Delicate spider takes down tough prey by attacking weak spots

Like a miniature martial artist, the Loxosceles gaucho recluse spider can sneak up on a heavily armored harvestman (a type of arachnid), identify its weak spots and quickly disable its meal, a new study reveals.

This species of recluse spider lives in and around Sao Paolo, Brazil. Often the spiders are found near and inside people’s homes, dwelling under tree trunks, dead palm fronds and bricks. They eat a variety of arthropods, including crickets and Mischonyx cuspidatus harvestmen.

Harvestmen aren’t typical meals for such delicate-bodied spiders because their thick cuticle has only a few spots, such as their joints, that would be vulnerable to a spider’s bite. But after finding dead harvestmen on L. gaucho spiders’ webs, Julio Segovia of the University of Sao Paolo and colleagues suspected that the spiders somehow overcome the harvestmen’s defenses.

The researchers came up with a series of hypotheses about how the spiders might accomplish the feat: The spider tracks down a harvestman via chemical cues. It detects its prey through vibratory cues. And the spider’s web helps it handle the harvestman so that it can bite the arachnid in its vulnerable spots. The researchers tested each of these hypotheses and reported their findings in the March Animal Behaviour.

First, they put hungry spiders into an arena with one spot that had chemical cues from harvestmen and another with chemical cues from crickets. The spiders had no preference for either one. Nor did the spiders prefer either of the cues over that of a blank piece of paper. So scent is probably not leading them to a meal.

The scientists next tested the spiders’ ability to sense vibrations by letting them prey upon harvestmen on paper and on granite. There was no difference in the spiders’ hunting ability — so they’re probably not using vibrations to sense their prey. And in a test of the third hypothesis, the presence of silk had no effect on the spiders’ ability to capture the harvestmen; that hypothesis was wrong as well.

The behavior of the harvestmen during tests was unusual, as well. “Harvestmen have a pair of scent glands used to repel predators,” notes study coauthor Rodrigo Hirata Willemart of the University of Sao Paolo. “It came as a surprise that these harvestmen did not release their smelly secretions against a predator with such a high success in eating harvestmen.”

Videos of the spiders finally revealed their hunting strategy: The spider spots a harvestman and then approaches or brushes by its prey. At that time, the spider is probably identifying weak spots to attack. Then it delivers a series of short and/or long (up to several minutes long) bites until the harvestman stops trying to get away. Finally, the spider weaves silk threads around its meal.

“Sometimes the harvestmen freeze,” Willemart says. “Sometimes they attempt to scurry away. But the spider is faster and quickly manages to move and stay in front of him. The harvestman goes right, the spider goes right, always in front of the prey.”

The recluse spider can disable a cricket faster and with fewer bites, probably because that insect lacks the harvestman’s heavy armor and require less venom to subdue, the scientists note. That may explain why earlier researchers reported finding more crickets than harvestmen in the spiders’ webs — the crickets are the easier meal.