An assessment of function, intraspecific variation, and taxonomic reliability of eremobatid ctenidia (Arachnida: Solifugae)

Camel spiders, members of the order Solifugae, are a diverse but poorly understood order of arachnids. The conserved morphology within many groups in the order makes species delimitation and identification challenging. In the North American family Eremobatidae, swollen setae called ctenidia are a common character employed in species delimitation, diagnoses, and identification. Unlike many other arachnid systems, the traditional use of ctenidia characters in eremobatid taxonomy and species identification does not allow for intraspecific variation, despite variation being common in this character system. To access and document the extent of intraspecific variation in this system, a combination of light microscopy and scanning electron micrographs (SEM) were used to survey and document the inter- and intraspecific variation of ctenidia shaft number, shape, and relative length. Additionally, these characters were also evaluated for species-group level and/or genus level taxonomic utility by evaluating the phylogenetic signal for each using a previously published molecular phylogeny as a context. Lastly, as ctenidia have no known biological function, we also assessed ctenidia shaft morphology for evidence of mechano- and/or chemoreceptive function. Observations from nearly 800 museum specimens indicate that ctenidia characteristics are generally far more variable within individual species than previous taxonomic literature indicates, necessitating increased caution for utilization in species diagnoses, delimitation and identifications. Phylogenetic signal was detected for shaft number and shape, but shaft length was not constrained by phylogenetic proximity. Scanning electron micrographs did not reveal morphology consistent with arachnid mechano- or chemoreception, as evidenced by a lack of pores or shaft mobility. Although the biological function of ctenidia remains elusive, the presence of phylogenetic signal and shifts in shaft number and shape may indicate some functional significance yet undiscovered.