Monday, June 20, 2011

Looks Like a Spider

Drassodes cupreus
Spiders live in a very three dimensional world. The web spinning and hunting techniques employed by various species requires excellent visual awareness of their surroundings. It is remarkable therefore that this is achieved with simple eyes (1). Unlike some other arthropods, spiders do not have compound eyes, yet these simple eyes are ranked as the best eyes among the arthropods, with a visual acuity to rival primates (2).

Eyes of Drassodes cupreus

Spiders possess four pairs of eyes in all, that are grouped in two pairs: a single pair of principle eyes and three pairs of secondary eyes (1).
Drassodes cupreus
In a species of ground spider, Drassodes cupreus, one of a pair of these secondary eyes does not form images at all. It uses a built in polarization filter in the eyes to determine the direction of skylight polarization. Amazingly, D. cupreus uses this as a compass, allowing it to find its way back to its nest after foraging (3).

  1. O'Carroll et al., 2001. The Journal of Experimental Biology 204 pp. 2481–2490
  2. Land, 1985. Neurobiology of Arachnids (ed. F. G. Barth), pp. 53–78
  3. O'Carroll et al., 1999. Nature 401 pp. 470-473

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