tapetum

Size and eyes

Yesterday we collected 17 live opilioacarids from under stones near the Chamela station greenhouses. ¬†Their light purple colour makes them stand out relatively well against the reddish soil encrusted on the bottoms of the rocks. But they are pretty small and getting one off a rock can involve quite a chase with a paintbrush as they dash in and out of crevices. In case you want to know how big they are, here is a photo of one in a vial with my left index fingerprint for scale. The closeup shows how the camera’s flash is reflected by a tapetum in the eye of the mite – a feature unique to opilioacarids among both parasitiform and acariform mites. In spiders, presence of a tapetum seems be an adaptation for night vision (but not all nocturnal spiders with good vision have a tapetum according to this ¬†Australian Museum post). Does the tapetum in opilioacarids mean that they come out from under rocks at night? Or that they use these fancy eyes to better navigate their dark sublithic world?

opilioacarid with fingers for scale and tapetum compilation16 Feb 2014