ABSTRACT
A review of the main cytological types of arthropodan eyes leads us to suggest their possible evolutionary changes across the major groups of arthropods. Simple eyes of the ocellus type, typically with many photoreceptor cells beneath a single corneal lens, display plesiomorphic features which have probably been inherited from some non-arthropodan phylum. Ocellus-type eyes are present in many chelicerate lineages, both in the median and the lateral position on the prosoma, while only median ocellus-type eyes have been retained in most mandibulates. Compound eyes consisting of numerous contiguous highly organized ommatidia are found in many arthropod lineages. They most probably derived from simple ocellus-like eyes. The ommatidia of Mandibulata appear to be more evolved than those of Xiphosura, having acquired a more elaborate dioptric apparatus including a crystalline cone, and retinulae typically composed of eight photoreceptor cells. In different mandibulate taxa, a regressive evolution of the compound eyes has led to variously modified visual organs, such as ‘dispersed eyes’ and more highly modified stemmata which are found in particular in many holometabolous insect larvae. The evolution of the lateral eyes in some hexapods may even have led to ocellus-like lateral eyes, for instance in Cicindela larvae and in some adult insects exhibiting unicorneal eyes. The latter cases thus appear to result from a reversal to a plesiomorphic state. The lateral simple eyes present in several myriapod groups appear to be either similar to isolated and simplified ommatidia, or not very different from more complex stemmata such as those found in certain insect larvae.
