Q: Some species of spiders are known for “ballooning”. On a hot day, they climb to the top of a tall object and run out a thread, gripping with their legs until the thread is long enough to provide them with sufficient lift. Then they release their grip and “fly”. The lift has been attributed to crosswinds, but this seems unlikely, as there is no aerofoil involved and ballooning has been observed in still air.
It may be that the lift is provided by convective air dragging along the length of the thread. But how does such an insubstantial thread generate enough lift? If it did, wouldn’t this cause problems with webs, or is there a different kind of thread used for ballooning?
A: Paradoxically, it is drag, not lift, that enables a ballooning spider to drift with the air current. Of the forces which act on a flying animal – lift, weight, drag, and perhaps thrust – the spider’s silk increases the frictional drag of air in relative motion with respect to the spider and silk filament combination. This is similar to the way the down on a dandelion clock seed increases drag. In such small objects, drag slows descent and air currents provide horizontal or vertical motion.
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To balloon, the spider raises its abdomen into the air at the top of a grass stem or something similar and squirts out a stream of silk from its exposed spinnerets. This silk filament continues to be drawn out by air drag until ballooning occurs when the combined drag forces on the spider and the silk filament are sufficient to overcome both the spider’s grip on the stem and the combined weight of both spider and silk. Remarkably, the silk represents less than 0.1 per cent of the system’s weight, yet contributes more than 75 per cent of the drag.
Spider ballooning is constrained by the laws of physics. Spiders cannot be lighter than 0.2 milligrams (a body diameter of about 1 millimetre) or bigger than 8 milligrams (about 2.5 millimetres) and typically weigh between 0.5 and 1 milligram. Relative air motion must be at least 0.25 metres per second and no more than about 3.5 metres per second (11.2 kilometres per hour, a very gentle breeze). Once the spider is airborne, it will rise as long as the wind’s vertical vector component exceeds the system’s freefall terminal velocity. Spiders can therefore balloon on rising thermal currents in “still” air. Relative air velocity affects the length of the ballooning silk filament and travel speed. Spiders attached to short filaments in strong winds travel further and faster than the same spider attached to a long filament in slower winds.
Ballooning spiders have been seen at altitudes of more than 4000 metres and have been carried more than 300 kilometres.
A: Ballooning is not confined to spiders; some mites and the young caterpillars of some moths also use silk threads. The threads reduce fall speed by increasing drag, not by increasing lift. Longer threads induce slower fall, as has been measured in the laboratory. Upward and downward movement while airborne is caused mainly by atmospheric turbulence – the balloonists simply fall down.