VISION IN THE CTENID SPIDER CUPIENNIUS SALEI: SPECTRAL RANGE AND ABSOLUTE SENSITIVITY

Abstract

Electroretinograms were recorded from all eyes of the wandering spider Cupiennius salei (Ctenidae) and were found to be simple cornea-negative potential differences with amplitudes of up to 10 mV. In both the principal eyes and all of the secondary eyes, the spectral response curves show a prominent green peak at 520 or 540 nm and a shoulder in the ultraviolet between 340 and 380 nm. The largest response in the ultraviolet measures between 65 % and 80 % of the green peak. Selective chromatic adaptation to either green or ultraviolet monochromatic light does not change these relative response levels and fails to indicate the presence of more than one spectral type of receptor. In the range 450–500 nm, however, the Dartnall curve clearly deviates from the spectral sensitivity (SS) curve. Since the SS curves of all eyes have a small shoulder in the blue at 480 nm, the existence of two or even three visual pigments is a possibility. Intensity curves were determined with white and monochromatic light. For white light, absolute corneal illuminance thresholds were clearly below 0.01 lx. For monochromatic light stimuli, a corneal illuminance of approximately 3×1012 photons cm-2 s-1 is needed to elicit a half-maximal response. At threshold, the equivalent value is 3×109 photons cm-2 s-1, which corresponds to a retinal illuminance of 5.9×109 photons cm-2 s-1. Consequently, Cupiennius salei should be able to use its visual sense not only shortly after sundown but also under much poorer light conditions, such as those provided by moonlight. The log-linear response range of all eyes covers a stimulus intensity range of 4 log units. The sensitivity of the principal eyes increases by up to 0.81 log units at night as compared with daytime. The chromophore of the visual pigment of all eyes is 11- cis retinal.