Water strider leg fans help them navigate streams
Certain species of water striders grow fans on their legs, allowing them to navigate habitats that other water striders cannot.
You may have seen water striders skating around on a calm pond or lake, making tiny ripples in the water’s surface. Water striders, also known as pond skaters or Jesus bugs, are semi-aquatic insects whose long legs allow them to distribute their body weight wide enough to balance on top of water without breaking the surface tension. Their legs are covered with microscopic hairs, which prevent them from getting wet and therefore weighed down, and trap tiny air bubbles under their feet, allowing them to actively repel the water beneath.
In addition to the hairs, some water striders grow feathery fans on the middle pair of their six legs. There are over 2,000 kinds of water striders, but only one genus — Rhagovelia — grows the fluffy fans. Unlike the hairs, the function of these fans is a mystery. This mystery captivated biologist Abderrahman Khila, who filled his lab with aquariums containing dozens of different water strider species in order to study them.
Khila, who researches at the University of Lyon in France, first speculated that the leg fans might give extra support to the hairy legs on Rhagovelia, allowing them to grow bigger while maintaining their ability to stay on top of the water. However, Rhagovelia are smaller than some other types of water strider that lack the leg fans. (Water striders in Southeast Asia can have legspans the diameter of CDs.)
The fans also didn’t appear to give Rhagovelia an edge in their fiercely competitive lives. Water striders often engage in violent feeding frenzies and exhibit cannibalistic tendencies. “In the lab, they eat each other all the time,” Khila explains. Most cannibalized water striders are attacked just after molting, when they are no longer shielded by a protective exoskeleton.
To solve the mystery of the fans, Khila examined the other trait that sets Rhagovelia apart from other water striders: their ability to make quick changes in direction and stay atop the water’s surface even when the water is fast-flowing. In some Rhagovelia specimens, Khila and his colleagues suppressed the genes that caused the fans to grow; for others, they removed parts of the fans. The mutated Rhagovelia were no better at making quick turns and navigating fast currents than other species of water strider that naturally lack the leg fans.
Khila considers the leg fan trait “one of those examples of key evolutionary innovations” that “just pop up” and turn out to be highly beneficial to a species. The emergence of this trait allowed Rhagovelia to literally explore new territory: to move from ponds and lakes to fast-flowing streams.
The results of the water strider experiments were published last month in Science.