Assassin bug sneaks up on spiders

10 Sep BBC

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.

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. 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.