Reducing Surface Tension to Travel on Water

a black insect stands on a green leaf against a blurry green background

Biological Strategy

Rove beetle

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Move in/on Liquids

Water is not only the most abundant liquid on earth, but it’s vital to life–so it’s no surprise that the majority of life has evolved to thrive on and under its surface. Moving efficiently in and on this dense and dynamic substance presents unique challenges and opportunities for living systems. As a result, they have evolved countless solutions to optimize drag, utilize surface tension, fine tune buoyancy, and take advantage of various types of currents and fluid dynamics. For example, sharks can slide through water by reducing drag due to their streamlined shape and specially shaped features on their skin.

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Modify Surface Tension

Water molecules are attracted to each other, a phenomenon known as cohesion. The molecules at the surface of water, such as in a pond, do not have other water molecules on all sides of them. Therefore they cohere more strongly to molecules next to and below them. This strong horizontal cohesion creates a “tension” on the surface of water that is more difficult to break through, allowing various items and organisms such as insects to perch on top of without sinking through.

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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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Abdominal glands of the rove beetle help it skim quickly across water via secreted chemicals that locally reduce surface tension.

“A small beetle, Stenus, normally walks slowly on the surface. When speed is desired, through, it secretes a substance from its last abdominal segment that locally reduces the surface tension. The result is an asymmetrical force on the beetle, which moves forward at up to 0.7 meters per second (Chapman 1982).” (Vogel 2003:108)

“Perhaps the most unusual way of getting around is demonstrated by the semiaquatic rove beetle. Jumping onto the surface of the pond, this beetle excretes a chemical which reacts so violently with water that the insect is sent skimming across the pond at high speed.” (Forsyth 1992:29)

Vogel S. Comparative Biomechanics: Life’s Physical World. Princeton: Princeton University Press; 2003.

Last Updated August 18, 2016

References

Book

Comparative Biomechanics: Life's Physical World, Second Edition

Princeton University Press | 17/06/2013 | Steven Vogel

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Reference

Book

Exploring the World of Insects: The Equinox Guide to Insect Behaviour

Camden House | 30/10/1992 | Adrian Forsyth

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