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Scientists Find New Twist in Spider-and-Fly Conflict

April 18, 1987

WASHINGTON (AP) _ Some spiders dislike other spiders as much as they like flies, and some of the flies have worked out a way to use this to their advantage, scientists report.

With flips of their unusual wings and some fancy footwork, several species of fruit flies can do a passable impersonation of a spider - good enough in many cases to keep them from being eaten by the real thing.

Taking on the guise of their worst enemy is a type of predator mimicry rarely seen in the camouflage-filled world of nature, researchers said Friday.

Separate research groups reported that their experiments prove earlier speculation that tephritid fruit flies can masquerade as jumping spiders to avoid capture by the predators.

The flies have a banding pattern on their wings resembling that of spider legs, along with false eyespots on the end of the abdomen. When threatened, the flies extend the wings and do a side-to-side dance, similar in appearance to the gait of a jumping spider, the scientists found.

The scientists, in reports published in the April 17 issue of the journal Science, said there are many examples of predators mimicking prey to aid in capture. Also, some animals copy color patterns to avoid detection and some adopt the more aggressive appearance of other animals.

″However, the case in which prey mimic their predators to avoid predation is rarely reported,″ wrote Monica Mather and Bernard Roitberg of Simon Fraser University in Burnaby, British Columbia, in one of the reports.

Erick Greene of Princeton University, lead researcher in the other group, said the spiders have special detectors in their eyes that only recognize patterns like leg waving.

″These spiders are very territorial and don’t like to see other spiders,″ Greene said in a telephone interview. ″When they see the movement and leg- like display on the fly’s wings, they think it’s another spider.″

Greene, working with Larry Orsak of the University of Georgia and Douglas Whitman of a U.S. Agriculture Department insect laboratory in Tifton, Ga., also found that the defense is very specific for jumping spiders.

″Other spiders were not deterred by this, and neither were the praying mantises or small lizards we tested,″ Greene continued. ″They ate the flies anyway.″

Mather and Roitberg tested snowberry flies found in Vancouver against their major enemy, the zebra spider. Spiders stalked their prey in a plexiglas arena and researchers recorded whether they fled from target insects or pounced.

The spiders pounced on house flies with clear wings 60 percent of the time, but only attacked the displaying snowflies on 20 percent of the occasions.

Greene and his colleagues did similar experiments with fruit flies captured in New Mexico. They used microsurgery to transplant the wings of house flies onto some fruit flies and vice versa.

The spiders attacked the normal fruit flies less often than all others, but aggressively pounced on house flies, house flies with striped wings and fruit flies with clear wings, the researchers found.

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