Comparative Morphology and Functional Significance of Setae Called Papillae on the Pedipalps of Male Camel Spiders (Arachnida: Solifugae)

Abstract

Abstract Some male camel spiders (Arachnida: Solifugae) in the families Eremobatidae, Karschiidae, and Solpugidae have clusters of specialized conical or acuminate setae called papillae, on the ventral surface of the metatarsus of the pedipalps. We compared the overall structure of the papillae found on representatives of the three families using scanning electron microscopy (SEM). We examined the ultrastructure of these setae using transmission electron microscopy (TEM). We also used extracellular electrophysiological recording techniques to examine the electrical properties of these sensory structures and test the hypotheses that they function as mechanoreceptors, olfactory receptors, and chemoreceptors.Wefound similarities in the structure of papillae among genera within a family or distinct family-level differences in structure. Thus, the papillae are phylogenetically informative; similar within family but differing between families. TEM results demonstrated the cuticular wall of a papilla is divided into three sublayers: endo-, meso-, and exocuticle. Mechanoreceptive dendrites are evident at the base of the setal shaft. Other dendrites innervate the shaft of the papilla and penetrate through the cuticular layers near the setal apex. Two SEM images show what appear to be pores on the branches of the papillae, and we found what appears to be a pore tubule extending from the distal portion of the dendrites through the exocuticular layer. Electrophysiological data support the hypothesis that the papillae function as mechanoreceptors and provide no support for chemosensory, thermoregulatory, or hygroreceptive functions. Our data suggest that the papillae function as mechanoreceptors and may also function as chemoreceptors.