Cats are famous for landing on their feet after a fall, but they aren’t the only animals that do so. The tiny pea aphid can also right itself in mid-air, and it does so in a way that’s far simpler than a falling feline.
Pea aphids face many dangers, including parasitic body-snatchers and predators. And since they spend their time sitting on plants, they could be inadvertently eaten by hungry grazing mammals. The aphids have no aggressive defences to deploy. Instead, they escape by falling. If they smell the breath of a grazer, they’ll release their hold on their plant, and tumble to safety.
Moshe Gish from the University of Haifa was studying this behaviour when he noticed that the aphids always rotate their bodies during their descent to land on their feet. To test this ability, he teamed up with Gal Ribak from the Israel Institute of Technology. They placed aphids on fava beans hanging over a layer of petroleum jelly, and threatened them with a ladybird—one of their natural predators. The aphids fell off and the jelly preserved the outline of their impact. Up to 95 percent of the insects landed on their feet.
How do they manage? Cats do it by twisting their flexible backbones, but the aphids have no such specialised structures. Instead, the secret is in the way they hold their limbs. When Ribak and Gish dropped dead aphids from a height, only 52 percent landed feet-first. If the duo amputated the insects’ limbs, things got even worse—just 28 percent landed the right way up. A limbless aphid is even worse at landing than a dead one.
With high-speed videos, Ribak and Gish found that a falling aphid always move its antennae forward and upwards, while holding its back legs above its body. This odd posture makes it look like a tiny insect base-jumper. It also ensures that the only stable orientation is a feet-down one. As the aphid falls, the forces acting upon it automatically rotate its body so that it’s the right way up. Unlike a cat, it doesn’t need to actively twist anything. By splaying out its appendages, it relies on physics to passively right itself.
Why does this matter? At their small size, the fall won’t kill or injure them. However, the ground is full of danger too, including a different set of predators and a lack of food. But falling aphids might never hit it. Ribak and Gish found that when the aphids right themselves, they can use their sticky feet to grab onto lower parts of their host plant as they fall. If they hit those same parts with their backs, they simply bounce off. By righting themselves mid-air, they escape from danger above, without falling into danger below.
Many other insects can control their falls. For example, some species of gliding ants and bristletails, which live in the rainforest canopy, can steer their descent well enough to land upon the trunk of their home tree. It’s possible that these primitive aerobatics were the forerunners to true insect flight, providing a stage for the evolution of wings.
Reference: Ribak, Gish, Weihs & Inbar. 2013. Adaptive aerial righting during the escape dropping of wingless pea aphids. Current Biology http://dx.doi.org/10.1016/j.cub.2012.12.010