The troglobitic harvestmen Megalopsalis tumida and Hendea myersi cavernicola inhabit the Waitomo Caves in New Zealand with their luminescent prey, the glow-worm Arachnocampa luminosa. A distribution map of the harvestmen in the caves is presented. Both species of harvestman possess two prominent eyes on the cephalothorax, with lens diameters of ca. 500 $\mu $ m and 250 $\mu $ m for Megalopsalis and Hendea, respectively. The eyes are of the everted (direct) type, with axons leaving the retina peripherally in a single optic nerve. Retinal organization is typical for harvestmen: rhabdomal groups are made up of three or four photoreceptive cells. Rhabdoms in Megalopsalis are ca. 250 $\mu $ m long and possess diameters of up to 20 $\mu $ m, whereas the corresponding figures for Hendea are 150 $\mu $ m and 26 $\mu $ m. Movement of screening pigment granules into dark or light-adapted positions could be induced at any time of day. f-Numbers of 1.06 for Megalopsalis and 0.97 for Hendea lenses indicate considerable light-gathering power of the dioptric system, which does not appear to be wasted because of the massively developed, voluminous rhabdoms in the retina of both species. Electrophysiological recordings showed that both species are sensitive to a range of light intensities covering at least 5 log units of magnitude. In terms of electroretinogram (ERG)-determined spectral sensitivity Megalopsalis displayed high ultraviolet sensitivity and a secondary broad peak ranging from blue to green light, whereas Hendea possessed a clear green peak and secondary sensitivity to blue and ultraviolet radiation. Both species demonstrated an overall negatively phototactic response to a bright ultraviolet light source and a positively phototactic response to a dim, artificial `glow-worm' light. Only three individuals of Megalopsalis and no Hendea were caught in six automatic traps a few metres outside the cave entrance over a period of five months, but within the cave 92 Megalopsalis and 174 Hendea were sampled in 12 collecting trips of ca. 4 h each, spread over one year. The relative lack of photoreceptor regression, despite the cave existence of the two species and their inability to produce light, is interpreted as a consequence of the light produced by the cave populations of glow-worms.