Cupiennius salei (Araneae, Ctenidae) has, like most spiders, eight camera-type eyes. The anterior median eyes are called principal eyes and have a movable retina; all of the other eyes are referred to as secondary eyes and are equipped with a reflecting tapetum. The photoreceptors in the secondary eyes are arranged in rows on the tapetum and the inter-receptor angle along such a row is smaller than normal to it. In this study, the vertical and horizontal spatial cut-off frequencies of moving gratings were measured for the posterior median (PM) eyes, and the data were then compared with the anatomical data reported in the literature. Detection of moving objects in the secondary eyes enhances the eye muscle potential frequency in the principal eyes. We thus recorded the eye muscle activity with a telemetric unit as a monitor for motion detection while moving stimuli - sinusoidally modulated bright and dark stripes - were presented to the PM eyes on a computer screen. A significant increase in the eye muscle activity was measured for gratings at an angular wavelength of 2.0 deg in the vertical orientation and of 2.7 deg in the horizontal direction. In the vertical orientation the critical wavelength is twice the inter-receptor angle; in the horizontal orientation the spiders responded to wavelengths that are smaller than twice the corresponding inter-receptor angle. The cut-off frequency seems thus to be limited by the visual field of the photoreceptors rather than the inter-receptor angle. The relative intensity modulations modelled for the two different grating orientations in single photoreceptor cells were in line with our data.