Fig. 4

Ground plan and development of the insect ommatidium. Lateral view of a cross-section along the differentiating retina epithelium. In the differentiating retina of Drosophila and other insects, the cellular components of each ommatidium assemble in a highly regulated sequence, which is shown here increasing in cellular organization from right to left in the wake of an inductive differentiation front that moves in anterior direction (right). The first cell fates to be established are those of photoreceptors 1–8. Subsequently, cone cells (cc) are specified from peripheral cells and move with their main cell body on top of the photoreceptor-cell assembly (cone cell cluster). Further accessory cell types including the pair of primary pigment or corneageneous cells (1′) join during terminal differentiation of the ommatidium. Labeled ommatidial cluster stages, as well as the labeled components of the differentiated ommatidium, are conserved between insects and crustaceans as demonstrated by studies in Drosophila and the tadpole shrimp Triops cancriformis (Crustacea, Notostraca) (Melzer et al. 2000)