Mechanosensory pathways of scorpion pecten hair sensillae—Adjustment of body height and pecten position

Abstract

Scorpions' sensory abilities are intriguing, especially the rather enigmatic ventral comb-like chemo-and mechanosensory organs, the so-called pectines. Attached ventrally to the second mesosomal segment just posterior to the coxae of the fourth walking leg pair, the pectines consist of the lamellae, the fulcra, and a variable number of pecten teeth. The latter contain the bimodal peg sensillae, used for probing the substrate with regard to chemo-and mechanosensory cues simultaneously. In addition, the lamellae, the fulcra and the pecten teeth are equipped with pecten hair sensillae (PHS) to gather mechanosensory information. Previously, we have analyzed the neuronal pathway associated with the peg sensillae unraveling their somatotopic projection pattern in dedicated pecten neuropils. Little is known, however, regarding the projections of PHS within the scorpion nervous system. Behavioral and electrophysiological assays showed involvement of PHS in reflexive responses but how the information is integrated remains unresolved. Here, we unravel the innervation pattern of the mechanosensory pecten hair afferents in Mesobuthus eupeus and Euscorpius italicus. By using immunofluorescent labeling and injection of Neurobiotin tracer, we identify extensive arborizations of afferents, including (i) ventral neuropils, (ii) somatotopically organized multisegmental sensory tracts, (iii) contralateral branches via commissures, and (iv) direct ipsilateral innervation of walking leg neuromeres 3 and 4.Our results suggest that PHS function as sensors to elicit reflexive adjustment of body height and obstacle avoidance, mediating accurate pecten teeth alignment to guarantee functionality of pectines, which are involved in fundamental capacities like mating or navigation.

No evidence for regeneration of pectines in the scorpion Euscorpius italicus (Herbst, 1800)

“…The pectinal teeth are equipped with numerous bimodal peg sensilla, which detect mechanosensory as well as chemosensory information of the substrate and increase in number during post-embryonic development (e.g., Brownell, 1988;Farley, 2011;Foelix & Müller-Vorholt, 1983;Gaffin & Brayfield, 2017;Hughes & Gaffin, 2019;Melville, 2000;Wolf, 2008Wolf, , 2017. Besides these bimodal peg sensilla, the pectines are covered with mechanosensory hair sensilla, which are hypothesized to elicit reflexive adjustment of body height and obstacle avoidance (Drozd et al, 2022). Due to the impressive number of peg and hair sensilla as well as the elaborated neuronal innervation (Brownell, 1998;Drozd et al, 2020Drozd et al, , 2022Melville, 2000;Wolf, 2008), the pectines are the most prominent sensory structures in scorpions.…”

“…Besides these bimodal peg sensilla, the pectines are covered with mechanosensory hair sensilla, which are hypothesized to elicit reflexive adjustment of body height and obstacle avoidance (Drozd et al, 2022). Due to the impressive number of peg and hair sensilla as well as the elaborated neuronal innervation (Brownell, 1998;Drozd et al, 2020Drozd et al, , 2022Melville, 2000;Wolf, 2008), the pectines are the most prominent sensory structures in scorpions. They are proposed to be essential for detecting prey and finding mates (Gaffin & Brownell, 1992;Krapf, 1986;Melville, 2000;Melville et al, 2003;Steinmetz et al, 2004).…”