28 May 2004

Flying spiders and water striders

Forrest M. Mims III

Most everyone has seen water striders darting across the still surface of a puddle or pond. But have you ever seen a flying spider?

Water striders accomplish with ease what ordinary mortals can do only on frozen water. Their achievement doesn't seem all that unusual, since even twigs and bits of dead grass can float. But water striders don't float in the conventional sense. These agile insects walk on top of the water's surface.

Water striders have a special gift. So do spiders, which are very abundant along Geronimo Creek in South Texas. Most insects, including some water striders, are equipped with wings. But spiders are not insects, and they have no wings. Yet spiders can take to the air when the conditions are right.

Several summers ago when I was making daily measurements of the Sun and sky, I spotted a relatively large object gliding through the sky around 20 feet overhead. Soon it dropped rapidly from the sky and landed in the tall grass nearby.

I grabbed my camera and took the photograph shown nearby. It shows a medium sized black and yellow argiope (Argiope aurantia) seconds after it alighted on the unopened bud of a native cone flower. The silk strand that served as its balloon on the way up and its parachute on the way down is still visible emerging from its abdomen.

The first time I saw a flying spider was October 5, 1995, a clear fall day. As I have done virtually every noon since May 1988 when the Sun is not blocked by clouds, I was in the field by my office measuring the ozone layer and the Sun's ultraviolet radiation.

While looking toward the nearby tree line, I noticed a small, dark object rise straight up from the tall grass about 200 feet south of my measurement site. The mysterious craft reached a height of about 15 feet and then drifted around 75 feet to the west. Then the little UFO suddenly turned to the northeast and headed in my direction.

The mini-UFO floated over my site and then suddenly dropped into the grass behind me. Forgetting about the measurements, I ran to the landing spot and was amazed to find a large female jumping spider perched high on a blade of grass. She had ballooned in on a strand of silk exactly like I had often seen baby spiderlings do. Only this was no weightless spiderling. This was a full-grown adult!

One Thanksgiving Day I escorted a group of relatives down to Geronimo creek to show them the damage caused by a major flood. Suddenly I noticed a full-grown crab spider hanging in the air. But there was no branch! It was sky riding a strand of silk not more than a foot long. As we watched, it spread its legs, let out some silk and dropped gently down to the grass.

Often in late summer and fall I notice dozens of shimmering strands of silk drifting by high overhead as I measure the Sun's rays. I've even seen this phenomenon from high atop Pike's Peak in Colorado.

Are spiders attached to each of those shiny threads? Do both spiderlings and adult spiders ride in on cold fronts? Do birds dine on them as they drift? Can they control their altitude by reeling out more silk?

Flying insects consume prodigious energy, and they can only fly over a specific temperature range. But drifting spiders consume no energy. They just climb atop any tall place where there's a breeze, do a head stand, and let out some silk.

Once airborne, they are as free as the wind. Having flown in hot air balloons, I know how they feel.

Forrest M. Mims III and his science are featured online at http://www.forrestmims.org/.

This feature was originally published in Forrest Mims's weekly science column in the Seguin Gazette-Enterprise, Seguin, Texas. The column is written for a general audience.