Assassin bug sneaks up on spiders

A Stenolemus bituberus assassin bug (Image: Anne Wignall) Assassin bugs use the wind as a "smokescreen" to cover the vibrations of their footsteps

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Assassin bugs hunt spiders on their webs by stalking or luring their victims before stabbing them with their long, sharp snouts.

Researchers studying these aggressive arthropods have now found that they use noise to cover their tracks.

The bugs wait for the wind to rustle the web, then take the opportunity to sneak up on their prey.

The findings will be published in the journal Animal Behaviour.

The bugs they studied, Stenolemus bituberus, are found in eastern and northern Australia. They live and hunt in spider webs on tree trunks or in rocky crevices.

The team, led by Dr Anne Wignall from Macquarie University in Sydney, brought juvenile assassin bugs into their laboratory.

Start Quote

It's very exciting thing to see a predator using vibrations to dupe their prey”

End Quote Dr Stimson Wilcox Binghampton University

They had spiders, of several species known to be preyed on by the bugs, build their webs in special wooden frames in the laboratory. The frames allowed the scientists to vibrate the webs, creating background "noise" and observing how the bugs reacted.

Dr Wignall and her team used the webs to find out exactly how the bugs avoided being spotted as they stalked their prey.

"Web-building spiders have only rudimentary eyesight, so avoiding being seen is not an issue for web-invading arthropods," the team explained in the paper.

"The main sensory system of web-building spiders is based on interpreting vibrations in the web - web silk is exquisitely proficient at transmitting vibrations from potential prey and predators in the web."

'Smokescreen'

The researchers placed the bugs onto the spider webs and used a desk fan to simulate a breeze on the web.

When the fan was on, "the assassin bugs stepped more often and walked in a more continuous manner", the team explained. They described this tactic as "opportunistic smokescreen behaviour".

"Generally, noise is considered a big obstacle that needs to be overcome, such as when trying to communicate or when searching for predators," Dr Wignall told BBC Nature.

"The exciting thing in this study is that the assassin bugs can increase their chances of catching food by using wind noise as cover."

The breeze did not seem to trigger the assassin bug to move when it was in an unoccupied spider web. This, the team wrote, suggested that "noise-related timing of behaviour reflects decisions made as part of a predatory strategy", rather than just a response to the physical movement of the web.

The assassin bug stalks its victim with a strange, erratic gait - Footage courtesy of Anne Wignall, Macquarie University

Assassin bugs also have a strange way of moving, and the scientists think this bouncing gait - "gently erratically rocking in the web" - may make it more difficult for the spiders to identify the characteristic vibrations of footsteps on silk.

The bugs are perhaps "simulating debris moving in the wind", explained the researchers.

In a previous study, published in the Proceedings of the Royal Society B, the same team discovered that the bugs lured spiders by "pretending" to be prey tangled in the web. They tapped the silk to mimic the vibrations of a trapped insect.

When the spider approaches - or when the assassin bug gets within striking distance - the predator strikes with its tube-like mouthparts, through which it injects the victim with a toxin that liquefies its insides so they can be sucked out.

Dr Stimson Wilcox from Binghampton University in New York, one of the co-authors of the article said: "It's very exciting thing to see a predator using vibrations to dupe their prey in this way.

"The use of vibrations to hunt is a fairly new phenomenon [to science]," he told BBC Nature. "This suggests that it's probably quite widespread."

Dr Wignall added: "We're now starting to find more evidence for species using vibrations during hunting."

But, she said, there was "still a lot to uncover about how the assassin bugs use these strategies".

"We'd like to know how much the species of spider influences their strategy, and how the spider's perceive the stimuli that the assassin bugs are generating."

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