Ricinuleids are one of the least investigated groups of Arachnida. In particular, knowledge of their ultrastructure is poor. Observations of the distal tarsomeres of ricinuleids show differences in their shape and equipment of surface structures. Legs I and II are used by the Ricinulei to explore their surroundings with tentative movements. The tarsomeres of these legs show similarities in shape and surface structures that distinguish them from those of legs III and IV. In this study, 11 different structures of the tarsomere surfaces of two cave-dwelling species, Pseudocellus pearsei and P. boneti from Mexico, were investigated for the first time with scanning and transmission electron microscopy and discussed regarding their possible function: 1) a single treelike ramifying seta resembles a no pore single-walled (np-sw) sensillum
2) setae occurring in a small number and possessing a bipartite shaft represent terminal pore single-walled (tp-sw) sensilla. The surface of the proximal half of the shaft shows small branches. The distal half has a smooth surface
3) long setae with conspicuous longitudinal lamellae show characteristics of chemoreceptive wall pore single-walled (wp-sw) sensilla
4) frequent small wp-sw sensilla with flat and irregular lamellae
5) very short wp-sw sensilla occurring solitary or in groups
6) a few short setae with smooth surface correspond to wp-sw sensilla
7) a single short clubbed seta articulating in a flat pit is considered to be a np-sw sensillum
8) common long setae with a pointed tip show characteristics of mechanoreceptive np-sw sensilla
9) ventral setae with adhesive and mechanosensory function are accompanied by multicellular "class III" glands
10) slit organs with mechanoreceptive function
and 11) dome-like tubercles with no indication of sensorial function. Several of these sensilla form a sensory field on the dorsofrontal surface which is particularly pronounced on the distal tarsomeres of legs I and II.
