We often have great difficulties understanding how the world works on a miniature scale. For evidence, you have to look no further than the attempts of novels such as `Gulliver's Travels' and Hollywood films like `Honey, I Shrunk the Kids'. Despite being about the size of an insect, the humans in these novels and films move in almost exactly the same way as normalsized humans. Yet when you're as small as an insect, your movements become quite different from those of larger creatures, and the substrates that you walk on may have quite different properties too. One obvious example of this is the meniscus of water. Insects have overcome many problems to enable them to move about on the water surface, and several recent studies have provided insights into some of the mechanisms they use, such as the water repellent hairs on the legs of water striders. At the water's edge, the meniscus bends just like it does in a test tube. Although this bending might seem small to us, to an insect the bending of the meniscus is a significant obstacle. The meniscus is smooth, so to an insect it appears to be like a glass mountain that provides no purchase to allow it to climb using its normal walking gait. Yet insects do manage to `climb' this obstacle and get back onto land. How do they do it? The trick, it seems, is to keep perfectly still.
David Hu and John Bush from the Massachusetts Institute of Technology set out to discover the technique insects use to climb the meniscus. The pair filmed water-treading insects from several genera including Mesovelia(water treaders) moving on the meniscus and at the edges where the meniscus bends. Their high-speed videos revealed that the insects often made rapid movements to attempt to climb up the meniscus, but this strategy didn't get them anywhere. Instead, when the water-treaders held themselves completely rigid, Hu and Bush saw that the insects would glide effortlessly up the meniscus. In this posture, insects such as Mesovelia use small claws to pull up the meniscus at their front and back legs whilst pushing down with their middle legs – but why does this cause the insect to move forward?The answer lies in the lateral capillary forces that exist between small floating objects. These are the forces that attract bubbles towards one another on the surface of drinks, forming `bubble rafts'. The insects that move up the meniscus are making use of these forces by adopting the rigid posture that Hu and Bush observed. The theory that the pair developed to predict the speed of the insects as they approach the edge of the meniscus is remarkably accurate, suggesting that the lateral capillary forces really do account for the insects' movements up the meniscus. Insects specialised for life on the water surface are not the only ones to exploit the lateral capillary forces; Hu and Bush also filmed terrestrial insects using this mechanism to scale the heights of the meniscus at the water's edge.
Meniscus climbing is an unusual form of locomotion quite different from the mechanisms normally used by insects or other animals. Although this is one of the more surprising – and least dynamic – mechanisms insects use to move through the world, no doubt there are many more out there to be discovered.
