Field of Science

Taxon of the Week: Metarhaucus

Metarhaucus lojanus, from Roewer (1912).

This was a bit of an unfair ID challenge, I'll admit. The most distinctive feature of the Cosmetidae, the harvestman family to which this animal belongs, is their laterally flattened pedipalps, not shown in the picture above. Without that, you would have had to combine vaguer features such as the overall shape of the body, the lack of spines on the eyemound (most related families have at least small spines) and how the divisions between the regions of the dorsal shield are less sharply incised than in other families.

Metarhaucus is a genus of eleven species of cosmetid, mostly found in Colombia, Ecuador and Peru (a single species has been described from Costa Rica; Kury, 2003). The genus has not been revised since the days of Roewer so it suffers the usual problems with Roewer's usage of genera based on characters that may turn out to be unreliable with further study. Roewer's (1912) defining characters for Metarhaucus were the presence of six pseudosegments in tarsus I, more than six pseudosegments in tarsus III, third and fourth legs much thicker than the front two pairs and the presence of a median pair of tubercles on the second and fourth areas of the dorsal shield. The South American Cosmetidae are currently under review by researchers in Brazil; time will tell whether Metarhaucus survives their scrutiny.


Kury, A. B. 2003. Annotated catalogue of the Laniatores of the New World (Arachida, Opiliones). Revista Ibérica de Aracnología, special monographic volume 1: 1-337.

Roewer, C. F. 1912. Die Familie der Cosmetiden der Opiliones - Laniatores. Archiv für Naturgeschichte, Abteilung A 10: 1-122, pl. 1-2.


  1. Hmm seems that, having been rumbled on a number of occasions, you've now decided that the ID challenge should be played without showing the diagnostic features... Fair play Senor Taylor. Next time you show us a dinoflagellate, maybe it could be life-size, to make things a little tricker for your faithful audience.
    Now, what's the biggest prey a harvestman can take in relation to bodysize?

  2. Most harvestmen are scavengers so they can eat any size that doesn't run away. If taking live prey, they'll generally only take very small animals (such as springtails). A few are specialist snail predators; presumably they can tackle larger prey.

  3. Not sure where to put this but have you see the ***awesome*** high-res image of Phalangium opilio eyes over on Bioephemera Nov 19th 2010 post?
    I'm seriously thinking of buying it to stick above my workstation... Magnifique.

  4. And yet, Opiliones eyes can't actually resolve images. All they can make out is the distribution of light and dark.

  5. I have just some great males Metarhaucus in front of me. Now the task to find out which species!


Markup Key:
- <b>bold</b> = bold
- <i>italic</i> = italic
- <a href="">FoS</a> = FoS