FOR a stalking predator, the element of surprise is crucial. And for jumping spiders that sneak onto other spiders鈥� webs to prey on their owners, it can be the difference between having lunch and becoming it. Now zoologists have discovered the secret of these spiders鈥� tactics: creeping forward when their quarry鈥檚 web is vibrating.
Portia jumping spiders, which live mostly in tropical forests, hunt a range of other spiders, some of which could easily turn the tables on them. 鈥淭hey will attack something about twice their own size if they are really hungry,鈥� says Stimson Wilcox of Binghamton University in New York state.
Wilcox, his colleague Kristen Gentile and Robert Jackson of the University of Canterbury in Christchurch, New Zealand, wanted to know how Portia keeps the upper hand. Sometimes the spiders lure their victims by vibrating the web to mimic the struggles of a trapped insect. But many web-weaving spiders appear to be wise to these tricks, so stalking is often a better strategy.
Advertisement
Sometimes, the researchers found, Portia takes advantage of the vibrations created in the web by a gentle breeze. But if necessary, it will make its own vibrations to mimic the impact of a falling twig.
The researchers allowed various prey spiders to spin webs in the laboratory and then introduced Portia spiders. To simulate the shaking effect of a breeze, the zoologists used either a model aircraft propeller or attached a tiny magnet to the centre of the web which could be vibrated by applying a varying electrical field.
The stalking Portia spiders moved more when the webs were shaking than when they were still, and they were more likely to capture their prey during tests in which the webs were periodically shaken than in those where the webs were undisturbed. If the spiders were placed onto unoccupied webs, they made no attempt to restrict their movements (Animal Behaviour, vol 51, p 313).
But it is Portia鈥檚 tactic of making its victims鈥� webs shake that has most intrigued the researchers. They noticed that the spiders sometimes twanged their quarry鈥檚 web violently, then crept forwards up to 5 millimetres before the vibrations died down. 鈥淭hey make a big pluck with one of their hind legs,鈥� says Wilcox.
These twangs were much more vigorous than the gentler vibrations Portia uses to mimic a trapped insect, and the researchers were initially surprised that the prey spiders did not respond to them in any way. But they have since discovered that the violent twanging produces a pattern of vibrations that match those caused by a twig falling onto the web.
Other predators make use of natural 鈥渟mokescreens鈥� to hide from their prey: lions hunting at night, for example, move in on their prey when clouds obscure the Moon. 鈥淏ut this is the first example of an animal making its own smokescreen that we know,鈥� says Wilcox.