Spider-eating bug muffles web vibrations to sneak up on prey

An assassin bug (right) closes in

Fernando Soley/Macquarie University

By Alice Klein

Species: Giraffe assassin bug (Stenolemus giraffa)

Habitat: East Kimberley region, Western Australia

Silent assassin. A predatory Australian insect creeps up on spiders undetected by breaking their webs thread-by-thread and suppressing the resulting vibrations.

Spiders are highly attuned to disturbances in their webs, meaning that most predators try to minimise contact. Damselflies, for example, avoid touching the web by hovering nearby and plucking spiders off.

The giraffe assassin bug on the other hand, boldly captures spiders on foot. To understand how, Fernando Soley at Macquarie University in Sydney filmed dozens of the insects hunting spiders and used a laser to measure vibrations in web threads.

The long-necked insect is found in rocky escarpments in the Kimberley region of Western Australia, where spiders build complex webs in crevices. Its relatively large size – a 2-centimetre body and 5-centimetre leg span – means it has to break these webs to approach its prey.

To do so without generating giveaway vibrations, the bug carefully snaps individual threads with its forefeet and holds the ends loosely, Soley found. After anything between several seconds to a few minutes, it gently releases them.

Breeze in

The study also found that the insect prefers to break the threads when it is windy, perhaps because the wind impairs the spider’s ability to detect vibrations coming from the insect, Soley says.

After breaking the threads obstructing its path, the bug moves across the web with delicate steps.

This stealth is crucial because a single reckless move can be fatal. In up to 10 per cent of cases, the spider detects the bug and launches a counter-attack, Soley says. “Spiders have silk and venom, so they can work the prey from a distance, bite them and weaken them before finishing the job.”

The insect’s ability to trick spiders is unusual, says Mark Elgar at the University of Melbourne. “This is interesting because there are numerous examples of spiders using intriguing methods of capturing insect prey, but rather fewer instances of clever ways in which insects capture spiders.”

Journal reference: Royal Society Open Science, DOI: 10.1098/rsos.160573

Read more: Zoologger: Attack of the giraffe assassin bug