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Assassin bugs lure arachnid snack

Australian researchers have uncovered the sophisticated vibrating techniques used by the assassin bug to turn the tables on unsuspecting spiders.

The process, called aggressive mimicry, lures the spider within attacking range of the bug, immediately before turning it into a "spider milkshake".

Dr Anne Wignall and Associate Professor Phillip Taylor of Macquarie University in Sydney outline the spider-eating assassin bug's (Stenolemus bituberus) vibrating technique in a paper appearing today in the Proceedings of the Royal Society B.

It follows on from their previous paper, which focussed on the assassin bug's behaviour.

"This paper takes a more experimental approach," Wignall says, "we know that they're attracting the spiders somehow, but how are they doing that, what mechanisms are they using? So we looked at the structure of the vibrations, and at the spiders responses."

Under controlled conditions, Wignall and Taylor introduced a variety of things into webs of spiders from the Acharaeanea genus. They monitored the different vibrations created by male spiders, leaves, and two common prey animals: fruit-flies and aphids.

Treading a fine line

They found that the vibratory signature that the assassin bugs created most closely resembled that of prey insects.

But the would-be assassins must enter the spider's domain, and they tread a fine line between eating and being eaten.

Wignall says that it wouldn't be in the bugs' interests to be exact mimics, and that the surprising thing was to see just how finely controlled the vibrations were.

"For mimicry to work it doesn't have to be exactly the same, it just has to work. So they're not mimicking signals like the impact when a fly first hits a web. That produces a really loud high frequency vibration," she says.

"They're also not mimicking things like a fly buzzing its wings, a really fast movement, really high amplitude, higher in pitch; that would elicit a highly aggressive approach from the spider. This is not what the bug wants."

Sucked in, sucked dry

It all comes down to how well they can control the spider's responses, to slowly bring it closer. Wignall says the hunt can last for hours as the bugs slowly and carefully manipulate a spider's reactions.

"The assassin bug moves very carefully onto the web and it begins plucking the silk with its forelegs," she says.

"If it's not getting a reaction from the spider it will move a little further onto the web, move around, pluck a little bit more. Once the spider begins approaching it will often stop signalling and the spider will approach very slowly because it's not getting signals any more. Then it will resume plucking."

"Once it's [within striking range], the assassin bug will tap the spider gently with its antennae once or twice as it moves into position over the body of the spider. Then it grabs the spider really quickly and tightly with its forelegs and stabs it with its mouthparts, like a reinforced modified sharp straw.

"Usually the spider stops struggling within a few seconds. Then the bug starts to suck out the insides, kind of like a spider milkshake."

Wignall says the next step in their research is to look at the spider's decision making process.

"We're not sure what the spider's actually latching onto to say that this is prey, so we're going to concentrate on how the spiders actually recognise different stimuli in the web."