Spider and fly swap roles

JOHN WHITFIELD

Spiders are sensitive to the sound of approaching flies. © G. Uetz

It seems suicidal. The fly zooms into the heart of a mass of spiders, and then to the centre of a web.

This is cunning, not recklessness, ecologists have found. Twanging the web like a struggling insect, the fly lures the mother spider away from her clutch. The fly then nips in and lays a predatory grub on the eggs¹.

The Mexican spider Metepeira incrassata builds its webs in colonies up to 100,000 strong and 160 metres long. To the fly Arachnidomyia lindae these are more crèche than death-trap.

The fly larva needs about a quarter of an hour to chew into the spider's egg sac - then it can devour the eggs in peace. If the spider discovers the intruder before this, it will flick it away.

The spider recognizes an approaching Arachnidomyia from the buzz of its wings, George Uetz of the University of Cincinnati has discovered. "They zip around and try to keep their body in front of the eggs," he says.

To distract the spider, the fly hangs onto the centre of the web, where the silk is not sticky, and vibrates the web. This gives the spider a dilemma - she can either investigate or guard her eggs.

"After a number of fruitless back-and-forth runs, the spider severs the connection between the egg sac and the web," says Uetz - to cut herself off from further misinformation. Once the distraction seems to have passed, she repairs it.

The spider's powers of discrimination are impressive - and so are the fly's powers of mimicry, says entomologist Linda Rayor of Cornell University. "Spiders are really into vibrations - they pay attention to them and they're good at reading them." Some other species, including spider-eating spiders, do similar impersonations, Rayor adds.

Pull the other one

To test the spider's early-warning system, Uetz's team held tethered, buzzing flies near to the spiders. In another experiment, they played the sound of fly wing-beats through tiny speakers.

Spiders are good at reading vibrations Linda Rayor Cornell University

Arachnidomyia's wing-beat puts the spider on alert, they found. A housefly has no effect - Metepeira can tell a threat from a meal on sound alone.

The researchers have become so expert at spider-scaring that they can spread panic by blowing across the top of a soft drink bottle, once the right amount has been drunk from it. "You can get the whole colony spinning," says Uetz.

1. Hieber, C. S., Wilcox, R. S., Boyle, J. & Uetz, G. W. The spider and fly revisited: ploy-counterploy behavior in a unique predator-prey system. Behavioral Ecology and Sociobiology, advance online publication, doi:10.1007/s00265-002-0547-2 (2002). |Article|

© Nature News Service / Macmillan Magazines Ltd 2003