Journal of Comparative Physiology A (2021) 207:99–103 https://doi.org/10.1007/s00359-021-01478-2 EDITORIAL Locomotion and kinematics of arachnids Jonas O. Wolf1,2 Received: 17 February 2021 / Revised: 19 February 2021 / Accepted: 20 February 2021 / Published online: 18 March 2021 © The Author(s) 2021 Abstract A basic feature of animals is the capability to move and disperse. Arachnids are one of the oldest lineages of terrestrial animals and characterized by an octopodal locomotor apparatus with hydraulic limb extension. Their locomotion repertoire includes running, climbing, jumping, but also swimming, diving, abseiling, rolling, gliding and -passively- even fying. Studying the unique locomotor functions and movement ecology of arachnids is important for an integrative understanding of the ecology and evolution of this diverse and ubiquitous animal group. Beyond biology, arachnid locomotion is inspiring robotic engineers. The aim of this special issue is to display the state of the interdisciplinary research on arachnid locomo- tion, linking physiology and biomechanics with ecology, ethology and evolutionary biology. It comprises fve reviews and ten original research reports covering diverse topics, ranging from the neurophysiology of arachnid movement, the allometry and sexual dimorphism of running kinematics, the efect of autotomy or heavy body parts on locomotor efciency, and the evolution of silk-spinning choreography, to the biophysics of ballooning and ballistic webs. This closes a signifcant gap in the literature on animal biomechanics. Keywords Octopedal locomotion · Animal biomechanics · Animal performance · Movement ecology · Kinematic analysis Background with laterigrade leg confguration (Zeng and Crews 2018), gliding locomotion in canopy spiders (Yanoviak et al. 2015), Legged locomotion is a common feature of most terrestrial somersaulting escape movements in desert huntsmen (Jäger animals. One of the oldest lineages of terrestrial animals are 2014), sailing behaviour on water bodies (Hayashi et al. the arachnids. Extant arachnids conquered nearly all types of 2015), and ultra-fast power-amplifed movements in the terrestrial ecosystems and play signifcant ecological roles mouth parts of trap-jaw spiders (Wood et al. 2016) and some as predators and parasites. They are capable of a diversity harvestmen (Wolf et al. 2016). In spiders, the production of of locomotory modes, including running, climbing, jump- silk further extends movement capacities. For instance, the ing, but also swimming, diving, rolling and -passively- even use of a ‘kite’ from charged, foating silk lines is a frequent fying. Arachnid locomotor performance is often impres- and efcient mechanism for long-distance dispersal. Such sive. The fastest arachnids are cursorial hunters, such as ballooning spiders have been found as high as one thou- wandering spiders, solifuges and predatory mites—the lat- sand metres above the ground (Glick 1939). Abseiling and ter of which may reach speeds of up to 100 body lengths bridging behaviour facilitates movement through the three- per second by super-fast muscle contractions (Wu et al. dimensional space and opens up new niches (Gregorič et al. 2010; Spagna and Peattie 2012). Discoveries of outstand- 2011). Some spiders also use silk for elastic energy storage ing locomotory features in arachnids are ongoing: Recent to build special spring-loaded traps that pull the prey into the research found an extreme turning speed of some spiders vicinity of the resting spider (Argintean et al. 2006; Greco and Pugno 2021). Research on arachnid locomotion started with detailed * Jonas O. Wolf comparative anatomical investigations of their locomotory [email protected] apparatus (reviewed in Shultz 1989). These studies high- 1 Zoological Institute and Museum, University of Greifswald, lighted that extensor muscles are absent in most arachnid Loitzer Str. 26, 17489 Greifswald, Germany joints, leading to speculations about the biomechanical 2 Department of Biological Sciences, Macquarie University, function of arachnid legs (Petrunkevitch 1909; Ellis 1944). Sydney, NSW 2109, Australia Vol.:(0123456789)1 3 100 Journal of Comparative Physiology A (2021) 207:99–103 This triggered the integrative study of arachnid locomotion locomotion in extant arachnids with detailed morphologi- as a focal topic. For instance, the combination of morpho- cal studies, the movements of ancient chelicerates can be logical, physiological and biomechanical methods helped reconstructed with modern computational simulation tools, to test hypotheses on the hydraulic function and the role of advancing our understanding on the evolution and origins hemolymph pressure in arachnid leg extension (Parry and of the peculiarities of arachnid