Opiliones

Sections of Eophalangium sheari from the Rhynie Chert, copyright H. Hass. (A) Female in lateral section showing ovipositor. (B) Male opisthosoma in ventral section showing styliform penis and opisthosoma surrounded by long, slender legs. Abbreviations: cx = coxa; em = eye mound; gu = gut; ov = ovipositor; pe = penis. Scale bars = 1 mm.

Belongs within: Euchelicerata.
Contains: Cyphophthalmi, Palpatores, Insidiatores, Phalangodidae, Sandokanidae, Epedanoidea, Gonyleptoidea, Pyramidopidae, Assamiidae, Zalmoxoidea, Samooidea.

Stygophalangium: harvestman or mite?
Published 20 August 2015
The original illustration of Stygophalangium karamani, from Oudemans (1933).

In 1933, the Dutch zoologist Anthonie Oudemans described what he believed to be a remarkable new species of harvestman. Based on two specimens collected from an underground spring in modern-day Macedonia and dubbed Stygophalangium karamani, Oudemans regarded this as a highly degenerate form as a result of its habitat: small, soft-bodied, and eyeless. It exhibited some significant differences to other harvestmen: in particular, the body lacked obvious signs of external segmentation. Also, its apparent aquatic collection point stood in direct contrast to the otherwise terrestrial habitats of other species. Nevertheless, Oudemans placed this unusual animal in a new family, the Stygophalangiidae, and suggested that its reduced morphology compared to other harvestmen might be compared to the position of Eriophyes (a plant-feeding, four-legged genus) among the mites. However, due to its anomalous character, subsequent authors have not paid much attention to little Stygophalangium. Mello-Leitão (1944) briefly suggested that it might represent a primitive form, placing it at the base of a branch of the phylogenetic tree leading to the Cyphophthalmi (mite-like harvestmen) and Palpatores (long-legged harvestmen). A number of online sources, such as Wikipedia, refer to Stygophalangium as being classified with the Eupnoi (a subgroup of the Palpatores), but this claim seems to be baseless. It seems to be derived from Joel Hallan’s online list of harvestman species (which no longer appears to be available) but while Oudemans did compare Stygophalangium to the eupnoin Phalangium opilio (the common field harvestman) in his original description, he did not actually classify his new species with any particular subgroup of harvestmen. Eventually, Kury (2011) dismissed Stygophalangium from consideration in his summary of harvestman classification, stating that it ‘is probably a member of the Acari’.

Unfortunately, as much as Stygophalangium might not be a convincing harvestman, it is also not a very convincing mite. One of the primary features that lead Oudemans to see Stygophalangium as a harvestman was its possession of three-segmented chelicerae. Most arachnids have chelicerae with only two segments (the basal segment and an opposing mobile claw or fang); three-segmented chelicerae are only found in three groups, the harvestmen, the mite group Parasitiformes, and the palpigrades (the last of which bears no resemblance to Stygophalangium). Of the four main groups (Opilioacarida, Holothyrida, ticks and Mesostigmata) within the Parasitiformes, none are similar to Stygophalangium. The ticks have distinctly modified (and kind of terrifying) blood-sucking mouthparts. The Holothyrida and Mesostigmata are both armoured to varying degrees, and mesostigs also bear a branched structure called the tritosternum underneath the mouthparts that is not described for Stygophalangium. The Opilioacarida are large, superficially harvestman-like mites that also have visible indications of external segmentation. And while there are a number of known lineages of aquatic mites, none of them really looks anything like Stygophalangium. It would be surprising if Oudemans, one of the leading mite researchers of his time, failed to recognise a mite when he had one in front of him! It is true that Oudemans’ work underwent a precipitous decline in his last years as a result of problems with his mental health (Southcott 1961), but at the time of Stygophalangium‘s publication Oudemans remained alert and well.

Ventral view of Stygophalangium, with close-ups of chelicera, terminal pedipalp segments, and leg claw, from Oudemans (1933).

