The intricate soccer-ball structures in the image above are so tiny that you could pack few hundred of them into the width of a human hair. They’re probably the most beautiful non-stick coatings to have ever evolved.
These balls, known as brochosomes, were first discovered in the early 1950s. Scientists knew that they were found on the shells of leafhoppers—a group of colourful, sap-sucking insects that includes more than 20,000 species. They make the brochosomes within special glands in their guts, secrete them by the billions in a milky anal fluid, and spread them over their bodies using their legs. When the fluid dries, the brochosomes form a powdery coating, and the leafhoppers spread them even further using comb-like hairs on their legs.
Roman Rakitov from the Russian Academy of Sciences first saw the brochosomes when he put a leafhopper under a powerful microscope. His advisor (wrongly) told him: “Look, your leafhopper is coated with pollen!” “The published data on brochosomes were so scarce,” says Rakitov, “that pretty much everybody who later worked on them re-discovered much of the information about them independently.”
Rakitov became mesmerised by the brochosomes and wanted to work out what they were for—a mystery that had gone unsolved for almost 50 years. Since the 1970s, several scientists have noticed that the leafhoppers are great at repelling water, and suggested that the brochosomes might help. It was a nice idea, but Rakitov became the first to test it, with help from Stanislav Gorb from the University of Kiel.
If you put a drop of water on a table or plate, it will flatten out. The “contact angle” that it makes with the surface falls towards zero. But a leafhopper’s wing is so good at repelling water that a drop will sit upon it as a nigh-perfect sphere. It forms a contact angle of around 170 degrees—one of the highest ever recorded for a natural surface.
The brochosomes are responsible. The droplets sit upon their tips, and are cushioned by the pockets of air between them. Without these spheres to add roughness and texture, the naked leafhopper shell has a contact angle of just 120 degrees.
Other natural water-repellent surfaces like lotus leaves or guillemot eggs work in the same way, but they are made that way. The leafhoppers, however, waterproof themselves by actively applying a rough surface to their shells. There are a few other similar examples: Christoph Neinhuis from Dresden University told me about some plants that coat themselves with spores for a similar reason.
But leafhoppers are land-living insects—why do they need such good waterproofing? The brochosomes might help to repel rain, or even spider silk, but the classic explanation is that they protect the leafhoppers from their own waste. After sucking the sap of trees, these insects excrete any extra sugar as a sticky liquid. This is a serious hazard. If the liquid contaminates their shells and dries, it could stick the insect to a leaf, or glue its body parts together. Some bugs deal with this problem by shooting the waste away from their bodies at high speed. Others coat their droplets with a waxy powder. Perhaps the brochosomes are the leafhoppers’ solution?
Again, Rakitov and Gorb have tested this idea. They clipped the wings from some European leafhoppers (Alnetoidia alneti) and removed the brochosomes from half of them. They then placed the wings in a cage with 100 live leafhoppers, which rained a downpour of sticky waste upon them.
A week later, the duo found that 155 spots of dried waste had stuck to the wings, but only three had hardened on the ones with intact brochosomes. The naked wings had become dirtier; the brochosome-coated ones were almost totally clean. The duo had found strong evidence that the tiny spheres can indeed protect the leafhoppers from their own waste.
That’s one mystery down, but many more left to solve. How exactly do leafhoppers produce such intricate structures? Does their soccer-ball form make them more efficient at repelling water than other shapes? And how do they actually stick to the leafhoppers’ surfaces, and why do they often form long chains? They’re not glued, says Rakitov. “We don’t know but we plan to find out.”
Reference: Rakitov & Gorb. 2013. Brochosomal coats turn leafhopper (Insecta, Hemiptera, Cicadellidae) integument to superhydrophobic state. Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2012.2391
Rakitov & Gorb. 2013. Brochosomes protect leafhoppers (Insecta, Hemiptera, Cicadellidae) from sticky exudates. Interface. http://dx.doi.org/10.1098/rsif.2013.0445