Structure and function of the eyes of two species of opilionid from New Zealand glow-worm caves (Megalopsalis tumida: Palpatores, andHendea myersi cavernicola: Laniatores)

Abstract

The troglobitic harvestmenMegalopsalis tumidaandHendea myersi cavernicolainhabit the Waitomo Caves in New Zealand with their luminescent prey, the glow-wormArachnocampa 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 ofca. 500 μm and 250 μm forMegalopsalisandHendea, 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 inMegalopsalisareca. 250 μm long and possess diameters of up to 20 μm, whereas the corresponding figures forHendeaare 150 μm and 26 μm. Movement of screening pigment granules into dark or light-adapted positions could be induced at any time of day.f-Numbers of 1.06 forMegalopsalisand 0.97 forHendealenses 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 sensitivityMegalopsalisdisplayed high ultraviolet sensitivity and a secondary broad peak ranging from blue to green light, whereasHendeapossessed 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 ofMegalopsalisand noHendeawere caught in six automatic traps a few metres outside the cave entrance over a period of five months, but within the cave 92Megalopsalisand 174Hendeawere sampled in 12 collecting trips ofca. 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 glowworms.