So if Stygophalangium was not a harvestman, and not a mite, then what was it? It is possible, of course, that it represented some taxon that has never been recorded since, but such an agnostic interpretation simply leaves the question of its affinities open. We can still at least try and compare it to other animals as best we can. One quite important point that I have avoided mentioning so far is that Oudemans’ specimens were apparently not mature: Oudemans was unable to find indications of either a genital or anal opening. Though he described the body as unsegmented, it should be noted that his illustration is a reconstruction of what was apparently a not so smoothly mounted animal. Oudemans did note that a number of creases were visible on the bodies of his specimens, though he interpreted these as artefacts of slide-mountaing rather than segment boundaries because they did not appear to be placed evenly (with some creases even crossing over each other). Also, the supposed aquatic habitat may be a red herring. Subterranean samples are commonly collected by lowering sampling devices down a borehole, and it is not unknown for surface-dwelling organisms to fall in the borehole or be picked up when the traps are raised or lowered. So is Stygophalangium a larval harvestman or mite?

Again, we can rule out any arachnid except harvestmen or parasitiform mites due to the three-segmented chelicerae. The objections given above to adult ticks or Mesostigmata apply equally well to their juveniles, so they’re also out. Larval Holothyrida lack the heavy armour of the adults, but these large litter-dwelling mites are not found anywhere near Europe. On the harvestman side of things, most harvestmen as both adults and nymphs have the second pair of legs particularly long and filamentous, functioning in a similar manner to the antennae of insects. The only harvestmen to lack this feature are the Cyphophthalmi, and together with the Opilioacarida they are the only real candidates for comparison with Stygophalangium. Both are soil-dwelling animals, and both are known from the Balkan region.

Larva of Opilioacarus texanus, from Klompen (2000).

One point in favour of an opilioacarid identity is that Oudemans described the chelicerae of Stygophalangium as inserted more dorsally than in other harvestmen. Opilioacarids have similarly inserted chelicerae, with a hypostome extending underneath the chelicerae. Oudemans also described Stygophalangium as lacking setae dorsally (instead having a somewhat scaly texture); opilioacarids have dorsal setae on the prosoma only. The opiliacarid prelarva (the earliest stage of its life cycle) has a scaly texture very similar to Stygophalangium (Klompen 2000), but mite larvae and prelarvae have only three pairs of legs. If Stygophalangium is an opilioacarid, it would have to be one of the later nymphal instars in which the fourth pair of legs has developed. Other features of opilioacarid juveniles conflict with Stygophalangium, such as the two pairs of large eyes on the opilioacarid prosoma. Also, Oudemans illustrated the venter of Stygophalangium with the coxae (the basalmost leg segment) integrated with the underside of the body, whereas opilioacarids (like other Parasitiformes) have the coxae free from the venter and attached by sockets. As Oudemans indicated the coxae of Stygophalangium with dotted lines only, it is possible that he inferred their position under the assumption of harvestman affinities. However, even if we assume this to be the case and that what Oudemans took to be the trochanters (the second leg segment) were actually the coxae, then Stygophalangium is left with one leg segment too few.

Larva of Siro rubens, from Juberthie (1964).

The only information on the juvenile stages of Cyphophthalmi is a brief description of the larva of Siro rubens by Juberthie (1964). Cyphophthalmi lack obvious eyes, and their legs do have the right number of segments for Stygophalangium. Juberthie described the cyphophthalmid larva as lacking a developed anus, which correlates with Oudeman’s description of Stygophalangium (opilioacarid nymphs, in contrast, have a well-developed anal cone). He also recorded the presence of a pair of egg-teeth in the midline of the prosoma near the front of the body, in the same position where Oudemans described a distinctive pigmented spot on Stygophalangium. Points against a cyphophthalmid identification include the non-dorsal insertion of the chelicerae (though, again, one can’t help wondering about the possibility of distortion through slide-mounting) and the presence of sparse but distinct dorsal setae. Especially difficult are the pairs of large setae marking the positions of the repugnatorial tubercles on either side of the prosoma. Unfortunately, Juberthie did not describe the venter of the cyphophthalmid larva, or comment on the degree of sclerotisation (mature cyphophthalmids are heavily sclerotised, whereas Stygophalangium is explicitly soft-bodied).

