title: Gluing the 'unwettable': soil-dwelling harvestmen use viscoelastic fluids for capturing springtails.
abstract: Gluing can be a highly efficient mechanism of prey capture, as it should require less complex sensory-muscular feedback. Whereas it is well known in insects, this mechanism is much less studied in arachnids, except spiders. Soil-dwelling harvestmen (Opiliones, Nemastomatidae) bear drumstick-like glandular hairs (clavate setae) at their pedipalps, which were previously hypothesized to be sticky and used in prey capture. However, clear evidence for this was lacking to date. Using high-speed videography, we found that the harvestman Mitostoma chrysomelas was able to capture fast-moving springtails (Collembola) just by a slight touch of the pedipalp. Adhesion of single clavate setae increased proportionally with pull-off velocity, from 1 μN at 1 μm s(-1) up to 7 μN at 1 mm s(-1), which corresponds to the typical weight of springtails. Stretched glue droplets exhibited characteristics of a viscoelastic fluid forming beads-on-a-string morphology over time, similar to spider capture threads and the sticky tentacles of carnivorous plants. These analogies indicate that viscoelasticity is a highly efficient mechanism for prey capture, as it holds stronger the faster the struggling prey moves. Cryo-scanning electron microscopy of snap-frozen harvestmen with glued springtails revealed that the gluey secretions have a high affinity to wet the microstructured cuticle of collembolans, which was previously reported to be barely wettable for both polar and non-polar liquids. Glue droplets can be contaminated with the detached scaly setae of collembolans, which may represent a counter-adaptation against entrapment by the glue, similar to the scaly surfaces of Lepidoptera and Trichoptera (Insecta) facilitating escape from spider webs.
authors.affiliation: Department of Functional Morphology and Biomechanics, University of Kiel, Am Botanischen Garten 1-9, D-24098 Kiel, Germany jwolff@zoologie.uni-kiel.de.
authors.forename: Jonas O
authors.initials: JO
authors.lastname: Wolff
authors.affiliation: Department of Evolutionary Biology, Institute of Zoology, Johannes Gutenberg University Mainz, Joh.-von-Müller-Weg 6, D-55128 Mainz, Germany.
authors.forename: Axel L
authors.initials: AL
authors.lastname: Schönhofer
authors.affiliation: Department of Functional Morphology and Biomechanics, University of Kiel, Am Botanischen Garten 1-9, D-24098 Kiel, Germany.
authors.forename: Clemens F
authors.initials: CF
authors.lastname: Schaber
authors.affiliation: Department of Functional Morphology and Biomechanics, University of Kiel, Am Botanischen Garten 1-9, D-24098 Kiel, Germany.
authors.forename: Stanislav N
authors.initials: SN
authors.lastname: Gorb
mesh_terms: D000269:Adhesives / Q000737:chemistry*; D000818:Animals; D001097:Arachnida / Q000502:physiology*; D001181:Arthropods / Q000648:ultrastructure; D001696:Biomechanical Phenomena; D001826:Body Fluids / Q000737:chemistry; D020285:Cryoelectron Microscopy; D011235:Predatory Behavior / Q000502:physiology*; D014741:Video Recording; D014783:Viscosity
chemical_list: D000269:Adhesives
keywords: Adhesion; Collembola; Microstructure; Opiliones; Tensile test; Wetting