Today, we get an answer thanks to the work of Peter Gorham at the University of Hawaii. He says that one idea put forward in the 1830s is that the spiders use electrostatic forces to lift them into the air. However, this idea was rapidly rejected by biologists who claimed that thermal currents could provide all the necessary lift.

Now Gorham has re-examined the electrostatic theory and says that it can easily account for all the mysterious flying behaviours of ballooning spiders.

First, some background about electrostatics in the atmosphere. Meteorologists are well acquainted with the huge charges and electric fields that build up in the atmosphere. “This field exists globally in the atmosphere with an average surface magnitude of 120 Volts per metre pointing downward,” says Gorham.

He then goes on to calculate the charge that a strand must have to lift a spider of a certain weight. This turns out to range from 10 to 30 nanoCoulombs.

Is it possible for spider silk to become charged in this way? Gorham shows that it is entirely reasonable. He points out that the silk contains significant amounts of charge bearing molecules such as amino acids and that it becomes negatively charged when in frictional contact with other materials.

That should allow the silk to become charged as it leaves the spinnerets, a process known as flow electrification. Indeed, this kind of charging is well known in the petrochemical industry where it can produce high voltage discharges (although Gorham says it has not yet been studied in spider silk).

There must be a source for this charge, of course. Gorham thinks a likely origin is the Earth itself which has a negative charge density of about 6 nanoCoulombs per square metre on average. That’s more than enough to give the silk a healthy boost and spiders may well be able to pick out prominences where the charge density is much higher.

This idea accounts for all the previously unexplained ballooning phenomena. For example, it explains how spiders achieve such a high velocity in conditions of little or no wind.

It also explains how such large spiders are able to generate lift. These produce several strands that each acquire charge and generate lift.

And it explains why these strands fan out from each other—because their negative charges repel.

There is one final mystery, however. Darwin describes the spiders launching themselves horizontally. How can this happen if the atmosphere’s electric field is vertical?

Gorham solves this puzzle by calculating the electric field around a tall conducting object, such as a ship like the Beagle fitted with a lightning conductor, in the middle of the ocean.

It turns out that the ship significantly distorts the field. “The angles of the field near the ship’s deck level fall within 10-30 degrees of the horizontal,” says Gorham.

That explains Darwin’s observation more than adequately. “Such launches are very difficult to explain by thermal convection given the calm conditions noted by Darwin,” says Gorham.

Of course, Gorham’s ideas will need to be tested by actually measuring the charge on gossamer spider silk as it is generated. That’s an experiment for an enterprising biologist to take on.

Should it be confirmed, the discovery will be yet another fascinating episode in Darwin’s extraordinary journey. As Gorham puts it: “This remarkable behavior…will place the Gossamer spider’s electrification ability among the most striking evolutionary adaptations that Darwin encountered on his voyage.”

Ref:arxiv.org/abs/1309.4731: Ballooning Spiders: The Case for Electrostatic Flight