Fine structure of the epicuticular secretion coat and associatedglands of Pedipalpi and Palpigradi (Arachnida)

Abstract

Pedipalpi Latreille, 1810 is a poorly studied clade of arachnids comprising the whipspiders (Amblypygi Thorell, 1883), short-tailed whip scorpions (SchizomidaPetrunkevitch, 1945) and whip scorpions (Thelyphonida Cambridge, 1872). It hasrecently been shown that whip spiders coat their exoskeleton with a solid cementlayer (cerotegument) that forms elaborate microstructures and turns the cuticle intoa super-hydrophobic state. The amblypygid cerotegument provides taxonomic infor-mation due to its fine structural diversity, but its presence and variation in the sistergroups was previously unknown. The present contribution reports the surface struc-ture of the cuticle in species of Palpigradi, Thelyphonida, and Schizomida to deter-mine if these taxa possess a solid epicuticular secretion coat. Scanning electronmicroscopy revealed that in addition to Amblypygi only species of Thelyphonida pos-sess solid epicuticular secretion layers. Unlike in Amblypygi, in the Thelyphonida thislayer does not usually form microstructures and is less rigidly attached to the under-lying cuticle. A species ofTypopeltisPocock, 1894, which exhibited globular struc-tures analogous to the amblypygid cerotegument, was an exception. Glandularstructures associated with cement secretions in Amblypygi and Thelyphonida wereconsidered homologous due to similar structure. Solid epicuticular secretion coatswere absent from Schizomida, which is interpreted as a secondary loss despite thepresence of slit-like glandular openings that appear to produce such epicuticularsecretions. The micro-whip scorpion order Palpigradi Thorell, 1900 exhibited mark-edly different cuticular surface structures and lacked solid epicuticular secretions,consistent with the hypothesis that this order is not closely related to Pedipalpi.These results enhance the knowledge of the small, enigmatic orders of Arachnida.