Taxon of the Week: Stygnoplus

Cluster of an unidentified Stygnidae species. Photo from here.

South America is the current centre of described harvestmen diversity, with the bulk of this diversity represented by Laniatores - the shorter-legged, heavily armoured and often quite spiky suborder of Opiliones. Of the twenty-six families of Laniatores recognised in Pinto-da-Rocha & Giribet (2007), sixteen are found in the Neotropics (excluding the Podoctidae, whose sole "Neotropical" representative, Ibantila cubana from Cuba, was described from a specimen collected in a botanical garden and probably represents an introduction from parts unknown). The majority of research on South American harvestmen has focused on the largest family, the Gonyleptidae, but the remaining families are all waiting their turn.

The genus Stygnoplus belongs to one of these families, the Stygnidae. A number of features support the Stygnidae as a monophyletic group, perhaps the most apparent being their disassociated eyemound. Unlike most other harvestmen that have the two eyes located on a central eyemound, stygnids have the eyes placed some distance apart without a central mound - still fairly close in the subfamily Nomoclastinae, further apart in the subfamilies Stygninae and Heterostygninae (to the latter of which Stygnoplus belongs). About eighty species of Stygnidae have been described to date, but the existence of a number of undescribed species is known (Kury & Pinto-da-Rocha, 2002). Like most South American Opiliones, the stygnids have mostly only been studied from a taxonomic point of view, with very little known about their natural history. However, the Stygnidae are ahead of the game in that, unlike most other South American Opiliones (or, for that matter, Opiliones in general), they have been the subject of an actual phylogenetic analysis (Pinto-da-Rocha, 1997).


Another unidentified stygnid. Photo by artour_a.

The known centre of diversity for the Heterostygninae (including Stygnoplus) is the Lesser Antilles, though Stygnoplus was recorded from the mainland of South America by Villarreal-Manzanilla & Rodríguez (2004). (The type species, Stygnoplus forcipatus, had originally been described from the mainland, though with the completely uninformative and decidedly untrustworthy locality citation of "Colombia".)



In the absence of any other images of Stygnoplus online, here's Villarreal-Manzanilla & Rodríguez's (2004) figures of the Venezuelan species Stygnoplus lomion. Feel free to print them off, cut them out and see if you can assemble your own model of a South American arachnid. The appendages shown in the lower part of the plate are the pedipalps - stygnids, like many Laniatores, have absolutely terrifying raptorial pedipalps. If you look closely at the second photo on this post, you'll see that in this family the femora (the first large segment) of the pedipalps are quite long, giving these animals a quite impressive reach, perfectly designed to strike terror into the hearts (or other significant circulatory organs) of small invertebrates everywhere.

REFERENCES

Kury, A. B., & R. Pinto-da-Rocha. 2002. Opiliones. in Amazonian Arachnida and Myriapoda (J. Adis, ed.) pp. 345-362. Pensoft: Sofia.

Pinto-da-Rocha, R. 1997. Systematic review of the neotropical family Stygnidae (Opiliones, Laniatores, Gonyleptoidea). Arquivos de Zoologia 33(4): 163-342.

Pinto-da-Rocha, R., & G. Giribet. 2007. Taxonomy. In Harvestmen: The Biology of Opiliones (R. Pinto-da-Rocha, G. Machado & G. Giribet, eds.) pp. 88-246. Harvard University Press: Cambridge (Massachusetts).

Villarreal-Manzanilla, O., & C. J. Rodríguez. 2004. Descripción de una nueva especie y dos nuevos registros del género Stygnoplus (Opiliones, Stygnidae) para Venezuela. Revista Ibérica de Aracnología 10: 179-184.