An insect retina without microvilli in the male scale insect, Eriococcus sp. (Eriococcidae, Homoptera)

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Abstract
Male scale insects of an undescribed Australian species of Eriococcus have no compound eyes but show an extraordinary arrangement of three pairs of ocelli: One pair is positioned dorsolaterally where most insects have their compound eyes. Another pair looks ventrally and is placed where insects usually have their mouthparts, and there are two small lateral ocelli. Corneal nipples and a spherical lens with an estimated F-number only 0.55 are structural adaptations considered to increase the overall light sensitivity in order to compensate for the poor quantum capture of the shallow retina whose rhabdomes are only 3 μm long. The outer segment of each receptor cell consists of a central core of cytoplasm containing mitochondria and a peripheral cylinder of about 16 “rhabdomeres”. There is no optical separation between neighbouring outer segments. Uniquely in arthropod eyes, the light sensitive structures are not composed of cylindrical microvilli, but consist of membrane stacks whose configuration is analogous to the stacked plates of vertebrate cones. At present no conclusive answer can be given as to why the photoreceptors have plates instead of microvilli. Comparative calculations show that they do not contain more photosensitive membrane per unit volume than rhabdomeres of fly ocelli.

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