locomotion (Garwood and Brown 1959; Shultz 1991; Sensening and Shultz 2003). Oth- Dunlop 2014). ers focused on the sensory control of locomotion (Blickhan These examples showcase the richness of the research and Barth 1985), or on respiratory physiology and the ques- on arachnid locomotion and its far reach into various dis- tion why spiders show such rapid fatigue (Prestwich 1988; ciplines. At the same time, it shows that an understanding Schmitz 2005). Early on, also the question of how some of locomotor function is most successful by the application spiders can scale smooth vertical surfaces with adhesive foot of elaborate interdisciplinary approaches, which makes this pads have been approached experimentally (Homann 1957), topic both appealing, innovative and challenging. which, however, could not be resolved before the develop- The aim of this special issue is to display the state of ment of modern nano-mechanical measurement techniques research on arachnid locomotion, linking physiology and (reviewed in Wolf and Gorb 2016). biomechanics with ecology, ethology and evolutionary biol- The advent of high-speed videography brought an impor- ogy. The idea arose in the context of the 20th International tant tool for the kinematic study of fast arachnid movements, Congress of Arachnology in Golden, Colorado in 2016. such as the jumping of salticid spiders (Nabawy et al. 2018) Linda Rayor, one of the organizers and also a contributor and box mites (Wauthy et al. 1998), or the predatory strikes to this special issue, asked me to compile a symposium on of spiders (Eggs et al. 2015; Wood et al. 2016; Zeng and arachnid locomotion, which resulted in a vivid full-day ses- Crews 2018) and amblypygids (Seiter et al. 2019). Detailed sion. Before organizing this symposium, I was not aware of kinematic studies of running spiders also highly advanced how active this feld has become in recent times. Clearly, the understanding of the biomechanical function of the what was missing was a synthesis of this advancing feld that arachnid locomotor system and overturned some long upheld due to its cross-disciplinary nature and diverse foci was com- beliefs—for instance they questioned hydraulics as the main municated highly scattered across very diferent journals and drive of locomotion in large spiders (Weihmann et al. 2012). communities. I thank Friedrich Barth for inviting me to edit The combination of kinematics and fuid mechanics uncov- this special issue for the Journal of Comparative Biology A. ered the mechanisms by which fshing spiders can move on Especially I thank all authors and reviewers who contributed water (Gorb and Barth 1994; Suter et al. 1997) and how their research and ideas to this special issue, making it a spitting spiders eject a sticky web on their prey by passive signifcant and representative reference of the feld. oscillatory movements of their chelicerae (Suter and Strat- The content is structured in three topic blocks, each con- ton 2009). taining fve articles. Note that these blocks do not represent More recently, the integration of robotics and material stringent categories and the content is often relevant for science into the study of arthropod biomechanics brought diverse felds. another leap in the understanding of the arachnid locomo- tor system and attracted engineers to it. For instance, the underactuated locomotory system of arachnids has been Kinematics of arachnid movements repeatedly picked up by biomimetic robotics (reviewed in Landkammer et al. 2016). Further, the octopedal locomotory Bøhm et al. (2021) explored the scaling of locomotor speed mode has been studied as a way to achieve a high stability and limb kinematics in wandering spiders with a size range and robustness of walking in robots when moving through spanning three orders of magnitude. Using a cutting-edge complex terrains (Klaassen et al. 2002; Spagna et al. 2007). automated markerless tracking approach, they measured Beyond the biomechanical aspects, arachnid locomotion limb flexion and extension speeds and compared them has also been in the focus of many ecological and behav- with spider mass. Their data does not support the hypoth- ioural studies, facilitated by the development of computa- esis that hydraulic leg extension poses a limit to maximal tional video tracking methods. This encompasses questions, running speed. Telheiro et al. (2021) studied the running such as how arachnids navigate through complex environ- kinematics in fat-tailed scorpions. These arachnids bear a ments (reviewed in Gafn and Curry 2020), disperse (Pow- massive telson that is elevated in a defensive posture. The ers and Aviles 2003), utilize distinct movement patterns for study revealed that the defensive posture changes the loca- mimicry (Shamble et