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Summary 1.Movement behaviour has become increasingly important in dispersal ecology and dispersal is central to the development of spatially explicit population ecology. The ways in which the elements have been brought together are reviewed with particular emphasis on dispersal distance distributions and the value of mechanistic models. 2.There is a continuous range of movement behaviours and in some species, dispersal is a clearly delineated event but not in others. The biological complexities restrict conclusions to high-level generalizations but there may be principles that are common to dispersal and other movements. 3. Random walk and diffusion models when appropriately elaborated can provide an understanding of dispersal distance relationships on spatial and temporal scales relevant to dispersal. Leptokurtosis in the relationships may be the result of a combination of factors including population heterogeneity, correlation, landscape features, time integration and density dependence. The inclusion in diffusion models of individual variation appears to be a useful elaboration. The limitations of the negative exponential and other phenomenological models are discussed. 4. The dynamics of metapopulation models are sensitive to what appears to be small differences in the assumptions about dispersal. In order to represent dispersal realistically in population models, it is suggested that phenomenological models should be replaced by those based on movement behaviour incorporating individual variation. 5. The conclusions are presented as a set of candidate principles for evaluation. The main features of the principles are that uncorrelated or correlated random walk, not linear movement, is expected where the directions of habitat patches are unpredictable and more complex behaviour when organisms have the ability to orientate or navigate. Individuals within populations vary in their movement behaviour and dispersal; part of this variation is a product of random elements in movement behaviour and some of it is heritable. Local and metapopulation dynamics are influenced by population heterogeneity in dispersal characteristics and heritable changes in dispersal propensity occur on time-scales short enough to impact population dynamics.

The behavioural responses of Delia brassicae to host plant odour were observed in a large wind tunnel (6 times 2.3 times 1.8 m). Only mated, gravid females showed oriented upwind responses to host plant odour; visual stimuli had no effect. There was a marked optimum in the odour concentration for upwind flight but higher concentration stimulated flight activity in both sexes. The response levels were higher and the degree of positive phototaxis lower in females released in the tunnel 1 hr before test, than in those tested without this preconditioning. The effects of handling on response level were complex; physical disturbance inhibited responses at intermediate intensities but increased it at high levels. RÉSUMÉ FACTEURS INTERVENANT DANS LES RÉACTIONS COMPORTEMENTALES DES ADULTES DE LA MOUCHE DU CHOU, DELIA BRASSICAE, À L'ODEUR DE LA PLANTE‐HÔTE Les réactions comportementales de Delia brassicae à l'odeur de la plante‐hôte ont été étudiées dans un grand tunnel parcouru par un courant d'air (6 times 2,3 times 1,8 m). Les femelles fécondées et gravides ont, seules présenté une réaction orientée contre le vent à l'odeur de la plante‐hôte; les stimulations visuelles ont été sans effet. La concentration en odeur a présenté un optimum marqué pour le vol contre le vent, mais une concentration plus élevée a stimulé le vol des deux sexes. Les niveaux de réaction étaient plus élevés et le degré de phototaxie plus bas chez les femelles libérées dans le tunnel une heure avant les expériences, que chez celles étudiées sans ce préconditionnement. Les effets des manipulations sur le niveau de réaction étaient complexes; les perturbations physiques ont inhibé les réactions aux intensités intermédiaires mais les ont augmentées aux intensités élevées. Bien que le comportement des femelles aurait pu être envisagé comme étant sous l'influence de deux systèmes de motivation contrôlant la ponte et la fuite, une étroite liaison entre les deux a paru exister, une perturbation physique stimulant simultanément les comportements de fuite et de ponte.

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Adult cabbage root fly (Erioischia brassicae (BouchC)) exhibited a diurnal periodicity in behaviour. Trapping tests indicated that the flies fed from hedgerow flowers in the morning, visited the crop in the early afternoon and returned to the hedges in the late afternoon. In the laboratory the flies also showed a cyclical pattern in behaviour. Feeding preceded oviposition by 3 days and this was reflected in the field by a relative increase in the movement of females away from the hedges and to the crop when they became gravid. The diurnal periodicity and the cycle in behaviour of males was similar to that of the females, indicating that some of the flies' movement was non-appetitive.The tendency of flies to stay for most of the day at hedges resulted in progressive declines in the numbers captured with increasing distances from hedges.

The behavioural mechanisms by which Delia brassicae (Hoffm.) locates its host plants are reviewed. In a large wind tunnel, the principal mechanism appears to be upwind orientation and flight stimulated by host plant odour. Gravid females landed frequently while in an odour plume, the mean flight length being 0.5 m for cultured flies under a variety of conditions. On leaving the odour plume while overshooting the source or moving across the air flow, the majority of females turned downwind, circled and re-entered the plume.These conclusions are seen as an hypothesis of host plant finding and adequate means of testing its validity in field situations are required. The approach developed here was to identify features which were likely to be peculiar to this type of host plant finding. A computer model was built to simulate the events downwind of an odour source and this predicted that the "average" cultured female moves upwind at the relatively slow rate of 65m/day. In two mark and recapture experiments in the field, cultured females moved upwind at estimated mean rates of 80 and 95m/day when released downwind of a brassica plot.Analysis of the behavioural mechanisms underlying the location of host plants by insects has been carried out largely under laboratory conditions. The progression of research on the cabbage root fly, Delia brassicae (Hoffm.) appears to be typical, having passed through similar stages to that on other species. Some field experiments have been carried out (de Wilde, 1947;Traynier, 1965;Hawkes, 1974Hawkes, , 1975 but when detailed observations of behaviour or an analysis of the causation of behaviour are required, the difficulties of field experimentation have forced a retreat to laboratory situations (Traynier, 1967 a, b;Coaker & Smith, 1970; Nair & McEwen, 1976;Hawkes & Coaker 1976, 1979 Hawkes, Patton & Coaker, unpubl.). The present knowledge on host plant finding in D. brassicae has then been outlined by field experiments but elaborated in the laboratory. This paper reviews the existing knowledge and suggests that the conclusions of laboratory experiments should be seen as an hypothesis and not as an endpoint. Such hypotheses need to be tested in the field to determine whether they provide a valid basis for the understanding of the ecological process of host plant finding. This paper illustrates an approach to the problem of validation of hypotheses. *) Present address: --Fasanv/igen 4, 752 52 Uppsala, Sweden.

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