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A survey of proximal sensilla associated with denticle subrows on scorpion pedipalp fingers (Arachnida: Scorpiones), with observations on scorpion fluorescence
A survey of proximal sensilla associated with denticle subrows on scorpion pedipalp fingers (Arachnida: Scorpiones), with observations on scorpion fluorescence
Abstract
Sensilla on the dentate margins of scorpion pedipalp fingers, associated with denticle subrows or terminal denticles, were surveyed by white light, UV fluorescence and scanning electron microscopy. A total of 163 species belonging to 103 genera from 19 families were examined, representing all major phylogenetic lineages in the order. In the majority of species (141/152) with dentition organized into linear subrows of median denticles, a small socketed sensillum with a short shaft was observed at the proximal terminus of each subrow, termed a ‘subrow proximal sensilla’ (SPS). With few exceptions, SPS were non-fluorescent (‘Type N’) in buthids, and fluorescent (‘Type F’) in non-buthids. SPS were present in buthids, chactoid families, and vaejovids; they were absent in Pseudochactas, bothriurids and scorpionoid families. Subterminal sensilla (STS) were found proximal to terminal denticles. Buthid fingers typically had two pairs of Type N STS, a proximal filiform pair and a distal spatulate pair, herein termed ‘Cruz-Armas sensilla’ after their discoverers; in a minority of species, the spatulate pair was missing. Non-buthid fingers typically had a single Type F STS, similar to Type F SPS. Type N sensilla studied here may be mechanoreceptive ‘macrosetae’, and Type F sensilla chemo-/mechanoreceptive ‘microsetae’, both specialized for detecting close-contact stimuli. In order to correctly interpret fluorescence images of sensilla, we reviewed the optics of UV excitation and fluorescence emission in scorpion cuticle. We propose a dielectric waveguide model of the cuticle, in which fluorescence emitted in the hyaline exocuticle is trapped by total internal reflection, and is able to propagate tangential to the surface. We suggest that this phenomenon could enhance topographic contrast of small surface features in fluorescence imaging, and increase the efficiency of UV fluorescence detection by ocular or extraocular photoreceptors. We observed highly effective block of UV light by the cuticle, and strong UV attenuation by non-fluorescent endocuticle. Scorpion cuticle may serve a dual role, as a protective sunscreen and as a UV wavelength converter and fluorescence collector for photoreceptors.