The Physiological Optics of a Nocturnal Semi-Aquatic Spider, Dolomedes aquaticus (Pisauridae)

The optics of the posterior eyes of the amphibious spider, Dolomedes aquaticus, are described. The lenses have strongly-curved spherical front and rear surfaces, and therefore, a fairly high light capturing ability — they have F-numbers of 0.9 — yet, the large amount of spherical aberration predicted by their shape is mostly corrected. The receptive segments of the receptors are arranged in rows, each row separated by pigmented glial cells, but within a row, rhabdomeres from neigh­ bouring cells are contiguous so that coupling between cells from the same row seems a possible way of increasing absolute sensitivity for night vision. When the spider submerges, an air-layer is held over the comeal surfaces — as with a diver’s face mask — so that light is focused on to the receptive layer of the retina under water as well as above. These features are considered as adaptations for the nocturnal and semi-aquatic habits of the spider.