Structure and phylogenetic interpretation of diplopod eyes (Diplopoda)

Summary The structural organization of the ocelli of several diplopod species has been studied by means of electron microscopy. The results provide evidence that diplopodan ocelli are derived from typical mandibulate ommatidia, which consequently had been present in diplopod ancestors. The recent representatives of the two sister groups, Pselaphognatha and Chilognatha are characterized by two essentially different types of eye morphology: The eyes of the Pselaphognatha comprise a bilayered rhabdom (built up by 3+4 retinular cells), a few corneagenous cells, a corneal lens, and two vitreous bodies. The latter probably represent relics of a former crystalline cone. On the contrary, the ocelli of the Chilognatha consist of a multilayered rhabdom (built up by a large number of retinular cells), numerous corneagenous cells, and a corneal lens. The dioptric apparatus lacks a crystalline cone. Further structural elements, the distribution of which varies, are the covering cells and processes of hypodermal cells which contain screening pigments. Whereas the eye of the Pselaphognatha can be traced back to a single ommatidium, the ocellus of the Chilognatha can only be interpreted as a merging product of several associated ommatidia or as the result of multiplication and rearrangement of former ommatidial elements. This concept is substantiated by analogous phenomena which occur within other arthropod groups and thus serve as models for the phylogeny of the diplopodan eyes. The comparison of the morphology and the ecology of palaeozoic and recent diplopods demonstrates that the disintegration of former facetted eyes and the modification of ommatidia were induced by the adaptation to cryptic modes of life.