Org Divers Evol (2016) 16:549–557 DOI 10.1007/s13127-016-0278-2 ORIGINAL ARTICLE Evolution of hyperflexible joints in sticky prey capture appendages of harvestmen (Arachnida, Opiliones) Jonas O. Wolff1 & Jochen Martens2 & Axel L. Schönhofer2 & Stanislav N. Gorb1 Received: 23 October 2015 /Accepted: 27 March 2016 /Published online: 4 April 2016 # Gesellschaft für Biologische Systematik 2016 Abstract The rigid leg segments of arthropods are flexibly efficiency of the pedipalp as a means of prey capture, because connected by joints, which usually consist of two ball-and- in springtails detachable scales limit the action of the sticky bowl hinges, permitting a uniaxial pivoting up to 140°. secretion of pedipalpal setae. Here, we report the occurrence of hyperflexible joints (range of movements = 160–200°) in the pedipalps (second pair of Keywords Prey capture . Adhesion . Predator-prey appendages) of some harvestmen (Sabaconidae and interaction . Biomechanics . Torsion . Kinematics . Nemastomatidae), representing some of the most flexible leg Arachnida . Soil communities joints among arthropods. Hyperflexion is achieved by a reduc- tion of hinges and a strong constriction of the joint region. We demonstrate that hyperflexion occurs during prey capture and Introduction is used to clamp appendages of the prey, in addition to attach- ment by glue secreted by specialized setae. By means of high- The coevolution of predator and prey frequently leads to an speed video recordings, we found that in the Sabaconidae the increase in speed and efficacy of means of prey capture or tibiotarsal joint of the pedipalp can flex extremely rapidly defense (Dawkins and Krebs 1979;Vermeij1994;Abrams (<5 ms), limiting prey escape. This is the fastest reported 2000). In arthropod predators, there are numerous exam- predatory strike in arachnids and caused both by leverage ples of limb modifications that enhance the ability to over- and a click mechanism. By comparative analysis of different whelm and/or secure prey, including raptorial clamps (e.g., related taxa, we retraced joint evolution and found that mantids and amblypygids) (Ass 1973;Weygoldt2000), hyperflexion has independently evolved in Sabaconidae and capture baskets (e.g., dragonflies and asilids) (Gorb Nemastomatidae, with totally different joint kinematics. We 1999), sticky pads (e.g., solifuges) (Willemart et al. hypothesize that (rapid) hyperflexion evolved to enhance the 2011), snap-traps (trap-jaw ants) (Gronenberg et al. 1993), harpoons (mantis shrimps) (Murphy and Patek 2012), smashing clubs (mantis shrimps) (Patek and Electronic supplementary material The online version of this article Caldwell 2005), or even hydraulic pistols (snapping (doi:10.1007/s13127-016-0278-2) contains supplementary material, shrimps) (Versluis et al. 2000). The fastest movements which is available to authorized users. are enabled by a click mechanism, in which a high amount of energy is accumulated by the elastic deformation of a * Jonas O. Wolff [email protected] cuticular structure, which is suddenly released by a de- arresting or buckling mechanism. Such mechanisms have evolved both in predators like trap-jaw ants, which close 1 Functional Morphology and Biomechanics, Zoological Institute, Kiel their mandibles in less than 1 millisecond (Gronenberg University, Am Botanischen Garten 9, D-24118 Kiel, Germany et al. 1993), and in arthropods that are frequent targets of 2 Department of Evolutionary Biology, Institute of Zoology, Johannes predation, like springtails (Collembola) being capable of Gutenberg University Mainz, Joh.-von-Müller-Weg 6, catapulting rescue jumps with a take-off speed of 1.4 m/s D-55128 Mainz, Germany (Christian 1978). Springtails are, besides mites, a 550 J.O. Wolff et al. ubiquitous and dominant part of soil arthropod communi- Material and methods ties and an important food source for arthropod predators (Hopkin 1997). Their catapult mechanism has been hy- Anatomical studies and phylogenetic analysis pothesized to be a result of an evolutionary Barms race^ between them and their predators (Hopkin 1997). For ex- Material ample, some ground beetles have evolved an antennal cap- ture basket that closes faster than the springtail can take off For anatomical studies, we used conserved material held in the (Bauer and Völlenkle 1976; Bauer 1982; Hintzpeter and private collections of the authors (f (females), m (males), juv Bauer 1986). However, speed is not only determined by (juveniles)): Caddidae: Caddo agilis BANKS 1892, Macon Co., the mechanism of movement generation, but also the time NC, USA, 2012, f, m; Acropsopilionidae: Acropsopilio of reaction, dependent on neuronal signal transmission. neozealandiae FORSTER 1948, South-Island, New Zealand, Thus, another predatory