Legs Reversibly Stick to Surfaces Underwater

macro photo of black and yellow beetle on white surface

Biological Strategy

Diving beetle

Jeanette Lim

Image: Udo Schmidt / Flickr / CC BY SA - Creative Commons Attribution + ShareAlike

Attach Temporarily

Living systems must sometimes, temporarily, stay in one place, climb or otherwise move around, or hold things together. This entails attaching temporarily with the ability to release, which minimizes energy and material use. Some living systems repeatedly attach, detach, and reattach for an extended time, such as over their lifetimes. Despite being temporary, these attachments must withstand physical and other forces until they have achieved their purpose. Therefore, living systems have adapted attachment mechanisms optimized for the amount of time or number of times they must be used. An example is the gecko, which climbs walls by attaching its toes for less than a second. Other examples include insects that attach their eggs to a leaf until they hatch, and insects whose wings temporarily attach during flight but separate after landing.

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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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Legs of male diving beetles reversibly stick to surfaces underwater using suction and viscous properties of water.

Diving beetles are aquatic insects that live in ponds, lakes, and other bodies of stagnant water. Their paddle-like hind legs enable them to swim through the water, and in males, enlarged and adhesive sections of the forelegs enable them to attach temporarily to the backs of female beetles during courtship. Like many insects, diving beetles use small projecting structures called setae on their legs to aid in adhesion. In terrestrial insects, these setae are often hair-like structures. In male diving beetles, however, the adhesive setae are modified into spatula or suction-cup-like shapes that can function underwater.

Each spatula-shaped seta on the diving beetle Cybister rugosus has a thin contact surface and a stalk. The contact surface is a ~400 µm-long, roughly oval-shaped pad with a small sucker and a series of parallel grooves running lengthwise on the underside. The short, flexible stalk connects the pad to the foreleg. These structures contribute to several mechanisms for effective and easily reversible underwater attachment. The small sucker on the pad likely functions as a suction cup, while the grooves can fill with water and adhere to a surface through viscous resistance. This is resistance to fluid flow due to the internal stickiness (viscosity) of water on a small scale, and would prevent the seta from slipping off a wet surface. The setal stalk is flexible and can twist to reorient the pad so that its water-filled grooves line up with exerted forces and enhance adhesion.

Finally, the setae can easily detach because adhesion is velocity-dependent, meaning the speed with which setae contact a surface can affect the strength of adhesion. Adhesion is stronger if movement between the seta and surface is fast. This might occur when a male diving beetle is holding onto a female beetle that is swimming erratically. Adhesion is weaker, though, if movement is slow, making it easier for the foreleg to detach if the setae are peeled off slowly.

Image: Udo Schmidt / CC BY SA - Creative Commons Attribution + ShareAlike

This is a different species, but same genus, as the beetle studied in the journal article by Chen et al.

Image: Udo Schmidt / CC BY SA - Creative Commons Attribution + ShareAlike

This is a different species, but same genus, as the beetle studied in the journal article by Chen et al.

The left foreleg is visible in the upper left corner of panel (a). Panel (c) shows the rows of setae on the foreleg’s underside. Panel (d) shows the stalk and contact surface of each seta. Panel (e) and (f) show the suction cup and grooves on each seta. Adapted from Chen Y; Shih M-C; Wu M-H; Yang E-C; Chi K-J. 2014. Underwater attachment using hairs: the functioning of spatula and sucker setae from male diving beetles. J. R. Soc. Interface 11: 20140273, by permission of the Royal Society.

Last Updated April 19, 2018

References

“Spatula setae with modified surface structures including a shallow sucker and channels can adhere surfaces using the combined mechanisms of suction and viscous resistance. Their velocity-dependent adhesion structures provide better control during movement and mating, and direction-dependent shear resistance helps reorient setae surfaces into preferred directions for the purpose of attachment.” (Chen et al. 2014:11)

“Under a pull-off force, the seta surface slides proximally and fluid must flow through the channels into the sucker before surface separation, and the faster the pull-off motion, the greater the generated resistance. Therefore, these grooves not only increase the effective contact area during preloading, but also create multiple flow pipes to increase viscous resistance. Consequently, when a male diving beetle mounts female elytra for pre-courtship, its spatula setae resist the female’s erratic swinging motion, whereas a slow pull-off motion allows setae to detach easily.” (Chen et al. 2014:11)

Journal article

Underwater attachment using hairs: the functioning of spatula and sucker setae from male diving beetles

J. R. Soc. Interface, 11: 20140273 | 06/08/2014 | Chen Y; Shih M-C; Wu M-H; Yang E-C; Chi K-J

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