Sensory structures and sexual dimorphism in the harvestman Dicranopalpus ramosus (Arachnida: Opiliones)

Abstract

A survey on sensory organs of both sexes of the harvestman Dicranopalpus ramosus classifies structure and frequency of campaniform sensilla, falciform setae, sensilla basiconica, slit sensilla, solenidia, spines, sensilla chaetica, trichomes (simple hairs) and plumose setae. Sensilla are equally distributed on the pedipalp tarsi of both males and females, but females show higher counts of campaniform and falciform setae than males. Females furthermore have about 1000 glandular plumose setae on each pedipalp, that at the same positions in males are replaced by sensilla chaetica. The walking legs of both sexes show a similar distribution of sensory organs, with females showing more sensilla basiconica at the legs I and II and more solenidia on the first pair of legs. Males have a large number of bipterate setae (about 2200 per specimen) at the metatarsi and tarsi of the third and fourth pair of legs. In females these are replaced by simple hairs. Although females show a similar (or slightly higher) number of leg sensilla than males, their density is higher due to their shorter legs. In both sexes the second pair of legs has the largest number of falciform setae, sensilla basiconica, chaetica and solenidia, followed by the legs I, III and IV. The first pair of legs has the highest density of falciform setae, sensilla basiconica and solenidia, followed by the legs II, III and IV. The genital operculum, sternites and tergites show a multitude of slit sensilla. The slit sensilla of the genital operculum and sternites are associated with insertion plaques of muscles operating the penis/ovipositor and regulating opisthosomal volume and hemolymph-pressure.