And that is about as far as we can go without looking at the original specimens. Personally, I suspect the issues with a cyphophthalmid identification are easier to overcome than those with an opilioacarid one (perhaps Oudemans did indeed mistake segment boundaries for mounting artefacts, and perhaps the dorsal setae had been lost post-mortem and Oudemans overlooked their sockets) but any such judgement requires the original description to be at least partially erroneous. Oudemans said that his type specimens were deposited in the Rijksmuseum van Natuurlijke Historie in Leiden; I wonder if they’re still there?

Hastocularis: a fossil harvestman allows us to see
Published 1 March 2017

Sometimes the fossil record just gives us a gift, something that moves our understanding to an all-new level. One such gift saw publication a couple of years ago, but unfortunately I didn’t have time to write about it then. I think it’s about time I corrected that lacuna.

Reconstruction of Hastocularis argus, from Garwood et al. (2014).

By this point in time, we have a pretty good understanding of the basal framework of harvestmen evolution. The mite-like harvestmen of the Cyphophthalmi are well established as the sister group to all other Opiliones (which form a clade called the Phalangida). Unique features of the Phalangida include an intromittent penis in the males (phalangids are one of the few groups of arachnids to possess such a feature) and a central eyemound with a single pair of eyes. The Cyphophthalmi are more heavily armoured than most phalangids, and have a characteristic pair of raised cones (the ozophores) on either side of the carapace near the front that support the openings of odour-producing repugnatorial glands. Until recently, it was thought that most Cyphophthalmi lack eyes, but tiny, lens-less remnant eyes are now known to be present at the base of the ozophores in many cyphophthalmid subgroups.

There had long been questions about the nature of the cyphophthalmid eyes. The original arachnids possessed multiple pairs of eyes, and there is a good case to be made that the basal arrangement for arachnids as a whole is a single pair of larger median eyes in the middle of the carapace, and a number of pairs (up to three) of smaller lateral eyes at the margin. In some arachnid groups the median eyes have been lost; in others, the lateral eyes have become reduced in number or lost. In spiders, the lateral eyes have become enlarged and shifted about so the lateral/median distinction is less applicable (for the record, the posterior median eyes in spiders correspond to the original median eyes). Mites, of course, being mites, mess the whole system up entirely. Most mite eyes correspond to the original lateral eyes, but some mites possess a single median eye whose relation to the original arachnid median eye pair is up for grabs.

Phalangids, with their single central eyemound and single pair of eyes, had obviously kept the original median eyes and lost the lateral eyes. But what had happened with the Cyphophthalmi? Did their single pair of eyes near the edge of the carapace represent a single remnant pair of lateral eyes, or did they correspond to the median eyes of other Opiliones? It should be noted that some derived groups of undoubted Phalangida have lost the eyemound and have their eyes sitting directly on the carapace, and in some cases these unraised eyes may be widely separated. Arguments for both interpretations of cyphophthalmid eyes had been put forward by different authors, but the matter had certainly not been decided.

A representative member of Phalangida, Platybunus pinetorum, showing the central eyemound, from Opiliophilia.

That was until the description by Garwood et al. (2014) of Hastocularis argus, a remarkably preserved fossl harvestman from the Carboniferous of France. The appearance of this animal was established in some detail by the use of microtomography, allowing a number of details about it to be established. It was a heavily armoured animal with long legs, and like modern Phalangida it possessed a central eyemound on which there had been a pair of eyes (the eyes themselves were not preserved, but the sockets that had originally contained them were). The use of microtomography also allowed the identification of an intromittent penis like a phalangid. But Hastocularis also possessed a pair of raised ozophores like modern Cyphophthalmi, and at the base of those was preserved another socket indicating the presence of a second pair of eyes. There really could not be a more perfect answer to the cyphophthalmid eye question: the immediate ancestor of the Opiliones possessed two pairs of eyes, and the eyes of Cyphophthalmi do indeed correspond to the lateral eyes of other non-harvestmen arachnids and not to the median eyes of phalangids*.

*Pedantically speaking, Hastocularis is not the first four-eyed taxon assigned to the Opiliones. In 1875, an Austrian biologist by the name of Stecker described a remarkable animal from the Sudeten Mountains of Bohemia under the name of Gibbocellum sudeticum. Gibbocellum bore an overall resemblance to the Cyphophthalmi, except for possessing two pairs of eyes on raised cones, as well as two pairs of spiracles (other Opiliones possess a single pair). Despite enthusiastic searches, no other naturalist was ever able to find further specimens of Stecker’s species, and at least one author suggested that it might be a poorly interpreted pseudoscorpion. However, a close criticism of various irregularities in Stecker’s publications on Gibbocellum eventually lead Hansen & Sørensen (1904) to the conclusion that it had not merely been misrepresented, but was in fact a complete fabrication on that author’s part.

