The feet of the palm leaf beetle protect it from predation by capillarity-based adhesion.

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The author noticed that when he tried to pick this beetle off of a palmetto leaf, he had to exert considerable force to pry them loose. "What the beetle does have on its feet is bristles, thousands of bristles, sticking out from the sole of each foot--the ventral tarsal surface--like hairs on a brush. Bristles are a common feature of beetle tarsi generally, but I had never seen them in such quantity per foot…Each tarsus was subdivided into three bristle-bearing subsegments, called tarsomeres. We counted the bristles and found that there were about 10,000 per tarsus, making a total of 60,000 per beetle. Each bristle was forked at the tip, which means, if we assumed the bristle endings to be the contact points with the substrate, that the beetles had the option of relying on 120,000 such points…The bristle endings of H. cyanea did indeed turn out to be padlike, and they were wetted…We now know that fluid is an oil, consisting of a mixture of long-chain hydrocarbons (specifically C22 to C29 n-alkanes and n-alkenes). An oil is ideally suited to secure adhesion to a leaf, since the outermost surface of leaves is waxy (palmetto fronds are no exception), and waxes make good contact with hydrocarbons." (Eisner 2003:134-135)

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"Hemisphaerota cyanea is a small blue beetle (Chrysomelidae; Cassidinae) found on palmetto plants in the southeastern United States. Anyone who has attempted to collect this insect knows that it is able to cling tenaciously to the frond when one attempts to pick it off. Ordinarily the beetle walks or rests with a loose hold, but if it is disturbed, it clamps down with such vigor that considerable force is required to pry it loose." (Eisner and Aneshansley 2000: 6568)

The beetle…activates a tarsal adhesion mechanism by which it secures a hold on the substrate. Its tarsi are oversized and collectively bear some 60,000 adhesive bristles, each with two terminal pads…small circular pores are irregularly distributed amidst the base of the bristles…glandular openings from which tarsal oil is secreted. When the tarsus is lifted, the bristles clump together and oil flows [by capillarity] to the clustered tips, wetting the bristle pads…We presume the oil to act as a thin-film adhesive that provides for the attachment of each individual bristle, by its two pads, to the substrate…When the tarsus then touches down, the bristles are splayed, and the prewetted pads make contact with the substrate…While walking, the beetle commits but a small fraction of the bristles to contact with the substrate. But when assaulted, it presses its tarsi flatly down, thereby touching ground with all or nearly all of the bristles. Once so adhered it can withstand pulling forces of 0.8g (~60 times its body mass) for 2 min and higher magnitudes, up to >3 g for shorter periods. (Paraphrased from Eisner and Aneshansley 2000: 6568, 6570, 6571)

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Book
For Love of InsectsOctober 31, 2005
Thomas Eisner

Journal article
Defense by foot adhesion in a beetle (Hemisphaerota cyanea)Proceedings of the National Academy of SciencesJuly 26, 2002
T. Eisner, D. J. Aneshansley

Journal article
Structural Design and Biomechanics of Friction-Based Releasable Attachment Devices in InsectsIntegrative and Comparative BiologyMarch 13, 2006
S. N. Gorb


Capillarity-based switchable adhesion

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