Zoologger: Attack of the giraffe assassin bug

Zoologger is our weekly column highlighting extraordinary animals ? and occasionally other organisms ? from around the world

Species: Stenolemus giraffa
Habitat: rock escarpments in Australia, sneaking up on spiders

Every so often, a species sticks its neck out ? literally. Over the aeons, a few groups have evolved disproportionately long necks, most famously modern-day giraffes and the long-extinct sauropod dinosaurs.

Long necks are a useful tool for finding food. Giraffes eat plants, and their elongated necks help them reach tall trees that other animals can't. Some sauropods probably did the same thing. This sort of lifestyle is sedate and ? barring predators or massive incoming meteorites ? rather peaceful.

The giraffe assassin bug has an altogether more active life. This little-known insect has a neck that makes up about half its total body length, but it doesn't use it to munch leaves. It uses it to lean into spider webs and take their owners by surprise.

Assassin's feed

Assassin bugs are aptly named. They are predatory insects, many of which use sneaky tactics to bring down prey much larger than themselves.

First described in 1956, the giraffe assassin bug is one of the most peculiar. Its neck ? technically called the pronutum ? can make up half of its 4-centimetre length. "No other species of assassin bug has this remarkable elongation," says Fernando Soley of Macquarie University in Sydney, Australia.

The giraffe assassin bug uses its neck to pull off a particularly dangerous trick: eating spiders. For most insects, spiders are dangerous predators, but the bug turns the tables. It targets spiders on their webs, despite the risk of being caught in their sticky threads. With Phillip Taylor, Soley investigated how the bug does it.

On the prowl

Taylor pitted captive bugs against two different web-building spiders, both of which they eat in the wild. Argiope katherina builds the familiar orb web, anchoring it between two vertical rocks, then perching in the centre of the web. Meanwhile, Trichocyclus arawari constructs dome-shaped webs underneath overhanging rocks, then sits on the underside of the dome.

Soley found that the giraffe assassin bugs did everything possible to avoid stepping onto the spider webs. "Webs are very sensitive to touch and are also sticky traps, so it makes sense that they want to avoid them," he says.

When pursuing T. arawari, the bugs often climbed onto the overhang. That got them close to the spider, but the web was still in the way. Worse, to get there they had to break some of the mooring threads holding the web in place. The spiders often detected this, and fled.

Faced with these obstacles, the bugs' favourite tactic was to clamber onto the surrounding rocks, getting as close as possible to either A. katherina or T. arawari without touching their webs. Then they leaned in, using their necks to stretch across the distance and reach the spider. After carefully tapping it with their antennae, they struck, stabbing the spider with their mouthparts.

Let's just hope giraffes never find out about this.

Journal reference: Animal Behaviour, DOI: 10.1016/j.anbehav.2012.04.016

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