Here’s why some water striders have fans on their legs

For an animal already amazing enough to walk on water, what could growing feather fans on its legs possibly add?

These fans have preoccupied Abderrahman Khila of the University of Lyon in France, who keeps some 30 species of bugs called water striders walking the tanks in his lab without getting their long, elegant legs wet.

“Walk” may be too humdrum a word. The 2,200 or so known species of water striders worldwide can zip, skim, skate and streak. Among such damp-defying acrobats, however, only the Rhagovelia genus grows a fan of delicate feathers on the middle pair of its six legs. Even little hatchlings head-banging their way out of underwater eggs have a pair of feathery microfluffs for their perilous swim up to cruise the water’s surface.
A first guess at a function — maybe plumes help support bigger adults — would be wrong, Khila says. The Rhagovelia are not giants among water striders. In a jar of alcohol in his lab, he treasures a specimen of a much bigger species, with a body about the size of a peanut and a leg span that can straddle a CD. Yet this King Kong among striders, found in Vietnam and China, slides over the water as other species do, cushioned by air trapped in dense hydrophobic leg bristles. No froufrou feathers needed.
Fans aren’t required either for water striders’ action-packed, often violent lives. “In the lab, they eat each other all the time,” Khila says. A newly molted strider, still soft and weak after 10 minutes of wriggling out of its old external skeleton, can get mobbed by cannibals. Any other insect, such as a mosquito, that lands on the water surface also triggers a frenzy. Small striders “start to attack the legs of the mosquito,” he says, “and seconds later there are 50 water striders gathered around.” With their tubelike mouthparts, the striders stab holes in the victim and inject enzymes to liquefy flesh into a meat shake to suck out.

For these Rhagovelia, Khila sees the fans as “one of those examples of ‘key evolutionary innovations,’” traits that just “pop up” in evolutionary history with no clear line of precursors or partial forms, he says. Now he and his colleagues have identified a fan benefit. When they removed plumes from the bugs or suppressed genes for fan formation, the mutant striders couldn’t turn as deftly or run upstream against the current as fully fanned Rhagovelia can, the researchers report in the Oct. 20 Science. Striders in a closely related but fanless genus were likewise hampered. The innovative fan opened up new territory, helping the insects navigate flowing water, the researchers conclude.

Fan-maker genes were intriguing in another way. Evolutionary biologists have long debated whether such evolutionary innovations just repurpose and recombine old developmental genes or actually rely on new ones. In the case of the fans, two genes, which the researchers named geisha and mother-of-geisha after geisha fans, are unique to this genus, but three other genes are repurposed. So in a twist on an old debate, Khila says, “neither hypothesis is wrong.”