The Bug That Stalks Spiders on Their Own Webs

It walks stealthily, slices through defenses, and cloaks itself in wind.

Fernando Soley

Spiders don’t make for easy prey. They are almost all venomous and almost all predatory. Many build webs whose silken lines ensure a sticky end for blundering insects. And those webs are, in a very real way, extensions of the spiders: By carrying the telltale vibrations of intruders, they act as both burglar alarm and death trap.

So it’s a bold kind of insect that hunts spiders for a living, a positively foolhardy one that hunts them on their own webs, and a seemingly suicidal one that does so on foot.

And yet, that’s exactly what the giraffe-necked assassin bug does.

It’s a bizarre-looking creature from northern Australia, with a spindly inch-long body, arms like a praying mantis, and a long neck that makes up half its body length. It kills by ramming its sharp snout into its prey and sucking the juices out. But to do so, it must get close. And since this assassin bug targets web-building spiders, that means venturing onto the silken strands.

Fernando Soley, from Macquarie University, the first person to properly study this species, has shown that it succeeds by using stealth. If it can, it will stay on a nearby rock and lean into the web, snatching the spider from afar. If it’s forced to actually enter the web, it does so with gradual steps, punctuated by lengthy pauses.

Soley recently quantified the assassin’s stealth by capturing some and releasing them onto artificial webs—strands of spider silk that he had strung up in front of a laser that can measure vibrations. And the lasers measured nothing. The bugs walked over the web and tapped the strands with their antennae without creating the slightest rustle. “By simply stepping onto a spiderweb, the assassin bug is stepping onto the sensory system of its intended prey,” says Fiona Cross, from the University of Canterbury, who studies spiders. “In such situations, a cautious approach would be highly advisable!”

More surprisingly, the bugs can also break the web strands with impunity. If a thread stands between them and their spider prey, they’ll simply grab it with their front legs and pull it apart. It’s a risky move. The threads of a web are held in tension, like taut rubber bands. If you cut them, the two ends ought to spring back, creating vibrations that would give away an interloper’s presence. And yet, astonishingly, the spiders don’t notice. The bug can even break threads that are right next to a spider without drawing attention to itself.

Soley noticed that when a bug snaps the threads, it holds on to the disconnected ends and lets them to sag toward their points of origin before releasing them. It does so one at a time, and often separated by long delays. All of this allows the bug to vandalize the web without creating vibrations, or at least, none that are distinguishable from background noise and wind. In fact, Soley also found that the bugs are more likely to break threads in breezy conditions. It’s an assassin that camouflages itself with wind.

“This shows that the assassin bugs are sensitive to the risk of detection, and adjust their behavior accordingly,” says Anne Wignall, from the University of Massey. “They’re not just automatons going through the motions of a fixed predatory strategy.”

She should know. Wignall studied a close relative of the giraffe-necked assassin bug—and one with an even riskier strategy. Rather than stalking its victims by muting its vibrations, it summons its prey by causing vibrations. It strums a spider’s web with its legs to mimic the vibrations of prey. The spider rushes in to investigate, and the bug pounces.

Wignall showed that the bug’s vibrations are very precise. They’re short, high-pitched, and gentle—very similar to those created by a small wriggling insect, and distinct from falling debris. The vibrations aren’t too strong either—that would tell the spider that a large insect had hit the web, and compel it to rush in aggressively. The assassin bugs, by not triggering such attacks, lure in their meals without becoming meals themselves.

Spiders remain dangerous prey. The assassin bugs, for all their stealth, only catch their prey around 20 percent of the time, and about one in ten of them become meals for the spiders. They also occasionally throw caution to the wind: Around 7 percent of the time, the giraffe-necked bug will snap a web thread forcefully, with no attempt to dull the resulting vibrations. “It would be really interesting to learn more about when this bug uses the reckless tactic,” says Cross.