trick is the usage of sticky secre- 1990, f; Ischyropsalididae: Ischyropsalis luteipes SIMON tions, since these act directly on the prey without the ne- 1872, Cataluña, Spain, 2009, f, m, juv; Taracidae: cessity of a sensory feedback (Betz and Kölsch 2004). Hesperonemastoma modestum BANKS 1894, CA, USA, Viscid glue can be found in springtail hunting rove beetles 2011, f, m, juv; Sabaconidae: Sabacon simoni DRESCO 1952, (Bauer and Pfeiffer 1991; Schomann et al. 2008), mites Alpes-Maritimes, France, 2008, f, m, juv; Dicranolasmatidae: (Alberti 1973, 2010), harvestmen (Wolff et al. 2014, Dicranolasma pauper DAHL 1903, Trentino-Alto, Italy, 2014, 2016), and carnivorous plants (Verbeek and Boasson f, juv; Nemastomatidae: Mitostoma chrysomelas HERMANN 1993). However, the efficacy of glue is decreased by de- 1804, Mainz, Germany, 2014, f, m. tachable scales that help the springtail freeing from the glue trap (Bauer and Pfeiffer 1991; Wolff et al. 2014). Such scales evolved multiple times among springtails, in- Light microscopy (LM) dicating their role in defense from glue-using predators (Zhang et al. 2014). Light microscopical images of animals and their pedipalps In a previous study, we observed that the harvestman were made with a multifocus stereo microscope (Leica Mitostoma chrysomelas (Nemastomatidae) uses two M205 A, Leica Microsystems GmbH, Wetzlar, Germany) counter-strategies to reduce prey loss (Wolff et al. 2014). (1) equipped with a camera (Leica DFC420). The patello-tibial It stretches its legs to prevent ground contact and therefore joint and the tibiotarsal joint were studied in detail using dif- generation of opposing forces by the prey. (2) It highly flexes ferent illumination techniques. To understand the kinematics its tarsus against the tibia and thereby occasionally clamps of the joints, different specimens were studied that had their appendages of the prey in between. The latter must be based tibia and/or tarsus in a differently retracted/expanded state. on a more complex modification of the tibiotarsal joint, since arthropod leg joints usually do not exceed a range of motion (ROM) higher than 140° (extremes e.g., 120° in the femur- Scanning electron microscopy (SEM) patellar joint of spider walking legs (Parry and Brown 1959); 140° in the tibiotarsal joint in the raptorial legs of mantids Pedipalps were removed using fine forceps and dehydrated in (Corrette 1990)). In order to retrace the evolution of a series of increasing ethanol concentrations (80, 90, 100 and hyperflexibility (ROM > 160°), we comparatively studied 100 % on a molecular sieve), followed by critical point dry- the functional joint morphology in different representatives ing. Samples were glued on stubs using a carbon-rich double- of related harvestmen families, all of which are ground- sided adhesive tape. In some samples, the patella, tibia, and dwellers. The pedipalps of the monogeneric family tarsus were slightly pulled apart to uncover joint structures. Sabaconidae were of particular interest, since previous au- Specimens were sputter-coated with 10 nm Au-Pd and stud- thors remarked that the tarsus can be highly flexed against ied with a Hitachi S 4800 scanning electron microscope the tibia (Simon 1879; Shear 1975; Juberthie et al. 1981). In (Hitachi Ltd., Tokyo, Japan) at an acceleration voltage of fixative-preserved specimens of Sabaconidae, we often found 3.0 kV. the tibia and tarsus bent and twisted in an unusual manner. Glue secreting setae of M. chrysomelas before and after Previous authors wondered about the function of the uniquely being in contact with springtail cuticle were visualized by shaped pedipalps and setae in this family, but never observed Cryo-SEM. For that purpose, fresh specimens were attached their use during prey capture (Shear 1975, 1986;Juberthie to a sample holder using Tissue-Tek® compound, shock fro- et al. 1981). To reveal biological and mechanical functions zen in liquid nitrogen, directly sputtered with 10 nm Au-Pd of hyperflexible pedipalps, we studied the prey capture be- using the Gatan ALTO-2500 cryo system (Gatan Inc., havior of M. chrysomelas and Sabacon simoni by means of Abingdon, UK), and viewed in the SEM with the stage cooled high-speed video recordings. up to −120 °C (for details see Wolff et al. 2014). Evolution of hyperflexible joints in sticky prey capture appendages of harvestmen (Arachnida, Opiliones) 551 Microcomputed tomography (μCT) with dental wax (Polyvinylsiloxane) containing humid fil- ter paper and different species and sizes of living collem- Further anatomical observations on the distal pedipalp were bolans collected on the campus of Kiel University. Prey made on female S. simoni, by means of μCT. We used two capture events were filmed with a high-speed video cam-