Arthropods exhibit a vast variety of morphological forms. Developmental studies on diverse arthropod groups will help clarify how these forms evolved during arthropod evolution. The terrestrial isopod Porcellio scaber is a suitable novel animal model for developmental studies; it is well adapted to terrestrial habitats and these adaptations make the species interesting from the evolutionary perspective. Porcellio scaber is already extensively used in ecotoxicological and physiological studies and its biology is well known, but a comprehensive description of its ontogenetic development is still missing. Here, a staging system of P. scaber embryos and marsupial mancas based on morphological observations with light microscopy techniques is presented. An important adaptation of oniscids to terrestrial lifestyle is formation of specific structures in the digestive system, which enable effective water retention and good digestion of cellulose-rich plant material. Differentiation of the digestive system during P. scaber embryogenesis is described and the results show that digestive system is well developed already in late-stage embryos. The presented staging system should provide a useful basis for the expanding field of gene expression studies during arthropod embryogenesis and enable comparative developmental studies between arthropod groups.