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. 2021 Jan 18;207(2):191–204. doi: 10.1007/s00359-020-01458-y

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© The Author(s) 2021

Open AccessThis article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 1 — Setup of a tethered tarantula and b freely walking spiders. a Spiders were secured with tape on a brass holder; their second leg fixed with dental cement. At this site, the pressure at the dorsal hemolymph channel was registered. Trimmed strain gauges were affixed with cyanoacrylate. Here, we focus on two sites of gauge application: (1) the dorsal tibia and (2) the site of the lyriform organ HS8. Activity of the flexor M. met. bilobatus was registered. Using a needle fixed with beeswax to the metatarsus, the leg tip was deflected electromagnetically in three orthogonal directions. A custom-built force transducer allowed the registration of the force required to achieve the deflections. While moving the leg tip in the different directions against joint stiffness, the reaction force points in the same direction. However, flexion induced by muscles generates a dorsad, extension generated by hydraulic pressure a ventrad reaction force. α: dorso-ventral angle of the tibia–metatarsus joint; dash-dotted line: dorsal axis of the hinge joint. Ti tibia, MeTa metatarsus, Ta tarsus. Anteriad (x), ventrad (y), proximad (z) form a right-handed system of co-ordinates. b A strain gauge was bonded to the specified site at the tibia of the spider’s leg. Slack leads were guided across the leg to the prosoma and to a holder. In some cases, ground reaction force was measured in 3D with a custom-built force plate and kinematics registered with a video camera supported by mirrors (e.g., Barth 2002)