphids may be associated with less severe or intermittent water strain in the plant when the reduced quantity of sap obtainable is more than compensated by its improved quality. Zusammenfassung 1. Die Wirkungen beträchtlichen Wassermangels getopfter Wirtspflanzen auf Ansiedlung und Vermehrung von Aphis fabae Scop. wurde in Gewächshausversuchen mit im Labor herangezogenen geflügelten Virginoparen an Evonymus europaeus L. und mit Ungeflügelten an Vicia faba L. sowie in einem Feldversuch bei der natürlichen Besiedlung von E. europaeus untersucht. 2. Der Wassermangel verminderte Ansiedlung und Larvenproduktion und vergrößerte den Anteil geflügelter Nachkommen. Diese negativen Wirkungen können, die normalen Wirkungen des Blattalters auslöschen, waren aber durch Wiederversorgung der Topferde mit Wasser leicht reversibel. Sie traten trotz der Bereicherung des Phloemsaftes ein, die aus dem Altern und der veränderten Zusammensetzung der Blätter bei Wasserverknappung zu schließen war. Sie werden auf Reize zurückgeführt, die auf einer Reduktion der für die Blattläuse zur Verfügung stehenden Saftmenge beruhen, welche als Folge reduzierten Turgordruckes oder steigender Saftviskosität entsteht. 3. Die gegenteiligen günstigen Auswirkungen des Wassermangels, über die früher bei verschiedenen Blattläusen berichtet wurde, können auf weniger starkem oder vorübergehendem Wassermangel in der Pflanze beruhen, wenn die Verringerung der erreichbaren Saftmenge von ihrer erhöhten Qualität mehr als ausgeglichen wird.