Sir

Because of the contact between phases, transport of liquid droplets on solid surfaces is a complicated, non-efficient task requiring a considerable amount of energy. Increasing the contact angle between the liquid and solid (hydrophobicity) above 120° should ease the motion. By coating a small amount of liquid with grain spores of Lycopodium, Aussillous and Quéré1 report their creation of non-sticky liquid marbles (contact angle of 180°). The droplets are soft solid and elastic, and can bounce and roll on a solid surface without leaking. It was predicted in the News and Views article2 accompanying this paper that this brilliant trick will have a rapid practical use in wear-free micromachines and lubrication.

Yet tiny, relatively primitive insects faced and solved a similar problem millions of years ago. Homopterans (aphids, whiteflies, and so on) feed on plant sap (usually phloem) that contains a high concentration of sugars but low nitrogen levels. Therefore, the extra sugars are deposited by the insects as sticky droplets, commonly known as honeydew. Some aphids live in galls where they face the danger of contamination and drowning in their own honeydew. These insects coat the honeydew with wax secreted from specialized glands3.

Furthermore, some gall-forming aphid species clean their galls by pushing the coated honeydew outside4,5,6. The coated elastic honeydew droplets can be pushed, rolled and squeezed without leaking or wetting. Smith3 noted that this behaviour was initially observed by Buckton in 1876 and may have promoted the evolution of social organization in aphids7. Practically, it is worthwhile examining whether aphid's wax is better at coating liquid droplets than are the spores of Lycopodium.

In short, 'liquid marbles' are yet another example of how insects 'developed' a technology through natural selection long before humans got around to it6. Technologists (high and low) and engineers should look for new solutions with the eyes of biologists.