The water lily beetle (Galerucella nymphaeae) is nature’s sprinter. It flits between water lilies in a flash, leaving nothing but a thin trail of ripples.
Researchers from Stanford University in the US used high-speed videos to capture the beetle’s flight, and found they walk a very fine line between being airborne and in the drink.
After watching water lily beetles zipping on ponds in the woods in western Massachusetts, senior study author Manu Prakash wondered: how do they do it? So he trapped a few, brought them home and filmed their passage across a plate of water.
The beetles created ripples in the water as though they were water skiing, but travelled at incredible speeds of up to 0.5 metres per second – equivalent to a human travelling at around 500 kilometres per hour! – propelled by their wings alone, as if they were flying while remaining attached to the surface.
Before taking flight, the beetles raise their middle pair of legs and stow them away underneath, then balance on the tips of the other four legs.
When ready to go, they open their wing case and unfurl their wings. The wings beat in a figure-8 pattern, 115 times per second, which propels them forward.
But instead of a smooth glide, the insects looked as if they were careering along a roller coaster as they flew across the ripple ridges they generated as they moved. “Almost like going on a road full of potholes,” says study lead author Haripriya Mukundarajan.
Then Mukundarajan and Prakash analysed the forces acting on the beetles as they slid across the surface. They realised the insects were playing a finely tuned balancing act between the water’s surface tension clinging to their claws and the lift generated by their wings, with surface tension keeping them firmly anchored at the surface.
According to Prakash, each wingbeat generates a force that momentarily pushes the insect down, making it bounce along the surface of the water – a bit like a pogo stick.
They published their observations in the Journal of Experimental Biology.