SOME SPIDERS MAKE FIERCE PREDATORS

by Whit Gibbons

August 12, 2002

Spiders are all around us during late summer, and when an inattentive bug gets caught in a spider's web a fascinating predator-prey drama unfolds. If a type of spider known scientifically as Portia happens to be around, the ensuing event takes on added thrills as a spectator sport. The lifestyles of several species of these jumping spiders are the ultimate in stealth, deceit, and fierceness.

Most spiders eat mainly insects, but Portia spiders prey primarily on other spiders (which are arachnids, not insects). And they capture their prey not by ensnaring them in webs but by pouncing on them. The tactics they use to outwit and capture web-building spiders, as well as members of their own species, are intriguing.

Portia's strategy for attacking web-building spiders involves a deceptive appearance and some misleading behavior. As part of their tactical design, they are almost undetectable to their prey, yet have excellent visual abilities themselves--plus fangs and venom. Four pairs of eyes that detect motion and provide keen vision give them better eyesight than most spiders. With their dark brown color and hairy-looking legs, Portia spiders can look like a piece of debris, allowing them to go unnoticed when invading another spider's web.

Being inconspicuous is only part of the design that allows Portia to enter another spider's lair and attack the inhabitant. They also mimic trapped insects. Many web-building spiders stay in a protected area at the base of their web and monitor vibrations. A trapped insect makes predictable movements while trying to escape. The spider thus knows it has captured a particular prey item, like a fly or grasshopper.

Portia enters another spider's web, being careful not to make vibrations that are detectable as anything more than that of a breeze or a falling leaf. The cunning predator then creates vibrations that mimic an entangled insect. As the web-building spider emerges from its hiding place intent on making a meal of a trapped victim, the tables are turned. The would-be predator, unprepared for an attack, can become the prey. By assuming a home field advantage against a defenseless insect, the web-builder may be caught off guard and killed by the Portia.

The little invaders, however, are not always successful. Most spiders have fangs, and spiders intended as victims are often much larger than Portia. If a spider moves in quickly, Portia may be killed before it has an opportunity to attack. Hence, these small warriors of the spider world sometimes use other tactics.

The Portia may employ camouflage, by looking like a piece of trash. As the larger spider approaches, Portia may change its drumming pattern on the web in a manner that belies its presence. For example, it may make the web move as it would in a gentle breeze, so the bigger spider does not attack immediately, assuming that a piece of debris has fallen into the web. Awaiting the right moment, Portia can attack effectively by catching the other spider completely unawares.

Not content with eating other species of spiders, Portia spiders, especially females, will also eat their own kind. Sometimes when females encounter each other they will fight until one kills the other. Plus, both adult and juvenile females on occasion release chemical signals that attract males--as meals, not mates. This ploy works because a signaling female may actually have mating in mind. Apparently, the odds are good enough to keep attracting males, a phenomenon some might consider to be universal among all species of animals.

The Portia spider saga is impressive, and although none may live in your region, all spiders offer an intriguing glimpse into the hidden world of invertebrates, which few of us notice. Equally amazing is that scientists with patience and an understanding of natural history continue to uncover enthralling facts about the lives of plants and animals. Captivating natural events are happening in your backyard. Look around; you may be surprised at what you see.



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