How does a locust walk a ladder? A lot like you

When a person walks a ladder or perhaps a series of stepping stones, they rely on their vision to find each and every foothold. A new report published online on December 24th in Current Biology, a Cell Press publication, shows that locusts do something very similar. It’s yet another piece of evidence showing that when it comes to brains, size isn’t everything, according to the researchers.

“Visually guided limb control is often thought to be complicated and require sophisticated computations because you have to place your limb in a position you can only see, not touch,” said Jeremy Niven of the University of Cambridge. “The visual control of limb placement in the locusts suggests that this can be achieved by much smaller-brained insects. It’s another example of insects performing a behavior we previously thought was restricted to relatively big-brained animals with sophisticated motor control, such as humans, monkeys, or octopuses.”

There are some key differences in the way that insects make their way, the new study shows. Rather than relying on binocular vision as we do, the locusts apparently rely on visual input from a single eye to control the leg on the same side. People and other mammals including cats also use their vision both before and during a step. It appears that locusts commit to a particular foothold before a step; if something changes mid-step, they miss their target.

Niven said that he was driven to ask the question about the locusts’ ladder walking out of a long fascination for insect vision in all its enormous diversity. “Most studies of insect vision have concentrated on insects using vision during flight because insects such as bees and flies do spend a lot of time flying,” he said. “Other insects, such as stick insects, crickets, and cockroaches spend a lot of time walking, but they all have relatively small eyes and long antennae to ‘feel’ their way through the environment. Locusts spend time both walking and flying and have short antennae and large eyes. This started us thinking about whether it was possible for locusts to use vision to find footholds.”

The researchers set out to test this in a series of experiments in which they manipulated desert locusts and then filmed the locusts with high-speed cameras while they walked on a ladder, allowing the researchers to catch their mistakes and missteps. The researchers observed normal locusts, those with one eye painted over, those with antennae removed, and those lacking some sensors on their front legs. The researchers also watched what happened when a rung of the ladder was moved mid-step.

The bottom line, according to Niven: “The study really emphasizes how insects can achieve similar results to vertebrates like humans or cats with few neurons, probably by simpler mechanisms. Although the locusts place their legs using vision, the way they achieve this is quite different from the way humans or cats do.”

The researchers include Jeremy E. Niven, University of Cambridge, Cambridge, UK, Smithsonian Tropical Research Institute, Panama, Republica de Panama´; Christian J. Buckingham, University of Cambridge, Cambridge, UK; Sheila Lumley, University of Cambridge, Cambridge, UK; Matthew F. Cuttle, University of Southampton, Southampton, UK; and Simon B. Laughlin, University of Cambridge, Cambridge, UK.

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