Stridulation by cosmetid harvestmen (Arachnida: Opiliones)

Stridulatory organs have not been previously investigated for harvestmen in the family Cosmetidae. During a field study, we observed the infrequent production of vibrations by adult Cynorta marginalis Banks, 1909. Using SEM, we examined the surfaces of several appendages for potential stridulatory organs. Our observations indicate that C. marginalis has denticles on the mesal surfaces of the basichelicerites that when rubbed together may function as an isomorphous stridulatory organ. In addition, there are denticles on the ectal surfaces of the basichelicerites and furrowed ridges on the femora of the pedipalps that may represent heteromorphous stridulatory organs. We did not observe any sexual dimorphism in morphology at either anatomical location. We also examined the appendages of two additional cosmetid harvestmen: Paecilaema inglei Goodnight & Goodnight, 1947, a species that also stridulates when held and Erginulus clavotibialis (Pickard-Cambridge, 1905), a species that has not been observed to produce vibrations. As in C. marginalis, we observed denticles on mesal and ectal surfaces of the basichelicerites and a furrowed ridge on the mesal surfaces of the femora of the pedipalps of adult P. inglei. In contrast, the basichelicerites of E. clavotibialis had relatively fewer and smaller denticles on the external surfaces of the chelicerae and the mesal surface of the femora of the pedipalps lacked ridges and were relatively smooth. Our comparative morphological data supports the hypothesis that there are cosmetid harvestmen that may use surface features on the chelicerae and pedipalps to produce vibrations which may function as a secondary defense mechanism.