Clustered setation on the pedipalps of buthid scorpions - morphology, taxonomic significance and a cleaning tool

 

The skin (exoskeleton) of scorpions has many different types of hairs (e.g. trichobothrias and setae). These have many functions, especially as sensory detectors of chemical substances and mechanical vibrations. For taxonomists they are also important morphological characters that can be used to identify taxa.

Trichobothriotaxy is already in great use in diagnosis  and description of many taxa (from species level to family level), but the usefulness of the setae (chaetotaxy) is less investigated. Graeme Lowe and Victoria Tang have now published a extensive study of chaetotaxy in scorpions with morphological descriptions of the different setation patterns in different taxa and the possible applications of these in taxonomy.

Interestingly, observations of Olivierus martensii (Karsch, 1879) (Buthidae) seem to indicate that this species uses setae clusters on pedipalps to brush the median ocelli (eyes) during sponge-bathing (cleaning behavior).

Abstract:
Chaetotaxy of the external pedipalp femur and distal ventral pedipalp movable finger was studied in 120 species, 69 genera and 17 families of scorpions. Setation was generally denser in the ‘Buthus’ group, a major arid-adapted buthid lineage distributed across Palearctic deserts. On the external femur, macrosetae formed a prominent cluster, the ‘distal external macrosetal cluster’ (DEMC); on the distal ventral movable finger they formed a dense patch, the ‘distal ventral macrosetal cluster’ (DVMC). In other buthids and non-buthids, the DEMC and DVMC were mostly absent, except in a few arid-adapted genera. Relative setation densities of DEMC and DVMC in different species depended strongly on size, being denser in larger species and sparser in smaller species, while absolute density varied only weakly with size (mean spacing of setae ~200 μm in DEMC, ~40 μm in DVMC). Ontogenetic variation followed similar trends. Multivariate morphometric analyses revealed taxonomic differences in setation patterns. The ‘Buthus’ group, other buthids, and non-buthids, were partially separable according to their spatial profiles of setation. In the ‘Buthus’ group, major genera were separable by spatial and density profiles of setation. In buthids, there were taxonomic differences in external femoral trichobothriotaxy. The ‘Buthus’ and Tityus’ groups were largely separable by proximodistal positioning of trichobothrium e1. Relative setation densities of DEMC and DVMC were positively correlated, in that species with dense DEMCs also tended to have dense DVMCs. In the buthid Olivierus martensii, DEMC and DVMC were observed to brush the median ocelli during sponge-bathing. In all examined buthids, the DEMC was located where it would contact the ipsilateral median ocellus during femoral articulation. Both DEMC and DVMC may assist in the ocular grooming of desert buthids, by removing sand and dust from surfaces of the median ocelli.

Reference:
Lowe G, Tang V. Clustered setation on the pedipalps of buthid scorpions (Scorpiones: Buthidae). Euscorpius. 2024(398):1-77. [Open Access]