Why do leafhoppers reflect little light?

The leafhopper clings to the stem of the plant, enjoying its juice. But a ray of sunlight on its shell or wings can betray it and attract a predator. However, during evolution, the leafhopper has developed a solution that prevents it from being easily exposed. It secretes nanoscale, spherical structures, called brosomes, and applies them to its body. This acts as an anti-reflective layer. Although it has been known since the 1950s, experts did not know how it reduces light reflection. Tak Sing Wong of Pennsylvania State University and his colleagues have found the answer: the dimensions of these structures and the size of their holes are ideal for interacting with visible and ultraviolet light.

Under an electron microscope, these spheroids look like footballs that have had all their sides cut off, leaving just a bit of skin on either side of the seams. Its diameter is about 500 to 600 nanometers. Since their discovery, many hypotheses have been put forward about their optical properties. In fact, the wavelength of visible light ranges between 400 and 700 nanometers. “But at this size, the laws of optics indicate that the broccosomes must scatter light very strongly,” marvels Tak Sing Wong. Reverse the anti-reflective effect!

To investigate this, physicists recreated the brosomes by 3D printing before studying their optical properties. “It is a challenge because these structures are very small and have complex geometries,” the researcher warns. Using two-photon photopolymerization – which consists of building a polymer under the influence of two photons – they were able to reproduce the complex geometry of these structures, but not their nanoscale size. Their artificial brosomes are 20 micrometers in diameter, or 40 times larger. “The resolution of the print did not allow us to make it smaller. “So we built it larger and looked at the interaction with longer wavelengths of light.”

Through their experiments, they identified two aspects of procosome architecture that are involved in reducing light reflection. First and foremost, as they hypothesized, a broad band of visible light is strongly reflected by the outer surfaces of the brosomes. Since their size is similar to the wavelengths of visible light, they are effectively reflected in all directions. “This somehow creates an anti-reflection effect from the point of view of the observer – or predator – who only picks up a small portion of the light reflected by the leafhopper,” Tak Sing Wong explains.

In addition, by constructing brosomes with or without holes, they highlighted the importance of vacuole structure in reducing reflectance. Part of the UV radiation, whose wavelength is similar to or smaller than the diameter of the holes (150-200 nm), is trapped within the structure itself and is therefore not reflected. The combination of these two phenomena has the effect of reducing the reflection of visible and ultraviolet light by 80 to 94%.

In addition to better understanding the optical properties of these insects, researchers believe that the geometry of brocosomes could serve as an engineering model for designing anti-reflective, light-absorbing, or camouflaging layers and materials.

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