A phylogenetic analysis of Hastocularis lead Garwood et al. (2014) to believe that it was more closely related to Cyphophthalmi than to Phalangida; together with another Carboniferous fossil species, Eophalangium sheari, they placed it within a new taxon Tetrophthalmi (meaning, of course, ‘four eyes’). The main features cited in support of this relationship were the complete fusion of the dorsal surface (the only other harvestmen to show this feature are a southeast Asian family, the Oncopodidae, who are too deeply nested within the Phalangida to be a likely direct relative of Hastocularis) and the genital opening being a broadly open gonostome (in Phalangida, the genital opening is covered by an operculum). This implies that the immediate ancestor of all Opiliones was relatively long-legged, with the short legs of Cyphophthalmi a derived feature. However, I personally find the presence of an intromittent penis in Tetrophthalmi (it is also known to be present in Eophalangium) somewhat problematic in this regard. As noted above, the phalangid intromittent penis that directly injects sperm into the female ovipositor is highly unusual among arachnids. Cyphophthalmi do possess a penis-like structure (called the spermatopositor) but it is much shorter than in any phalangid and does not function as an intromittent organ. Instead, Cyphophthalmi males produce an encapsulated spermatophore that is attached by the spermatopositor to the female’s underside, a more typical sort of arrangement for arachnids as a whole. An intromittent penis in the cyphophthalmid stem group would imply that Cyphophthalmi somehow reverted towards a more primitive-seeming reproductive arrangement at some point in the past. One possibility is that the penis of Tetrophthalmi did not function in exactly the same manner as that of Phalangida: perhaps tetrophthalmids still produced a spermatophore but were able to insert it more deeply in the female than Cyphophthalmi? Another possibility may be that Tetrophthalmi are stem-phalangids rather than stem-cyphophthalmids; only further analyses can possibly tell us more.

