A review of the evolution and mechanisms of ballooning by spiders inhabiting arable farmland

This paper complements and updates a review of spider ballooning literature by WEYMAN (1993), with expanded discussion of the possible causative factors of ballooning by spiders that inhabit arable farmland. Results relating to other taxonomic groups (especially Acari) are also described, where these help to illuminate the processes underlying the phenomenon of ballooning. Changes in air movement are proposed as an immediate trigger for take-off behaviour, and food deprivation is confirmed as a short-term moderator affecting ballooning frequency. Seasonal changes in ballooning motivation of spiders inhabiting arable farmland have not been found. Within species, ballooning frequency can vary with the growth stage and sex of the spider but variations may also represent a mixed strategy within the population. Differences in ballooning frequency between species are difficult to detect if sample size is low, trapping period is short, or the results are not controlled for the physiological states of the spider populations being compared. Where real inter-specific differences in ballooning frequency occur, they may be related to foraging strategies, physiological adaptations to exploit transient food resources, or dispersal selection pressures in the origin habitats of each species. Ballooning by spiders found on arable farmland is suggested to have evolved primarily as a risk-spreading strategy to maximise survival in unpredictable habitats. In addition to reducing the risk of being confined in deteriorating habitats, ballooning could confer on spiders the opportunity to exploit (in advance of less dispersive predatory invertebrates) ephemeral blooms of prey organisms. Areas for further research are suggested.