Large animals think nothing of walking through a heavy mist: water droplets simply role off their hides. However, smaller insects are at constant risk of entrapment by the sticky forces of surface tension. Craneflies, which set up home in boggy settings and riverbanks, routinely encounter damp surfaces and mist that could prove fatal, yet they shrug off droplets with ease and can even stand on water. Jolanta Watson and colleagues from James Cook University and the University of Queensland, Australia, decided to take a close look at the insect's fragile legs and wings to find out how craneflies avoid getting stuck in water ().
Photographing cranefly legs at increasing magnification, the team could see that the insect's legs are covered with water-repelling hairs: thick long (90 μm) hairs with a rough grooved surface, shorter thick curved hairs, even shorter fine hairs and the shortest hairs of all found clustered at the base of the longest thick hairs. The insect's wings are also covered in fine hairs, with 12 μm long hairs distributed evenly across the membrane and 90 μm long hairs coating the wing veins.
To find out how repellent the hairy surfaces are, the team photographed water droplets on the insect's legs and wings. They saw that instead of spreading over the insect, the droplets formed perfect spheres, characteristic of the way water is repelled by a hydrophobic surface. And when they laid a cranefly leg on water, the hairs formed tiny dimples in the surface instead of piercing it.
Finally, the team tested how the grooves on the longer hairs help the insects repel water by coating the long hairs with hydrophobic polydimethylsiloxane to fill the grooves. Poking coated and uncoated hairs into water droplets, the team could see that the coated hairs no longer repelled water and penetrated the droplets with ease, while the uncoated hairs were unable to pierce the droplets.
So craneflies avoid getting trapped in sticky water with a coating of rough hydrophobic hairs. Watson and her colleagues are keen to design cranefly-inspired water-repelling and self-cleaning surfaces.
