Research Shows Spider And Fly Rivals In New Light

The proverbial pairing of the spider and the fly takes an unexpected turn in new research published by UC biologist George Uetz.

In a paper published in the journal "Behavioral Ecology and Sociobiology," Uetz and colleagues Craig Hieber, Stim Wilcox and former UC student Jay Boyle report upon an observed relationship where the eggs of a Mexican spider become the prey of a species of fly. "It is somewhat akin to the old headline of 'Man bites dog' making news," Uetz admits.

The Mexican spider Metepeira incrassata is unusual to begin with in that it lives in large colonies. Much of Uetz's long-term research has been directed at understanding why social behavior has evolved in so unlikely an animal.

Despite the safety in numbers afforded by colonial web-building, the predatory fly Arachnidomyia lindae has evolved a behavioral ploy -- to exploit the webs of Metepeira to deceive mother spiders into thinking they may have an opportunity for a meal.

In reality, the fly is opening an opportunity for its own larva to prey on the spider's egg sac.

"It makes for an interesting nature story, as well as one of those tales about how scientists have to work to unravel these mysteries," Uetz says.

The research consumed eight years, including many trips to Mexico, as the researchers uncovered just how specialized and co-evolved the relationship between the two species had become. The spiders respond with predator-specific defense behaviors only to the wing vibrations of the fly species in question, and can sense even the tiniest of differences that come from other fly species, which are their potential prey.

"It is so rare to have an example in nature where we see the steps of evolution," Uetz says. "In this case, we demonstrated the adaptive value of a sequence of three co-evolved behaviors."

They include:

1) defensive strategies (spider egg-sac guarding),

2) behavioral ploys that circumvent defense (fly mimicry of trapped prey);

3) counter-ploy behaviors (spider signal thread-cutting).

" The fact these spiders can recognize the difference between the vibrations of wing beats between Arachnidomyia lindae and houseflies is amazing evidence of adaptive specialization," Uetz says.

The UC-led team used both "high-tech" and "low tech" approaches, with computer digitization, analysis and playback of vibrations, and experimental presentation of tethered predatory Arachnidomyia and houseflies to test spiders' reactions.

Publication of the work was also personally important to Uetz. His long-time collaborator and the study's lead author, Craig Hieber, died unexpectedly last summer. A specialist on the parasites that attack spider egg sacs, he was on the faculty of Saint Anselm College in New Hampshire.

"Publication of this is bittersweet," Uetz says. "We were best friends and worked together for 17 years."

An article about the new work appears in Nature Science Update, the online extension of the respected journal, "Nature." It can be seen at: http://www.nature.com/nsu/021125/021125-2.html

More details and photos from the project are available from Uetz's Web site at: http://www.biology.uc.edu/faculty/uetz/groupforaging.htm