The harvestman tarsus and tarsal flexor system with notes on appendicular sensory structures in Laniatores

The tarsal flexor system, a novel system of retinacular structures, is described for the first time based on morphological and ultrastructural examinations of several Neotropical harvestmen (Opiliones: Laniatores). The tarsal flexor system is made up of many individual pulleys that function to maintain close apposition between the tendon and internal ventral surface of the cuticle in the tarsus. Pulley cells are specialized tendinous cells that form the semi-circular, retinacular pulley system in the tarsus; these cells contain parallel arrays of microtubules that attach to cuticular fibers extending from deep within the cuticle (i.e., tonofibrillae). The tarsal flexor system is hypothesized to provide mechanical advantage for tarsal flexion and other movements of the tarsus. This system is discussed with regards to other lineages of Opiliones, especially those that exhibit prehensility of the tarsus (i.e., Eupnoi). Comparing tarsal morphology of laniatorid harvestmen to other well-studied arachnids, we review some literature that may indicate the presence of similar tarsal structures in several arachnid orders. The general internal organization of the tarsus is described, and ultrastructural data are presented for a number of tarsal structures, including sensilla chaetica and the tarsal perforated organ. Sensilla chaetica possess an internal lumen with dendritic processes in the center and exhibit micropores at the distal tip. With respect to the tarsal perforated organ, we found no ultrastructural evidence for a sensory or secretory function, and we argue that this structure is the result of a large pulley attachment site on the internal surface of the cuticle. A small, previously undocumented muscle located in the basitarsus is also reported.