Abstract
BackgroundMany arachnids possess adhesive pads on their feet that help them climb smooth surfaces and capture prey. Spider and gecko adhesives have converged on a branched, hairy structure, which theoretically allows them to adhere solely by dry (solid-solid) intermolecular interactions. Indeed, the consensus in the literature is that spiders and their smooth-padded relatives, the solifugids, adhere without the aid of a secretion.Methodology and Principal FindingsWe investigated the adhesive contact zone of living spiders, solifugids and mites using interference reflection microscopy, which allows the detection of thin liquid films. Like insects, all the arachnids we studied left behind hydrophobic fluid footprints on glass (mean refractive index: 1.48–1.50; contact angle: 3.7–11.2°). Fluid was not always secreted continuously, suggesting that pads can function in both wet and dry modes. We measured the attachment forces of single adhesive setae from tarantulas (Grammostola rosea) by attaching them to a bending beam with a known spring constant and filming the resulting deflection. Individual spider setae showed a lower static friction at rest (26%±2.8 SE of the peak friction) than single gecko setae (Thecadactylus rapicauda; 96%±1.7 SE). This may be explained by the fact that spider setae continued to release fluid after isolation from the animal, lubricating the contact zone.SignificanceThis finding implies that tarsal secretions occur within all major groups of terrestrial arthropods with adhesive pads. The presence of liquid in an adhesive contact zone has important consequences for attachment performance, improving adhesion to rough surfaces and introducing rate-dependent effects. Our results leave geckos and anoles as the only known representatives of truly dry adhesive pads in nature. Engineers seeking biological inspiration for synthetic adhesives should consider whether model species with fluid secretions are appropriate to their design goals.
Highlights
Climbing animals use a vast array of attachment strategies to scale vertical and inverted surfaces [1]
Geckos and anoles adhere by dry, intermolecular adhesion [3,4], but other organisms secrete a fluid over their adhesive pads, including insects [5,6,7,8,9], frogs [10] and bats [11]
We found clear evidence of footpad secretions in every species we studied: the spiders Grammostola rosea (Theraphosidae, Fig. 1a,b), Salticus scenicus (Salticidae, Fig. 1d), and Cupiennius salei (Ctenidae, Video S1), the mites Gromphadorholaelaps schaeferi (Fig. 1c) and Balaustium murorum, and the solifugid Gluvia dorsalis (Fig. 1e,f)
Summary
Climbing animals use a vast array of attachment strategies to scale vertical and inverted surfaces [1]. We would predict that the very small size of the spiders’ spatulae (200–300 nm wide; [22]) allows them to make close contact with the substrate, attaching without the aid of a fluid as the gecko does [3,4,23]. This has been the consensus among researchers who have studied spider adhesion [19,20,21,24,25]. The consensus in the literature is that spiders and their smooth-padded relatives, the solifugids, adhere without the aid of a secretion
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