Brochosomes Offer Protection From Sticky Liquid

Biological Strategy

Silver leafhopper

Eleanor Banwell & AskNature Team

Image: Mick E. Talbot / CC BY SA - Creative Commons Attribution + ShareAlike

Protect From Excess Liquids

While water is essential to life, too much water or other liquids can overwhelm living systems. Excess liquids can, for example, decrease a living system’s access to oxygen, promote excessive bacterial or fungal growth, or strip away soil and nutrients. To prevent the accumulation of excess liquids, living systems must control the movement of liquids across their boundaries or surfaces. They do so using waterproofing materials or structures, slowing flow, and/or facilitating flow to move the liquid away. Plant leaves, for example, commonly have waxy surfaces comprised of water-repelling chemicals to keep water from engorging the leaves or facilitating bacterial and fungal growth.

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Protect From Dirt/solids

When dirt and other small solids adhere to living systems, they can slow them down, create blockages, reduce their ability to carry out vital functions, or cause surface wear and tear. Due to electrostatic forces, it’s easy for dirt and other solids to adhere to surfaces, so living systems must overcome those forces. An earthworm, for example, uses a small electric current to keep soil particles from adhering to its body as it moves through the soil. This enables it to move more efficiently by reducing drag.

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Insects

Class Insecta (“an insect”): Flies, ants, beetles, cockroaches, fleas, dragonflies

Insects are the most abundant arthropods—they make up 90% of the animals in the phylum. They’re found everywhere on earth except the deep ocean, and scientists estimate there are millions of insects not yet described. Most live on land, but many live in freshwater or saltwater marshes for part of their life cycles. Insects have three distinct body sections: a head, which has specialized mouthparts, a thorax, which has jointed legs, and an abdomen. They have well-developed nervous and sensory systems, and are the only invertebrate that can fly, thanks to their lightweight exoskeletons and small size.

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Renewable coating of brochosomes on leafhopper skin protects from sticky honeydew by repelling water.

Introduction

Leafhoppers are small insects that feed on sap from the leaves of deciduous trees. They excrete droplets of a liquid waste called honeydew. Excreted honeydew falls as a constant rain onto anything that happens to be underneath, including leaves and other leafhoppers that might be feeding from the same tree. Honeydew is very sticky and, for these tiny animals less than 8 mm long, becoming clogged up by it can be very dangerous.

Image: Roman Rakitov / Public Domain - No restrictions

Seen up close, the silver leafhopper's brochosomes appear like grains of pollen.

The Strategy

Brochosomes are tiny hollow football-shaped honeycomb spheres 400 nm across. Leafhoppers produce brochosomes from a special and then use a comb-like arrangement of setae, or hairs, on their legs to apply brochosomes across the whole surface of their bodies, including their eyes. Insect skin coated with brochosomes is superhydrophobic—meaning it is able to repel water as well as other water-based liquids like honeydew.

Many animals and plants have solid surfaces that are water-repellent, but leafhopper brochosomes are a powder coating. This means they adhere to the surface of droplets that contact the insect and move with them as they roll off. Brochosomes also form a rough surface as they aggregate to different depths. This creates hierarchical surface roughness, which is one of the keys to water repellency.

The often intricate structures that make a surface water repellent are susceptible to damage. Structures that are built directly into the solid surface of an organism, like those of the sacred lotus plant and springtail skin, are difficult to repair. But because leafhoppers use a renewable coating of spheres, if anything removes the brochosomes, the coat can be quickly replaced. Furthermore, spheres are connected into branching chains. This means that, if spheres at one end of a chain are dislodged, those at the other end might stay attached to the animal. By repeatedly and vigorously grooming themselves, leafhoppers heal their water-repellent coat in a way that isn’t possible for organisms with permanent repellent surfaces.

Image: Olga Karengina / CC BY SA - Creative Commons Attribution + ShareAlike

Chemically and structurally, brochosomes are hollow honeycomb spheres made from protein.

The Potential

Leafhoppers’ brochosome-based superhydrophobic skin provides a blueprint for the development of advanced water-repellent coatings. Engineers and materials scientists could explore replicating this mechanism to create self-renewing, water-resistant coatings for various applications, such as textiles, building materials, and even electronics. These coatings could potentially reduce the need for more costly maintenance and replacements, contributing to sustainability.

Surface Allows Self-Cleaning

Sacred lotus

Leaves of the sacred lotus are self-cleaning thanks to hydrophobic microscale bumps.

photo of blue and black butterfly on orange flower with a yellow flower to its left

Biological Strategy

Wing Surface Self-Cleans

Morpho butterfly

High surface-area wing texture of Morpho butterfly sheds water and dirt via hydrophobic microstructure.

Last Updated January 25, 2018

References

“Leafhoppers (Insecta, Hemiptera, Cicadellidae) actively coat their integuments with brochosomes, hollow proteinaceous spheres of usually 200–700 nm in diameter, with honeycombed walls. The coats have been previously suggested to act as a water-repellent and anti-adhesive protective barrier against the insect’s own exudates. We estimated their wettability through contact angle (CA) measurements of water, diiodomethane, ethylene glycol and ethanol on detached wings of the leafhoppers Alnetoidia alneti, Athysanus argentarius and Cicadella viridis. Intact brochosome-coated integuments were repellent to all test liquids, except ethanol, and exhibited superhydrophobicity, with the average water CAs of 165–172°, and the apparent surface free energy (SFE) estimates not exceeding 0.74 mN m−1.” (Rakitov and Gorb 2013(a):1)

Journal article

Brochosomal coats turn leafhopper (Insecta, Hemiptera, Cicadellidae) integument to superhydrophobic state

Proceedings of the Royal Society B, 280: 20122391 | 07/02/2013 | Rakitov R, Gorb SN

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Reference

Journal article

Brochosomes protect leafhoppers (Insecta, Hemiptera, Cicadellidae) from sticky exudates

Journal of the Royal Society Interface, 10: 20130445 | 06/10/2013 | Rakitov R, Gorb SN

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