Phylogeny of the Myriapoda-Crustacea-Insecta: a new attempt using photoreceptor structure

According to molecular sequence data Crustacea and not Myriapoda seem to be the sister-group to Insecta. This makes it necessary to reconsider how the morphology of their eyes fit with these new cladograms. Homology of facett ed eye structures in Insecta (Hexapoda in the sense of Ento- and Ectognatha ) and Crustacea is clearly supported by identical numbers of cells in an om matidium (two corneageneous or primary pigment cells, four Semper cells whi ch build the crystalline cone and primarily eight retinula cells). These ce ll numbers are retained even when great functional modification occurs, esp ecially in the region of the dioptric apparatus. There are two different po ssibilities to explain differences in eye structure in Myriapoda depending on their phylogenetic position in the cladogram of Mandibulata. In the trad itional Tracheata cladogram, eyes of Myriapoda must be secondarily modified . This modification can be explained using the different evolutionary pathw ays of insect facetted eyes to insect larval eyes (stemmata) as an analogou s model system. Comparative morphology of larval insect eyes from all holom etabolan orders shows that there are several evolutionary pathways which ha ve led to different types of stemmata and that the process always involved the breaking up the compound eye into individual larval ommatidia. Further evolution led on many occasions to so-called fusion-stemmata that occur con vergently in each holometabolic order and reveals, in part, great structura l similarities to the lateral ocelli of myriapods. As myriapodan eyes canno t be regarded as typical mandibulate ommatidia, their structure can be expl ained as a modified complex eye evolved in a comparable way to the developm ent to the fusion-stemmata of insect larvae. The facetted eyes of Scutigera (Myriapoda, Chilopoda) must be considered as secondarily reorganized later al myriapodan stemmata, the so-called 'pseudo-compound eyes'. New is a crys talline cone-like vitreous body within the dioptric apparatus. In the new c ladogram with Crustacea and Insecta as sister-groups however, the facetted eyes of Scutigera can be interpreted as an old precursor of the Crustacea - Insecta facetted eye with modified ommatidia having a four-part crystallin e cone, etc. as a synapomorphy. Lateral ocelli of all the other Myriapoda a re then modified like insect stemmata. The precursor is then the Scutigera- Ommatidium. In addition further interpretations of evolutionary pathways of myriapodan morphological characters are discussed.