Stygophalangium: Harvestman or Mite?

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 two groups, the harvestmen and the mite group Parasitiformes. 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?

REFERENCES

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.

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

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.