Saintly Harvestmen (Taxon of the Week: Equitius)

Features of Equitius formidabilis. Basically, a lot of spikes. From Hunt (1985).

The Australian harvestman genus Equitius was first named by the French arachnologist Eugene Simon in 1880. Now there was nothing particularly odd about that - for many years in the late 1800s, Simon was not so much an arachnologist as the arachnologist, achieving a reputation none of his contemporaries could match, and possibly none of his successors either (apparently, the Muséum National d'Histoire Naturelle still has his chair and desk on display). Simon used classical names for many of his genera, and Equitius was such a genus - there are numerous personages in Roman history by the name of Equitius, including an early Catholic saint. In this case, unfortunately, Simon seems to have dropped the ball somewhat in terms of getting the word out, because in 1903, the British researcher Pocock described a very similar species as belonging to a new genus, Monoxyomma. Then the baton was taken up by arachnology's favourite bête noire, Roewer, who in 1915 and 1931 added further genera, distinguished (as usual for Roewer) by the most superficial of features*, to the list. It wasn't until 1985 that the Australian Glenn Hunt combined all these genera into a single one, Equitius, found in southern Queensland and New South Wales.

*Hunt (1985) was later to refer to specimens for which the Roewerian system would have identified oneside as Equitius, and the other as Monoxyomma.

Equitius is a member of the Triaenonychidae, a family of Laniatores or short-legged harvestmen. Laniatores generally tend to be rather spiky, heavily-armoured creatures, but some triaenonychids have a tendency to be particularly baroque, with high spines ornamenting the eyemound and abdomen, and large spiny pedipalps in the males. The greatest diversity of triaenonychids has been described from Australasia (though southern Africa is fast catching up), and in my experience triaenonychids are the easiest harvestmen to find in New Zealand. That is, assuming that you can see them - they can readily be found by lifting logs and stones in moist areas, but the usual triaenonychid response when disturbed is to ball up their legs and freeze, at which point they become very difficult to spot against the background. Even if you do spot them, they still have the defense of the distinctive harvestman odour, which has on at least one occasion nearly (I stress nearly) fooled me into thinking that an individual I'd found was both dead and in a reasonably advanced state of decay.


An individual of a North American triaenonychid species, Fumontana deprehendor. Fumontana is something of a biogeographic enigma - despite its Appalachian distribution, it appears to be more closely related to Southern Hemisphere triaenonychids than to other North American species (which are quite possibly not correctly assigned to Triaenonychidae). Photo from the Marshal Hedin Lab.

One intriguing feature of a number of triaenonychid genera is the occurrence of male dimorphism, with one male form failing to develop the enlarged pedipalps and other secondary sexual characteristics of the other male form. In many similar cases in other animals, such male dimorphism is related to trade-offs between attractiveness to females and overall vitality, but it has not been demonstrated if that is the case with triaenonychids. Glenn Hunt studied the development of effeminate males in Equitius doriae for his PhD thesis, but unfortunately only the abstract was ever published (Hunt, 1981). Hunt found that effeminate males became dormant over winter an instar earlier than normal males, suggesting that dimorphism in this genus may be as much a matter of environmental factors as anything else.

REFERENCES

Hunt, G. S. 1981. Male dimorphism and geographic variation in the genus Equitius Simon (Arachnida, Opiliones). Dissertation Abstracts International B 41: 4375.

Hunt, G. S. 1985. Taxonomy and distribution of Equitius in eastern Australia (Opiliones: Laniatores: Triaenonychidae). Records of the Australian Museum 36: 107-125.