These insects fly with their legs. Physics explains how

DENVER — For these insects, flight is all in the legs.

Phantom crane flies float lazily on breezes, their six long legs splayed and wings held utterly still. Wind tunnel experiments are beginning to reveal how they stay aloft. The insights are helping researchers build miniature flying machines that mimic the crane flies’ easygoing flight strategy, physicist Sarahi Arriaga-Ramirez reported March 17 at the American Physical Society’s Global Physics Summit.

The Eastern phantom crane fly (Bittacomorpha clavipes) lives in the eastern United States. The flies live only about a week at full maturity, during which the insects mate, but don’t eat. “They have very limited energy, and they have to save it,” says Arriaga-Ramirez, of the University of California, Berkeley.

When viewed with high-speed cameras in still air in a laboratory, phantom crane flies flapped their wings to ascend, holding their legs vertically, presumably to reduce drag so that they can fly more easily. But that changed in a wind tunnel’s updraft: The insects held their wings still and splayed their legs out and up into an inverted cone shape, reminiscent of an inside-out umbrella or a fluffy dandelion seed — both of which are adept at catching the wind. The flies floated on the breeze due to the drag caused by their legs.

As conditions changed, the shape of the cone adjusted. In a more forceful updraft, the arrangement of the flies’ legs became narrower.

To understand the impact of the changing cone shape, the researchers made larger-than-life, 3-D printed models of a phantom crane fly and dragged them through a tank of mineral oil. Swapping air for oil simulated the conditions a real, smaller fly would experience, because on small scales, air acts as if it’s more viscous. The results revealed that a narrow cone produced less drag than a flatter one, by about 20 percent. That suggests that the crane flies’ legs adjust to provide a gentler lift.

The insect’s quirky mode of transport inspired the researchers to tinker with various designs for small, energy-efficient aerial vehicles. One option used a material called a shape memory alloy to allow the crane fly bot’s legs to bend on command. Such materials return to a previous shape when an electric current is passed through them. Coils made of these shape memory alloys moved the legs from straight to bent, and back again.

It’s unclear how much phantom crane flies control their legs, and how much they are just blowing in the wind. So the team also tried legs with flexible joints that allowed passive movement. As the wind speed increased, the legs automatically bent, giving the vehicle a narrower shape.

That results in very stable flight, Arriaga-Ramirez said. Even if the air flow around the aerial vehicle is disturbed, the vehicle, like the phantom crane flies, is unperturbed.