Possibly The Coolest Thing I Had Published This Year

Dorsal view of male Australiscutum hunti (missing a few legs, I'll admit). Photo by yours truly.

In 1991, Hunt and Cokendolpher published a paper describing the new harvestman subfamily Ballarrinae and comparing to a selection of long-legged harvestmen from around the globe. In that paper, they referred to an undescribed species of the subfamily Monoscutinae from eastern Australia. For whatever reason, Glenn Hunt never completed a description of this new species before his death but I am happy to say that it has finally been brought into print by Taylor (2009) with the description of three species of the new genus Australiscutum.

Except for the minor detail that Australiscutum is not actually a monoscutine. Monoscutines are small harvestmen from New Zealand; despite belonging to the Phalangioidea, which are known as long-legged harvestmen, monoscutines have relatively short, robust legs. They also have the entire dorsal surface sclerotised (hence the name 'monoscutine' or 'single shield'; in most other phalangioids, only the prosoma or cephalothorax is sclerotised) and ornamented with small nodules. Australiscutum resembles monoscutines in having relatively short legs and while the dorsal surface of the opisthosoma (or abdomen) is not sclerotised, it is covered in small spine-like setae. However, in other features such as genital morphology Australiscutum is quite distinct from monoscutines. Shorter legs and hardened bodies have evolved in a number of different harvestman lineages, such as troguloids and various groups of Laniatores, and are probably an adaptation to a soil-dwelling lifestyle. Though it hadn't been run at the time I submitted this paper, my phylogenetic analysis of Monoscutidae (for which I'll probably be submitting the manuscript in a few days' time) corroborates the lack of a connection between Australiscutum and monoscutines and that the short-legged morphology has arisen twice within the family.

Neat as that is, though, that's not the really cool thing about Australiscutum. This is:



These are the chelicerae (the pincers) of males of the three species of Australiscutum as seen from the front and to roughly the same scale. Except for the appearance of the chelicerae, the differences between each of the species are relatively minor (though A. hunti is the most distinct of the three). Australiscutum hunti has large swollen chelicerae; A. graciliforceps has much smaller slender chelicerae. But when we reach A. triplodaemon, we see that it has one larger, more swollen chelicera and one smaller, more slender chelicera*. This cheliceral asymmetry is unique among monoscutids and, so far as I've been able to find, is unique among harvestmen. In fact, I haven't yet found any previous record of asymmetrical pincers like these for any arachnid.

*The species name of Australiscutum triplodaemon is a reference to the Triple Demons of Compromise in Norton Juster's The Phantom Tollbooth. Of these three, one demon was very tall and thin, the second was very short and fat, and the third one looked exactly like the other two.

So far, I can only make a few vague inferences about why A. triplodaemon has these uneven chelicerae, based on comparisons with asymmetrical pincers in crustaceans (where they are known in a number of different lineages). One thing I can say for sure that it is a true morphological feature and not a pathology. If a crustacean such as a crab or lobster looses a pincer while growing, the new claw that grows back may be smaller, but A. triplodaemon can be inferred to have naturally asymmetrical pincers because (a) it's always the left chelicera that is smaller, and (b) of the other two species in the genus, the one that is closest in appearance to A. triplodaemon (and hence the one that would represent its non-pathological form if it was a developmental accident) is the small-pincered A. graciliforceps and not the large-pincered A. hunti. Many crustaceans develop differently-sized pincers as an adaptation for handling food (for instance, they may use one pincer to hold a prey mollusc and they other to break open its shell) but, again, this seems unlikely to apply to A. triplodaemon as the asymmetrical chelicerae are only found in males. Female Australiscutum have small discrete chelicerae like other monoscutids; unless the females were eating a different diet from the males (not impossible, but unusual), we might expect both sexes to have asymmetrical chelicerae if they were related to food-processing.

The third role that has been recorded for asymmetrical pincers among crustaceans is as part of mating displays (fiddler crabs are the most famous example) and their presence in males only suggests that this is also they role they play for A. triplodaemon*. Determining just how A. triplodaemon uses its odd-shaped pincers will require observation of living specimens.

*Though I say that in the full knowledge that I am largely invoking the old line that "if you don't know what it's for, then it's for display".

I'd like to share one more interesting thing that came out of the review process for this paper. The two most similar species of Australiscutum, A. triplodaemon and A. graciliforceps, are in fact identical except for their chelicerae. Their recorded ranges also overlap, and the possibility occured to me when I was first describing them that they might be different male forms of a single species. Male dimorphism has been recognised in a number of harvestman species, including monoscutids, but because the mode of dimorphism in Australiscutum would differ from modes described previously I decided that the safest approach for now was to treat the two forms as separate species. I hemmed and hawed as to whether I should point out the possibility of their being dimorphs of a single species in the manuscript but eventually decided that it was perhaps too much speculation on my part and left it out (I tend to be fairly cautious in drawing inferrences; I've never been able to decide whether this is a good or bad thing). However, when I received the reviewers' comments, two of the three reviewers had thought of the same possibility without any prompting from me and suggested that I comment on it. So it's there.

REFERENCE

Taylor, C. 2009. Australiscutum, a new genus of Monoscutidae (Arachnida: Opiliones) from eastern Australia, with the first record of asymmetrical chelicerae in Opiliones. Insect Systematics and Evolution 40 (4): 319-332.