Fluid mechanic constraints on spider ballooning.

Early investigations point to ballooning as an important mechanism for the dispersal of spiders. Most studies, however, have focused on collecting and interpreting observations with the aim of establishing the statistical characteristics of the dispersed population and their relation to biotic factors. With few exceptions, important physical factors that mechanically constrain the ballooning activity have been neglected or simply ignored. Especially important are various fluid mechanics phenomena that affect the initiation and maintenance of the ballooning activity. By reference to a simple mechanical model that simulates the essential drag characteristics of the ballooning spider-filament system, a region is defined, in terms of the relevant physical parameters, within which the ballooning activity can be initiated. Extension of the model allows a numerical investigation of the influence of vertical wind oscillations on the velocities and trajectories of ballooning spider-filament systems. Results are presented and discussed to illustrate the relative importance to ballooning of drag on the spider's body and on the silk filament to which it is attached.