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

Abstract

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.

Survey of cuticular structures on leg IV of cosmetid harvestmen (Opiliones: Laniatores: Gonyleptoidea)

“…They hypothesized that it is the one place on the tarsus that serves as a final anchoring site immediately prior to exuviation. Our observations revealed interspecific variation in the number of plates comprising the tarsal perforated organ (Table 1) but we did not observe any pores or glandular products in association with individual plates, and thus our observations are consistent with the functional hypothesis proposed by Proud & Felgenhauer (2013b).…”

“…Willemart et al (2007) first described the tarsal perforated organ as glandular structures composed of 10-20 plates and observed them occurring on the tarsi of all four legs of two gonyleptid species. Proud & Felgenhauer (2013b) examined the ultrastructure of the tarsal perforated organ in several gonyleptoidean harvestmen including cosmetid species and found no evidence to support either a glandular or a sensory function. Proud & Felgenhauer (2013b) hypothesized that the tarsal perforated organ likely aids in the molting process by providing a large attachment site for tendinous cells which maintain a connection with the old cuticle during the premolt stage.…”

“…Proud & Felgenhauer (2013b) examined the ultrastructure of the tarsal perforated organ in several gonyleptoidean harvestmen including cosmetid species and found no evidence to support either a glandular or a sensory function. Proud & Felgenhauer (2013b) hypothesized that the tarsal perforated organ likely aids in the molting process by providing a large attachment site for tendinous cells which maintain a connection with the old cuticle during the premolt stage. They hypothesized that it is the one place on the tarsus that serves as a final anchoring site immediately prior to exuviation.…”

Traction reinforcement in prehensile feet of harvestmen (Arachnida, Opiliones)

“…Flexion and extension of the tarsus is controlled by the pretarsus (claw) levator and depressor muscles, which are located in the patella and tibia and the basitarsus, respectively (Shultz, 1989(Shultz, , 2000. A system of pulleys guides the ventral tendon to keep proper motion control over the large number of adesmatic joints (Proud and Felgenhauer, 2013).…”