Spider weaves a killer catapult and the flies don’t stand a chance

The flies don’t have time to see their nemesis spring its trap.

A Hyptiotes cavatus spider holding its web in tension. Picture: AFP

The flies don’t have time to see their nemesis spring its trap. Barely have they hit the silken tripwire before eight legs of doom catapult towards them with an acceleration of 80 times gravity and they are enmeshed in the web.

Catapult is the operative word for what this spider does. By carefully observing the ­process by which the triangle weaver spider catches its prey, scientists have shown that it is the only non-human ever ­recorded using tools for power amplification.

A way of storing energy, power amplification is the basis not only of bows and arrows but also of medieval siege weaponry, which often used ratchets to increase the tension in machines such as ballistae, before releasing the energy in one deadly blast. In this way, such machines can provide a burst of energy far greater than could be generated by muscles alone.

Sarah Han, a specialist in biomimicry at the University of Akron in Ohio, had a hunch that something like this could be happening in this spider.

“I was going around the woods of Ohio, looking at spiders. When I came across this spider, I just watched it, and started noticing this behaviour,” she said. “I wasn’t too familiar then with the concept of power amplification. I just saw that it was doing something really fast, faster than its muscles should be capable of achieving.”

The spider has a very unusual hunting technique. It makes a triangular web, sitting at the apex and holding an anchor line that attaches it to something solid. When a fly lands on the web, it appears to an observer as though the web simply slackens very fast. For the fly, it is rather more dramatic.

By using high-speed photography Ms Han, who is studying for a PhD, worked out the full sequence of events. “The spider adds tension to the web, waiting for the moment to strike,” she said. This is the spider’s equivalent of ratcheting up a ballista.

When the fly lands, the moment to release arrives. “The fly panics, starts thrashing around,” Ms Han explained. “The spider feels the vibrations and releases its back legs from the line.” This suddenly unleashes the tension the spider has been building, causing both spider and web to jolt forward with an acceleration of almost 800m per second.

“All the stored energy is ­released, like letting go of the string of a bow,’’ Ms Han said. Although the spider stops before reaching its prey, shockwaves down the web cause other strands to whip around the fly.

Writing in the journal Proceedings of the National Academy of Sciences, Ms Han said that this showed an unlikely congruence between spiders and medieval siege weapons. “The spider uses its web like a catapult, loading multiple cycles of muscular contraction and then flinging its own body and the web forward to entrap prey,” she wrote. “This is the only known case of a non-human utilising an external device for power amplification.”

The Times