Systematics of Opiliones
<==Opiliones [Cyphopalpatores, Opilioni, Opilionidae, Opilionina, Phalangoideae]
    |  i. s.: Stygophalangium Oudemans 1933 [Stygophalangiidae]O33
    |           `--*S. karamani Oudemans 1933O33
    |         Arachnometa Petrunkevitch 1949SDG16
    |           `--*A. tuberculata Petrunkevitch 1949SDG16
    |--+--CyphophthalmiGS14
    |  `--Hastocularidae [Tetrophthalmi]GS14
    |       |--Hastocularis Garwood, Sharma et al. 2014GS14
    |       |    `--*H. argus Garwood, Sharma et al. 2014GS14
    |       `--Eophalangium Dunlop, Anderson et al. 2004GS14, DA04
    |            `--*E. sheari Dunlop, Anderson et al. 2004DA04
    `--Phalangida [Dyspnolaniatores, Opilionides, Phalangeae, Phalangides]GE02
         |  i. s.: Archaeometidae [Archaeometoidea]SDG16
         |           |--Eopholcus Fritsch 1904SDG16
         |           |    `--*E. pedatus Fritsch 1904F04
         |           `--Archaeometa Pocock 1911SDG16
         |                |--*A. nephilina Pocock 1911SDG16
         |                `--A. devonica Størmer 1976SDG16
         |--PalpatoresGV09
         `--Laniatores [Eulaniatores, Gonoleptides, Gonyleptomorphi, Lomaniatores, Mecostethi, Tricospilata]GE02
              |  i. s.: Ausulus minutus Roewer 1927KSP-G15
              |         Gunturius glaber Roewer 1949KSP-G15
              |         Johorella simplex Roewer 1949KSP-G15
              |         Kokoda luteoscutum Roewer 1949KSP-G15
              |         Pegulius parvulus Roewer 1949KSP-G15
              |         Peltamma sumatranum Roewer 1927KSP-G15
              |         Seblatus coxalis Roewer 1949KSP-G15
              |         Sergitius spiniger Roewer 1949KSP-G15
              |--InsidiatoresDL23
              `--Grassatores [Camptonoti]SG14
                   |  i. s.: Anamota Šilhavý 1979K03
                   |           `--*A. custodiens Šilhavý 1979K03
                   |         Belemarua Roewer 1949K03
                   |           `--*B. nitens (Soares & Soares 1946) [=Paramitraceras nitens, Timoleon nitens]K03
                   |         Bissopius Roewer 1949K03
                   |           `--*B. rugosus Roewer 1949K03
                   |         Caribula Šilhavý 1979K03
                   |           `--*C. longimana Šilhavý 1979K03
                   |         Cleombrotus Sørensen in Henriksen 1932K03, KM20
                   |           `--*C. minutus Sørensen in Henriksen 1932K03, KM20
                   |         Contuor Roewer 1963K03
                   |           `--*C. novum Roewer 1963K03
                   |         Detlefilus Roewer 1949K03
                   |           `--*D. trispinosus Roewer 1949K03
                   |         Ignacianulus Roewer 1957K03
                   |           `--*I. pumilio Roewer 1957K03
                   |         Jimeneziella Kury & Alonso-Zarazaga 2011KA-Z11
                   |           |--*J. decui Avram 1970K03
                   |           `--J. negreai Avram 1970K03
                   |         Alpazia Özdikmen & Kury 2006 [=Lapazia Roewer 1949 non Ferris 1937]OK06
                   |           `--*A. minima (Roewer 1949) [=*Lapazia minima]OK06
                   |         Manuelangelia Kury & Alonso-Zarazaga 2011KA-Z11 (see below for synonymy)
                   |           `--*M. tuberosa (González-Sponga 1998)KA-Z11 [=*Limonia tuberosaK03]
                   |         Liomma Roewer 1959K03
                   |           `--*L. laeve Roewer 1959K03
                   |         Micrisaeus Roewer 1957K03
                   |           `--*M. gracillimus Roewer 1957K03
                   |         Mirda Šilhavý 1973K03
                   |           `--*M. insulanus (Banks 1901) [=Stygnus insulanus]K03
                   |         Munis Roewer 1963K03
                   |           `--*M. multivirgatus Roewer 1963K03
                   |         Octophthalmus Wood 1869K03
                   |           `--*O. marginatus Wood 1869 (n. d.)K03
                   |         Ortizia Roewer 1952K03
                   |           `--*O. gracilipes Roewer 1952K03
                   |         Neoparalus Özdikmen 2006 [=Paralus Roewer 1949 non Rafinesque 1815]O06
                   |           `--*N. granitus (Roewer 1949) [=*Paralus granitus]O06
                   |         Pentos Roewer 1952K03
                   |           `--*P. pygoplus Roewer 1952K03
                   |         Pucallpana Avram & Soares 1983K03
                   |           `--*P. pullex Avram & Soares 1983K03
                   |         Siryseus Roewer 1949K03
                   |           `--*S. tibialis Roewer 1949K03
                   |         Tarmaops Roewer 1956K03
                   |           `--*T. koepckei Roewer 1956K03
                   |         Valifema Šilhavý 1979K03
                   |           `--*V. blanda Šilhavý 1979K03
                   |--PhalangodidaeGV09
                   `--+--SandokanidaeSG11
                      `--+--EpedanoideaDL23
                         `--+--GonyleptoideaDL23
                            `--+--PyramidopidaeDL23
                               `--+--AssamiidaeDL23
                                  `--+--ZalmoxoideaDL23
                                     `--SamooideaDL23

Manuelangelia Kury & Alonso-Zarazaga 2011KA-Z11 [=Limonia González-Sponga 1998 nec Meigen 1803 nec Agassiz 1846 nec Carvalho 1985K03]

*Type species of generic name indicated

References

[DL23] Derkarabetian, S., A. Lord, K. Angier, E. Frigyik & G. Giribet. 2023. An Opiliones-specific ultraconserved element probe set with a near-complete family-level phylogeny. Molecular Phylogenetics and Evolution 187: 107887.

