Hairy-necked tiger beetle. Credit: Daniel Zurek
Hairy-necked tiger beetle. Credit: Daniel Zurek

Meet The Predator That Becomes Blind When It Runs After Prey

The tiger beetle can run so fast that it blinds itself.

There are 2,600 species of these long-legged predatory insects, and the fastest can sprint at up to 5 miles per hour, covering 120 of its body lengths in a single second. For comparison, Usain Bolt covers just 5 body lengths per second. To match the beetle, he’d have to run at 480 miles per hour.

Tiger beetles use this incredible speed to run down both prey and mates. But as they sprint, their environment becomes a blur because their eyes simply can’t gather enough light to form an image. They have extremely sharp vision for insects, but when they’re running, the world smears into a featureless smudge. To compensate, the beetle has to stop to spot its prey again, before resuming the chase.

It seems like a bad evolutionary joke: a hunter that loses sight of its prey whenever it runs.

But tiger beetles don’t mind because… well… they are really, really fast. They can afford to stop in the middle of a chase because they are so ridiculously quick when they’re in motion. It’s like the aforementioned Bolt pausing at the 50-metre mark for a drink, and still winning.

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Hairy-necked tiger beetle. Credit: Daniel Zurek.

Cole Gilbert at Cornell University discovered the tiger beetles’ staccato hunting style in 1998. Now, together with Daniel Zurek, he has worked out how they cope with another problem: obstacles.

At high speed, it’s hard enough to avoid incoming obstacles. But try doing it when your eyes can’t make out anything, much less small pebbles or sticks. A running tiger beetle is permanently in “collision mode”, says Zurek. “It’s like when I’m driving a car really fast and not wearing my glasses. When something hops in the road, I can’t stop in time.”

He discovered how they cope by watching an American species—the hairy-necked tiger beetle, Cicindela hirticollis. When it runs, it always keeps its antennae in the same fixed position: straight ahead, angled at a V, and held slightly above the ground. The antennae can move, but they never do while the beetle’s in motion.

The antennae are obstacle-detectors. If they hit an obstacle, their flexible tips bend back before springing forwards again. The beetle moves too fast to change course, but it can tip its body slightly upwards so that it skitters over the obstacle rather than running headlong into it. It’s like a blind person holding two white canes (and wearing rocket skates).

“Because of their shape, the antennae can slip over the edge of an obstacle, which tells the beetles that there’s a top they can run over,” says Zurek. He saw how effective this is by filming tiger beetles running down a long track with a piece of wood in the middle. If their antennae were intact, they cleared the obstacle most of the time, even when Zurek painted over their eyes. But if he cut the antennae off, the beetles frequently face-planted into the wood.

This solution is not only effective, but cheap. The beetles could potentially deal with motion blur by evolving more sensitive eyes, but it takes a huge amount of energy to pay for an eye with good temporal resolution. They would also have to analyse that information, and their small brains probably don’t have the processing power. Fortunately, they don’t need anything that over-engineered.  Their antennae provide them with all the collision-detection they need.

Zurek thinks that human engineers should take note. One of the first autonomous robots—Shakey—found its way around with some “bump detectors”. If they hit an obstacle, they bent, and Shakey would back up.

But modern robots rely on cameras. NASA’s Curiosity rover, for example, is currently trundling over Mars with the help of eight hazard avoidance cameras, or Hazcams. “As humans, we tend to think first and foremost from a visual standpoint,” he says. “Many really sophisticated robots rely on an array of cameras that analyse on the fly, which is very computationally intensive.” The tiger beetle’s solution would be simpler, and might help robots to move much faster than Curiosity’s leisurely pace.

PS: How does one catch an insect that moves so quickly? With great difficulty at first, but Zurek says, “It’s pretty fun once you get the hang of it,” he says. “You have to fool them by coming up behind them really slowly and then lowering yourself. I get them around 60 percent of the time.”

Reference: Zurek & Gilbert. 2014. Static antennae act as locomotory guides that compensate for visual motion blur in a diurnal, keen-eyed predator. Proceedings of the Royal Society B http://dx.doi.org/10.1098/rspb.2013.3072