Sensory Recordings

Experiments were performed on adult cockroaches (Periplaneta americana males obtained from Carolina Biological Supply) and stick insects [Carausius morosus, females obtained from colonies maintained at the Bielefeld University, the University of Cologne or from commercial suppliers (Backwater Reptiles)]. All animals were adult and selected to have firm exoskeletons (“al dente”) based upon squeezing the thorax. The methods for recording activities of tibial campaniform sensilla (Fig. 1, A–C) have been previously described (23, 24). Briefly, animals were first restrained on a resin-coated platform using staples and all nerves to the leg under study were severed in the thorax (Fig. 1, A and ​andB).B). The leg was then positioned so that the plane of leg movement was parallel to the surface of the platform. The femoro-tibial joint was fixed using a pin and adhesive and the tarsus was amputated so that forces could be applied to the free end of the tibia. Small holes were made in the cuticle of the femur and fine silver wires (50 μm diameter) were inserted and positioned close to the nerves that contain the receptor axons (20, 23). Sensory activities were monitored during electrode placement, recorded using a custom-built amplifier (Michael Duebber, University of Cologne) and stored digitally via a Spike2 interface [Cambridge Electronic Design (CED), Cambridge, UK].

Preparations for recording discharges of tibial campaniform sensilla and responses to cap indentation vs. leg bending. A and B: recording preparations—activities of tibial campaniform sensilla were recorded through a pair of wires (placed adjacent to a nerve containing the receptor axons. In most experiments, forces were applied to the end of the tibia with a probe (containing strain gauges that was driven by a motor (A). In studies in stick insects, forces were also applied with an Aurora puller (B), which permitted independent control of force or displacement. C: structure of campaniform sensilla—(left): a campaniform sensillum consists of a sensory neuron whose dendrite inserts into a cuticular cap at the surface of the exoskeleton. Detailed drawing of cuticular cap [middle, after Gnatzy et al. (26)]. The cap of the campaniform sensillum is linked to the cuticle via a collar. Confocal fluorescence micrograph of cockroach tibial sensilla (right): the collars appear as bright rings of elastic cuticle surround the darkened (sclerotized) caps. D: indentation of cuticular caps—indenting the caps of individual stick insect tibial campaniform sensilla with a fine probe (time indicated by indent cap) produced discharges to force increases in 6B (left) and 6A (right) receptors but no firing to decreases in indentation force. E: brief application of bending forces (forced extension of the tibia) elicited firing of 6B sensilla to force increases and action potentials in 6A receptors to force decreases. F: application of bending forces at different rates of rise and decline produced more prolonged firing of 6A receptors that reflected the rate of force decrease (left). Plots of sensory firing frequencies vs. rate of force application show that similar rate sensitivities are found to decreasing forces and to increasing forces in the opposite direction (6A: n = 28, N = 2; 6B: n = 40, N = 4).