[DA04] Dunlop, J. A., L. I. Anderson, H. Kerp & H. Hass. 2004. A harvestman (Arachnida: Opiliones) from the Early Devonian Rhynie cherts, Aberdeenshire, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences 94: 341–354.

[F04] Fritsch, A. 1904. Palaeozoische Arachniden. Selestverlag: Prague.

[GS14] Garwood, R. J., P. P. Sharma, J. A. Dunlop & G. Giribet. 2014. A Paleozoic stem group to mite harvestmen revealed through integration of phylogenetics and development. Current Biology 24 (9): 1017–1023.

[GE02] Giribet, G., G. D. Edgecombe, W. C. Wheeler & C. Babbitt. 2002. Phylogeny and systematic position of Opiliones: a combined analysis of chelicerate relationships using morphological and molecular data. Cladistics 18: 5–70.

[GV09] Giribet, G., L. Vogt, A. Pérez González, P. Sharma & A. B. Kury. 2009. A multilocus approach to harvestman (Arachnida: Opiliones) phylogeny with emphasis on biogeography and the systematics of Laniatores. Cladistics 25: 1–30.

Hansen, H. J., & W. Sørensen. 1904. On Two Orders of Arachnida: Opiliones, especially the suborder Cyphophthalmi, and Ricinulei, namely the family Cryptostemmatoidae. University Press: Cambridge.

Juberthie, C. 1964. Recherches sur la biologie des opilions. Annales de Spéléologie 19 (1): 5–244.

Klompen, J. S. H. 2000. Prelarva and larva of Opilioacarus (Neocarus) texanus (Chamberlin and Mulaik) (Acari: Opilioacarida) with notes on the patterns of setae and lyrifissures. Journal of Natural History 34 (10): 1977–1992.

[K03] Kury, A. B. 2003. Annotated catalogue of the Laniatores of the New World (Arachnida, Opiliones). Revista Ibérica de Aracnología, volumen especial monográfico 1: 1–337.

[KA-Z11] Kury, A. B., & M. A. Alonso-Zarazaga. 2011. Addenda and corrigenda to the “Annotated catalogue of the Laniatores of the New World (Arachnida, Opiliones)”. Zootaxa 3034: 47–68.

[KM20] Kury, A. B., A. C. Mendes, L. Cardoso, M. S. Kury & A. A. Granado. 2020. WCO-Lite: Online world catalogue of harvestmen (Arachnida, Opiliones). Version 1.0—Checklist of all valid nomina in Opiliones with authors and dates of publication up to 2018. Published by the author: Rio de Janeiro.

[KSP-G15] Kury, A. B., D. R. Souza & A. Pérez-González. 2015. World checklist of Opiliones species (Arachnida). Part 2: Laniatores—Samooidea, Zalmoxoidea and Grassatores incertae sedis. Biodiversity Data Journal 3: e6482.

[O33] Oudemans, A. C. 1933. Ein neuer Stygobiont, Stygophalangium karamani Oudms. Zoologischer Anzeiger 103: 193–198.

[O06] Özdikmen, H. 2006. Nomenclatural changes for some Laniatores (Opiliones) genera: new substitute names and new combinations. Mun. Ent. Zool. 1 (1): 63–68.

[OK06] Özdikmen, H., & A. B. Kury. 2006. Three homonymous generic names in Araneae and Opiliones. Journal of Arachnology 34 (1): 279–280.

[SDG16] Selden, P. A., J. A. Dunlop & R. J. Garwood. 2016. Carboniferous araneomorph spiders reinterpreted as long-bodied harvestmen. Journal of Systematic Palaeontology 14 (2): 127–137.

[SG11] Sharma, P. P., & G. Giribet. 2011. The evolutionary and biogeographic history of the armoured harvestmen—Laniatores phylogeny based on ten molecular markers, with the description of two new families of Opiliones (Arachnida). Invertebrate Systematics 25: 106–142.

[SG14] Sharma, P. P., & G. Giribet. 2014. A revised dated phylogeny of the arachnid order Opiliones. Frontiers in Genetics 5 (255): 1–13.

Southcott, R. V. 1961. Studies on the systematics and biology of the Erythraeoidea (Acarina), with a critical revision of the genera and subfamilies. Australian Journal of Zoology 9: 367–610.

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