Tiny froghoppers and planthoppers are the elite of the insect athletic world, launching themselves into the air at an impressive 5 m s–1. However, hefty locusts still give them a respectable run for their money, pulling off impressive take-offs at 3 m s–1. Malcolm Burrows and colleagues Tim Bayley and Greg Sutton from the University of Cambridge, UK, explain that the forces exerted on the leaping insect’s hind legs come within 20% of wrecking the limbs. This puts the insects at risk of serious injury when things go wrong, either during take-off or when they misplace a kick. Filming locust take-offs, the team discovered that the leaping insects’ hindlimbs buckled when they slipped and lost their footing. Explaining that William Heitler had also noticed a region of the locust hind limb just below the knee that appeared to buckle when the insects kicked (Heitler, 1977, J. Exp. Biol.67, 29-36), Burrows and his colleagues decided to find out how much the insects’ legs deform and how they protect themselves from injury when a jump or kick misfires (p. 1151).
Filming leaping locusts at 1000 frames s–1 as they lost their footing, the team saw the slipping leg fly out before the insects took off, with the misplaced leg swinging 13 times faster than it had during a successful jump. And when they looked at the limb in closer detail, they saw that the tibia bent at angles ranging from 2 to 38 deg just below the knee joint, in the same location where Heitler had seen the tibia of kicking locusts buckle. Then the limb bounced up and down, repeatedly buckling at the same location until it had dissipated all of the misdirected energy.
Next, the scientists filmed the insects kicking and missing their targets. This time the limb swung even faster, bending in the same region by as much as 48 deg. However, when the insects aimed a successful kick, the tibia remained completely straight. Finally, the trio painted nail polish on the region of the tibia that buckled. This time, instead of bending, the tibia remained straight and bounced back and forth around the knee joint.
Having filmed the tibia’s deformation, the team calculated the amount of energy absorbed as the leg buckled and it was an impressive 88% of the kick’s energy. They also directly measured the amount of energy stored in the flexible region by bending the leg with a servo-motor, and found that the limb initially absorbed over 1000 μJ during the first kick, falling to 600 μJ during subsequent kicks, but recovering to the initial value after 24 h.
Intrigued by the tibia’s recoverable crumple zone, the colleagues shone UV light on the limb. Explaining that the elastic protein resilin fluoresces under UV light and that the remarkable material turns up in a wide range of flexible structures, Burrows saw the material’s tell-tale violet glow exactly where the tibia buckled. So the flexible zone contains elastic resilin, which allows the leg to bend without snapping.
Finally, knowing that there is a cluster of campaniform sensilla (mechanoreceptors) on the tibia, just below the buckling region, the team decided to find out whether these sensors respond when the tibia deforms. Recording nerve signals from the sensors, the trio reproduced the leg’s kicking movement and found that they did fire when the leg buckled. The team explains that the mechanoreceptors’ response is too slow to allow the locust to react when the leg buckles during a mistimed jump or kick. However, they suspect that the mechanoreceptors could influence the insect’s subsequent behaviour or participate during the preparatory phase of a jump, when the tibia flexes and the flexible region deforms slowly.