These beetles waterski, scientists say. How do they do it? (+video)
Waterlily leaf beetles don't have to lift off the water to move around a pond. Instead, they can use their wings to propel along the surface of the water as if waterskiing.
If you watch a waterlily leaf beetle hanging out on the surface of a pond, you might see it seemingly disappear and then reappear somewhere else. That may not seem all that strange for a flying insect, but a mysterious line of ripples hints otherwise.
The little beetle isn't flying from place to place precisely as you might think. It's actually moving as if waterskiing, flying straight over the pond with its feet still touching the surface of the water.
Curious about how the waterlily beetles do it, scientists used high speed video cameras to take a closer look. They describe their findings in a new paper published in the Journal of Experimental Biology.
"It blows your mind away when you look at these things," Manu Prakash, who runs the lab at Stanford University that conducted this study, tells The Christian Science Monitor in a phone interview.
How do they do it?
Before a waterlily beetle takes off, it prepares its body. The insect lifts its middle pair of legs off the water to limit drag and tilts its body up, so its chest is lifted. Then, with its front and back legs touching the water, the beetle flaps its wings to thrust its body across the surface of the water.
"This behavior is fascinating," Adrian Thomas, who studies the biomechanics of animal flight at the University of Oxford but was not part of this study, tells the Monitor in a phone interview.
Someone might expect that such a mode of transportation takes too much effort, says Sanjay Sane, who also studies insect flight at the National Centre for Biological Sciences in India, but was also not part of this study. And it's therefore puzzling as to why the beetles' waterskiing flight evolved, Dr. Sane writes in an email to the Monitor.
Perhaps the beetles are taking advantage of surface tension to support their body weight, the new study suggests. Or, Dr. Thomas adds, the cushion of air created in the space between the insect's wings and the water could also help buoy its weight.
That increased force supporting the insect in staying aloft is called the ground effect. Those same physics are employed to build vehicles, like the Ekranoplan, a vehicle that looks like a plane but cruises atop large bodies of water.
"To me, this looks like a beetle that is behaving like an Ekranoplan, like a ground effect vehicle," Thomas says.
The waterlily beetle's legs might also have something to do with how the insect waterskis.
Most of the beetle's underside is covered with water-repelling, hydrophobic, hairs. But at the tip of each leg is a hydrophilic claw that could help the animal adhere to the surface of a pond. This combination could create a balance between keeping the bug from becoming submerged and anchoring it.
Perhaps this behavior evolved in order to exploit an ecological niche, says Dr. Prakash.
"Water acts like a superglue" for insects because of their small size, he explains. "Once they attach, they cannot detach themselves," so going near the surface of a body of water is dangerous.
But the waterlily beetles have apparently evolved to be able to live in that very environment. The beetles depend on waterlilies for food and a place to reproduce, so they need to be able to survive close to the surface of the water.
"The fact that evolution ends up actually designing this solution is just absolutely beautiful," Prakash says.
Sane adds, this "greatly expands our horizons in terms of what insects (and other animals) are capable of achieving in their struggle for survival."