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1 July 2016 Lateral eye evolution in the arachnids
Sebastian T. Miether, Jason A. Dunlop
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The distribution of lateral eyes in arachnids and their relatives (Chelicerata) is reviewed, including novel data for selected taxa. Particular focus was given to camel spiders (Solifugae) and whip scorpions (Thelyphonida), for which there are conflicting reports about their eye morphology in the literature, and to the condition in some fossil scorpions (Scorpiones) and some extinct trigonotarbid arachnids (Trigonotarbida), which have lateral eyes with c. 30 or up to 15 individual lenses, respectively. Arachnid outgroups like horseshoe crabs (Xiphosura) and sea scorpions (Eurypterida) have compound lateral eyes, but the hypothesis that reduction of these eyes to five lenses or fewer is a synapomorphy of Arachnida can be rejected. Fossil data indicate, instead, that the arachnid lateral eyes were also originally (semi-) compound, and that reduction to only a handful of lenses must be a homoplastic character state. Note that camel spiders retain vestigial lateral eyes, while fossil ricinuleids had a pair of lateral eyes not seen in the living representatives. Among the Pantetrapulmonata group, i.e. spiders and their closest relatives, there is a clear trend towards the lateral eyes consolidating into triads of three lenses. A number of earlybranching spider families (e.g. Hypochilidae, Atypidae, but interestingly not Liphistiidae) retain clear evidence of triads. Whip scorpions unequivocally have two additional minor lenses, documented here photographically, while at least some Devonian trigonotarbids show more than five lateral eye lenses: implying a compound eye in the process of reducing to a simple lens-based system. Most arachnids rely more on non-visual sensory inputs, which may explain why reduction of the lateral eyes appears to be a common evolutionary trend.

Sebastian T. Miether and Jason A. Dunlop "Lateral eye evolution in the arachnids," Arachnology 17(2), 103-119, (1 July 2016).
Published: 1 July 2016
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