Palpigradi: Morphology & Physiology

Palpigrades are eyeless, pale, soft-bodied arachnids 0.7–2.7 mm in length (without appendages). The anterior part of the body, the prosoma, carries chelicerae with big shears, leg-like pedipalps with claws, and four pairs of legs including the very long, sensory forelegs. A dorsal shield with lanceolate sense organs covers the four anterior appendage-bearing segments. A waist marks the articulation between the prosoma and the 11-segmented opisthosoma, which terminates in a filiform subdivided flagellum. The last three opisthosomal segments are constricted and may be considered as a metasoma. 

The mode of sperm transfer is unknown. Measurements of oocytes indicate relatively big eggs. Laboratory observations of post-embryonic development are missing. As inferred from morphological studies, palpigrades pass one larval stage (Prokoeneniidae: two larval stages) and one juvenile stage. Males and females differ in the lobes that cover the genital opening and, occasionally, in the number of setae on opisthosomal sternites.  

Recent evidence suggests that Eukoenenia spelaea feeds (also?) on cyanobacteria. The traditional view of predatory feeding is supported by a video that shows a Eukoenenia capturing a tiny springtail seemingly by chance, without visible hunting strategy. The lack of chitinous arthropod remains in the guts of palpigrades may indicate selective uptake of soft tissues or extra-intestinal digestion. 

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