Abstract
BackgroundDespite ongoing interest in the neurophysiology of visual systems in scorpions, aspects of their neuroanatomy have received little attention. Lately sets of neuroanatomical characters have contributed important arguments to the discussion of arthropod ground patterns and phylogeny. In various attempts to reconstruct phylogeny (from morphological, morphological + molecular, or molecular data) scorpions were placed either as basalmost Arachnida, or within Arachnida with changing sister-group relationships, or grouped with the extinct Eurypterida and Xiphosura inside the Merostomata. Thus, the position of scorpions is a key to understanding chelicerate evolution. To shed more light on this, the present study for the first time combines various techniques (Cobalt fills, DiI / DiO labelling, osmium-ethyl gallate procedure, and AMIRA 3D-reconstruction) to explore central projections and visual neuropils of median and lateral eyes in Euscorpius italicus (Herbst, 1800) and E. hadzii Di Caporiacco, 1950.ResultsScorpion median eye retinula cells are linked to a first and a second visual neuropil, while some fibres additionally connect the median eyes with the arcuate body. The lateral eye retinula cells are linked to a first and a second visual neuropil as well, with the second neuropil being partly shared by projections from both eyes.ConclusionsComparing these results to previous studies on the visual systems of scorpions and other chelicerates, we found striking similarities to the innervation pattern in Limulus polyphemus for both median and lateral eyes. This supports from a visual system point of view at least a phylogenetically basal position of Scorpiones in Arachnida, or even a close relationship to Xiphosura. In addition, we propose a ground pattern for the central projections of chelicerate median eyes.
Highlights
Scorpions have two classes of eyes: one pair of large elevated eyes in the middle of the carapace commonly referred to as median eyes, and two to five pairs of small eyes along the anterior, lateral margin of the carapace, commonly referred to as lateral eyes [1]
General layout of the visual system The visual system in the studied scorpion species, Euscorpius italicus and E. hadzii, is composed of two median eyes located medially on top of the cephalothorax, and two pairs of lateral eyes located along the front corners of the cephalothorax
Our results show that the median eye retinula cells are linked to a first and a second visual neuropil, while some fibres connect the median eyes with the arcuate body
Summary
Scorpions have two classes of eyes: one pair of large elevated eyes in the middle of the carapace commonly referred to as median eyes, and two to five pairs of small eyes along the anterior, lateral margin of the carapace, commonly referred to as lateral eyes [1]. Due to the construction of the dioptric apparatus as well as the anatomy of the retina, the visual acuity is reduced They have been suggested to function mainly as extremely sensitive light detectors, e.g. for Zeitgeber stimuli to synchronize a circadian clock [3,4]. To shed more light on this, the present study for the first time combines various techniques (Cobalt fills, DiI / DiO labelling, osmium-ethyl gallate procedure, and AMIRA 3D-reconstruction) to explore central projections and visual neuropils of median and lateral eyes in Euscorpius italicus (Herbst, 1800) and E. hadzii Di Caporiacco, 1950
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