Appearance of ocular electrical responses during development of cavernicolous and epigaeic opilionids (Arachnida)

The appearance of the electroretinogram was established during the development of two species of opilionids, an epigaeic species, Pachylus chilensis, and a cavernicolous one, Ischyropsalis luteipes. In nymphs and adults of the two species, electroretinograms of about the same amplitude can be recorded. During embryonic and larval development they are quite different. In Ischyropsalis luteipes, it is impossible to record an electrical response before hatching. In Pachylus chilensis on the other hand an initial inconstant and very small response occurs from the 50th hour of development of the retina and can always be recorded from the 72nd hour. In older embryos, the response has a progressively greater amplitude, but remains a simple negative wave. On the tenth day, when the first synaptic relays appear, the electroretinogram takes the shape of that of the adult. Given that transmission electronic microscopy cannot detect differences between the microvilli of these two species, it is tempting to compare Ischyropsalis luteipes lack of response with that of the mutant norp A of Drosophila melanogaster in which transmission electron microscopy shows no marked difference, whereas freeze-etching shows a very low number of intramembranous particles.