I should probably entitle this post as an homage to Robert Burns’ poem ‘To a Mouse’, because this ‘best-laid schemes o’ mites an’ men, Gang aft agley’ (tacky true, but irresistible) has been an orphaned draft for more than two years, but then 2013 was truly gang aft agley. In any case, I wanted to congratulate Boyer & Router on their 2012 paper* advancing the study of a fascinating group of little-known arachnids, the Cyphophthalmi.
Interestingly, the openings to the tracheal system are positioned behind the legs and the genitalia (segment VIII) as one would expect, but the juveniles have a series of spiracle-like structures in the soft pleural region of the opisthosoma. These structures do not seem to connect to any tracheal or duct system, but presumably have some function.
What makes these structures particularly interesting is they seem to be similar to the mystery organ on adult Allothyrus (Acari: Holothyrida) mites called the peridium. I think it looks rather like the tool I used to use to clean sparkplugs, but I can’t think how the mites could insert anything in such an awkward position. Perhaps it is an organ for releasing pheromones or allomones, but I never found an obvious reservoir associated with the peridium.
Well, hard to rule out convergence, but it is an interesting similarity between these small, mite-like opilionids and the rather large (for a mite) and ‘primitive’ Allothyridae.
*Sarah L. Boyer and Catherine N. Reuter. 2012. New species of Austropurcellia mite harvestmen (Opiliones, Cyphophthalmi, Pettalidae) from Australia’s Wet Tropics, with commentary on biogeography of the genus. In press, Journal of Arachnology. Journal of Arachnology 40.1: 96-112.
See also Giribet & Shear 2010 http://scholar.harvard.edu/files/ggs/files/giribet_shear_2010.pdf