RHABDOM ADAPTATION AND ITS PHYLOGENETIC SIGNIFICANCE

The rhabdoms of arthropod compound eyes are structurally differentiated into open, fused layered and fused continuous. All are capable of perceiving polarized light. The fused layered, and under certain conditions the fused continuous, perform particularly well. The fused layered rhabdoms occur in malacostracan crustaceans and in various insect groups. This, together with the presence of open rhabdoms etc., in insects and crustaceans, indicates convergent development of organs and parts of organs. Elaborate visual organs of more than 1 kind occur in crustaceans, as is exemplified by the compound and nauplius eyes. More than 1 construction on the organ level is possible in a restricted taxonomical unit for the perception of light. The different rhabdom types performing well in receiving polarized light show parallel evolution below the organ level. Adaptation analyses indicate the need for a restricted use of the concept of homology basic to morphological investigations and a base for phylogenetic speculations. It envisages a fruitful approach to a peep into the workshop of evolution. A fully formed compound eye in arthropod ancestors is hardly conceivable. A realistic alternative is an inherent capacity of forming a compound eye. The ancestral compound eye could have ranged from nothing to a partly-developed stage. The recent eyes need not have originated from 1 source.