Deceiving an Orb Weaver

Portia sits near the edge of an orb web, looking across the sticky spirals toward the web's architect, Gasteracantha sp., sitting at the hub. Gasteracantha is a distinctive spider. It is large, powerful, and has long horny spines on its abdomen that make it difficult for Portia to hold on to.

Gasteracantha's eyesight is too poor to recognize Portia as a predator. However, Gasteracantha has an acute ability to detect and interpret web signals—displacements, even very small displacements, of its web's silk lines (Witt, 1975). Portia's task is to get within attacking distance without eliciting the wrong response from Gasteracantha. This large spider is fully capable of preying on Portia should it get the upper hand. Just walking across the web will not work for Portia. The resulting web signals will give it away.

So Portia moves slowly onto the edge of the web, reaches out with its forelegs, and begins to pluck on the silk; but Gasteracantha does not move. Portia continues to make signals, but varies them. It plucks with different legs, plucks with its palps, varies the speed and the amplitude at which its appendages move, and it shakes the web by vibrating its abdomen up and down. Complex patterns are made by simultaneously moving different sets of appendages, with different appendages moving in different ways. By using any combination of its eight legs, two palps, and abdomen, Portia is capable of generating an almost unlimited repertoire of web signals.

Eventually a signal may cause a reaction in Gasteracantha, and it may approach Portia. If the approach is not too fast, Portia will continue to signal, slowly drawing the prey spider closer (Jackson and Wilcox, 1993a). Portia avoids making web signals that elicit a fast approach because, when moving quickly, Gasteracantha is dangerous and more likely to become predator than prey.

Luring Gasteracantha is a slow process, and close to an hour has already passed. Then something happens to speed things up. A light tropical breeze gently rocks the web. The wind-induced web movements mask any fainter movements caused by Portia and the spider takes advantage of the smoke screen (Wilcox et al., 1996) by stepping rapidly across the web toward Gasteracantha. This time, however, when the breeze dies down, Portia is still several centimeters from its prey.

Now Portia creates a smoke screen of its own (Tarsitano et al., 2000). By violently and repeatedly flexing all of its legs at the same time, the spider shakes the web much as the breeze did. Cloaked by a succession of such diversions, Portia closes the remaining distance. However, when it is about 3 mm from Gasteracantha, something goes amiss. Gasteracantha suddenly turns on Portia, lunging forward and grabbing one of Portia's legs with its chelicerae. Portia leaps off the web, leaving the leg behind.

After landing on the forest floor half a meter below, Portia looks up at the web and then climbs back to it. Once there, it repeats the entire process and this time succeeds in lunging at Gasteracantha. Portia quickly punctures Gasteracantha's cuticle with its fangs and then lets go. Gasteracantha runs to the edge of the web and drops to the ground in an attempt to escape, but paralysis soon sets in. Portia drops to the ground on a dragline, walks in the direction of Gasteracantha, and scans the forest floor for the specific kind of spider it just attacked (R. R. Jackson, unpublished results). Portia will bypass other potential prey placed in its path, continuing to search for the expected prey, in this case, Gasteracantha.

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