A layered rhabdom in an isopod (Crustacea). A case of convergent development

The isopod species Astacilla longicornis (Sowerby) has a layered rhabdom, which is the first case reported in isopod crustaceans. The rhabdom comprises the rhabdomeres of six retinular cells. It is surrounded by an extracellular palisade. Usually, palisades are intracellular in arthropods. A layered rhabdom is found in taxonomically widely separated groups within the Arthropoda and no intermediate forms are found between the different, highly specialized rhabdom types present. It is concluded that the layered rhabdom represents an example of convergent development.