The visual system of harvestmen (Opiliones, Arachnida, Chelicerata) - a re-examination.

Roland R. Melzer,Eva Lodde-Bensch,Tobias Lehmann,Martina Metz

doi:10.1186/s12983-016-0182-9

Roland R. Melzer, Eva Lodde-Bensch

Open Access

https://doi.org/10.1186/s12983-016-0182-9

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Abstract

BackgroundThe visual systems in chelicerates are poorly understood, even though they show strong variation in eye and visual neuropil architecture, thus may provide valuable insights for the understanding of chelicerate phylogeny and eye evolution. Comparable morphological characters are desperately sought for reconstructions of the phylogeny of Chelicerata, especially with respect to Arachnida. So far, reliable data exist only for Pycnogonida, Xiphosura, Scorpiones, and Araneae. The few earlier studies of the organisation of the visual system in harvestmen are contradictory concerning the number, morphology, and position of the visual neuropils.ResultsWe undertook a descriptive and comparative analysis of the neuroanatomy of the visual system in several phalangid harvestmen species. Various traditional and modern methods were used that allow comparisons with previous results (cobalt fills, DiI/DiO labelling, osmium ethyl gallate procedure, and TEM). The R-cells (photoreceptor and arhabdomeric cells) in the eyes of Opiliones are linked to a first and a second visual neuropil. The first visual neuropil receives input from all R-cell axons, in the second only few R-cells terminate in the distal part. Hence, the second visual neuropil is subdivided in a part with direct R-cell input and a part without. The arcuate body is located in a subsequent position with direct contact to the second visual neuropil.ConclusionsThis re-examination comes to conclusions different from those of all previous studies. The visual system of phalangid Opiliones occupies an intermediate position between Pycnogonida, Xiphosura, and Scorpiones on the one side, and Araneae on the other side. The projection of the R-cells is similar to that in the former grouping, the general neuropil arrangement to that in the latter taxon. However, more research on the visual systems in other chelicerate orders is needed in order to draw inferences on phylogeny or eye evolution.

Highlights

The visual systems in chelicerates are poorly understood, even though they show strong variation in eye and visual neuropil architecture, may provide valuable insights for the understanding of chelicerate phylogeny and eye evolution
The arcuate body is with direct contact to the second visual neuropils
Phalangida have only one pair of median eyes, while the visual system of most other chelicerates consists of several pairs of eyes – median eyes and lateral eyes (e.g., Xiphosura, Scorpiones, Araneae, Uropygi, and Amblypygi)

Summary

Introduction

The visual systems in chelicerates are poorly understood, even though they show strong variation in eye and visual neuropil architecture, may provide valuable insights for the understanding of chelicerate phylogeny and eye evolution. The few earlier studies of the organisation of the visual system in harvestmen are contradictory concerning the number, morphology, and position of the visual neuropils. E.g., Stygnommatidae, Biantidae and Dibuninae, an eye tubercle is absent. It is not unequivocally clear whether the 'median eye' term often applied to harvestmen eyes is merely. Topological or informative in an evolutionary context as it is in many other arthropods, in which a distinction of median and lateral eyes and neuropils with respect to position, structure and function is evident (see Lehmann et al [9] for a recent review of chelicerate visual systems). The arhabdomeric cells are seen as non-photosensitive, secondary neurons, and are found in similar form in Xiphosura and Scorpiones [10, 12]

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