Thursday, October 31, 2013

Ghost Spiders In The Sky

  Spider parachute and dragline silks caught
on Montana power lines earlier this month
(All Hallow's Eve issue...)

Halloween is a great time to dwell over MILLIONS OF SPIDERS RAINING FROM THE SKY! This kind of nightmare-inducing headline gets a lot of play in backwards countries - like Brazil and Texas (1). Fortunately, hordes of hungry spiders also fall from the big skies of Montana, but for some reason you never read about it here.

Time to change that.

We normally think of spider silk as a static object - the web. But silk is useful for oh-so-much more than just catching food. Many spiders also use silken threads to lift and carry them to new places.

This manner of flight is called "spider ballooning." Careful experimentation has shown that the individual spider has no control over its flight direction. However, they do show some control over their rates of descent/ascent by changing their body posture to create more or less drag.

Ballooning is common in summer and fall when small, baby spiders are dispersing. The floating population is composed almost entirely of young (second instar) spiders weighing between 0.2 and 1.0 mg. The typical aeronaut climbs to the top of a plant, points its abdomen to the sky, checks the wind direction and velocity, and releases a meter or so of silk into the breeze. It can take many attempts before the spider becomes airborn.

Some of the larger adult spiders also float around during the warmer months, though usually for much shorter distances. A big ballooner from Missouri weighed in at 25.5 mg, and a number of female South African spiders (species Stegodyphus Dumicola) were documented ballooning, even though their average weight was a whopping 100 mg!

Every summer here at the end of the road, we see small spiders skating across the middle of the lake, pulled by a single airborne thread like some miniature water skier. Ballooning spiders can also float up into the jet stream, and some have even landed on atmospheric data balloons at 16,000 feet altitude.

In 1975, a researcher in Costa Rica somehow documented that some young spiders can survive for at least 25 days while floating around in the jet stream without food. Hungry, well-traveled spiders are one of the first arrivals on new volcanic islands, sometimes hundreds of miles from land, and ballooning spiders often land on ships that are many hundreds of miles out to sea.

The traditional explanation is that even the lightest air movement will raise the almost-weightless silk and lift a small spider off the ground. But modern physicists have found that there is a lot more science taking place than meets the eye.

Even back in the 1830's, eagle-eyed observer Charles Darwin noted that thermal convection in the air didn't provide a sufficient explanation for the speed achieved by spiders ballooning off Darwin's research ship, the H.M.S. Beagle, on a clear and calm day:

“I repeatedly observed the same kind of small spider, either when placed or having crawled on some little eminence, elevate its abdomen, send forth a thread, and then sail away horizontally, but with a rapidity which was quite unaccountable."

Darwin noticed that most of his spiders emitted 4-5 silks into the air before launching, and that these silken threads repulsed each other and spread out like, "...diverging rays of light." Darwin was one of the first naturalists to notice the electrostatic nature of spider balloon silks, which also applies to single "dragline" silks.

I'll try to explain the electrical part, but I barely survived my own aimless voyage through college physics (I was born in and dispersed from Texas, after all). The main concept is that spider silk actually is "static" in the sense that it is charged with static electricity - and so is the Earth.

The Earth maintains an electrical field that originates from friction against moving air masses, and from condensation of water vapor within clouds. The result is a net-positive charge in the atmosphere and a net-negative charge on Earth. The electrical intensity increases with elevation, and there is also a midday peak.

Yellow crab spider (c) John Ashley
Crab spiders make silk "parachutes" but not silk webs
On a much smaller scale, the spider web maintains a small negative charge because it is grounded to the Earth by a skyward plant. And, friction between the air and a flying insect's wings creates a positive electrical charge - up to 200 volts in a honeybee. So when a (positive) insect flies near a (negative) spider web, like two magnets, the lightweight web moves towards the heavier insect and increases the likelihood that lunch will be served.

This same magnetic effect also applies to spider balloon silks, but it's reversed to a negative-negative repulsion. Through a combination of several mechanisms, each spider balloon silk is negatively charged as it emerges from a tiny nozzle, and is "pushed away" from the negatively charged Earth. Launching from the top of a plant provides a slightly stronger push.

When a spider emits multiple balloon silks, each one negatively charged, they spread out away from each other and effectively increase the surface area to be "lifted" by air currents while also getting "pushed" away from the ground. Darwin was correct, spiders fly higher and faster than mechanical physics alone can explain. They're actually flying on two invisible currents - thermal and electrical. Small spiders need but a single dragline, while heavier spiders require dozens or hundreds of silken parachute silks just to make short flights at midday.

So, we finally figured out how spiders fly. Now we're starting to recognize that, as generalized predators of small insects, spiders are incredibly valuable to human agriculture. More carnivorous spiders means fewer vegetarian bugs, which in turn means lower pesticide use and higher crop yields. Even with our tendency to unintentionally reduce spider populations by tilling and spraying, the young storm-trooper spiders still parachute in to our benefit every growing season.

Until recently, the beneficial effects that spiders have on crop productivity has flown under the radar. But by using exclosures, a recent study found that 41-50% of the spiders on experimental plots actually arrived by air. Per hectare, that translated to a daily surge of roughly 1,800 spiders, or more than 216,000 spiders flying in and landing during a four-month growing season. The annual recolonization of crop fields is just as dependent on flying spiders as walking spiders.
So in spite of what you may have read, spiders falling from the sky is a very good thing indeed. Now, what should really prevent you from sleeping at night are visions of flying snakes!

(1) A few years ago I toured a new (second, third?) home in central Texas that was built along the river banks where I rambled as a child (and where my mother rambled as a child, too). The wealthy owner - from a Texas city - proudly pointed out his system of tubes that ran all the way around the house, up under the eaves. Once a week, this system sprayed a toxic mist all around his own habitat, with the specific purpose of killing spiders. I didn't know whether to laugh or cry, so I said nothing but died a little inside.

Behind the science. You can watch a mostly-correct video of spider ballooning here.

Below the spiders. In spite of my nature, I have never managed to photograph a ballooning spider, and probably never will. They're just too small and cover such large amounts of space. Photos of their parachute silks caught on power lines (above) is all I've managed to capture.