Fine structural organization of the lateral ocelli in two species of Scolopendra (Chilopoda: Pleurostigmophora): an evolutionary evaluation |
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Authors: | Carsten H G Müller V Benno Meyer-Rochow |
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Institution: | 1. Institut für Biowissenschaften, Lehrstuhl Allgemeine & Spezielle Zoologie, Universit?t Rostock, Universit?tsplatz 2, 18051, Rostock, Germany 2. Faculty of Engineering and Science (Biology), International University Bremen (IUB), Campus Ring 1, 28759, Bremen, Germany 3. Department of Biology, University of Oulu, 90014, Oulu, Finland
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Abstract: | The lateral ocelli of Scolopendra cingulata and Scolopendra oraniensis were examined by electron microscopy. A pigmented ocellar field with four eyes arranged in a rhomboid configuration is present
frontolaterally on both sides of the head. Each lateral ocellus is cup-shaped and consists of a deeply set biconvex corneal
lens, which is formed by 230–2,240 cornea-secreting epithelial cells. A crystalline cone is not developed. Two kinds of photoreceptive
cells are present in the retinula. 561–1,026 cylindrical retinula cells with circumapically developed microvilli form a large
distal rhabdom. Arranged in 13–18 horizontal rings, the distal retinula cells display a multilayered appearance. Each cell
layer forms an axial ring of maximally 75 rhabdomeres. In addition, 71–127 club-shaped proximal retinula cells make up uni-
or bidirectional rhabdomeres, whose microvilli interdigitate. 150–250 sheath cells are located at the periphery of the eye.
Radial sheath cell processes encompass the soma of all retinula cells. Outside the eye cup there are several thin layers of
external pigment cells, which not only ensheath the ocelli but also underlie the entire ocellar field, causing its darkly
pigmented. The cornea-secreting epithelial cells, sheath cells and external pigment cells form a part of the basal matrix
extending around the entire eye cup. Scolopendromorph lateral ocelli differ remarkably with respect to the eyes of other chilopods.
The dual type retinula in scolopendromorph eyes supports the hypothesis of its homology with scutigeromorph ommatidia. Other
features (e.g. cup-shaped profile of the eye, horizontally multilayered distal retinula cells, interdigitating proximal rhabdomeres,
lack of a crystalline cone, presence of external pigment and sheath cells enveloping the entire retinula) do not have any
equivalents in scutigeromorph ommatidia and would, therefore, not directly support homology. In fact, most of them (except
the external pigment cells) might be interpreted as autapomorphies defining the Pleurostigmophora. Certain structures (e.g.
sheath cells, interdigitating proximal rhabdomeres, discontinuous layer of cornea-secreting epithelial cells) are similar
to those found in some lithobiid ocelli (e.g. Lithobius). The external pigment cells in Scolopendra species, however, must presently be regarded as an autapomorphy of the Scolopendromorpha. |
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Keywords: | Centipedes Scolopendra Dual type retinula Retina Sheath cells Pigment cells Eye evolution Phylogeny Vision Mandibulata |
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