Al-Ultrabithorax ( Al-Ubx ) expression. All embryos are oriented with the anterior pole towards the left. (A) The earliest expression of Al-Ubx is in a small domain in the opisthosoma. (A') DAPI image of the same embryo as in A; scale bar is 50 μm. (B) Lateral view of the same embryo in A showing the small domain of Al-Ubx expression in the opisthosoma. (C) Close-up of the opisthosoma of the embryo in B showing Al-Ubx expression in O2. (D) Al-en expression in the opisthosoma used for comparison with Al-Ubx expression at the same stage, showing Al-Ubx expression only in O2, and its absence in O1 and in the Te. (E) Ventral view of a later-stage embryo showing that Al-Ubx expression is retained in the small opisthosomal domain. (F) Lateral view of the same embryo as in E. (G) Close-up of the opisthosoma at a stage between those shown in B and F to illustrate the position of Al-Ubx expression in the O2 segment during the formation of the caudal bend. (H) Close up of the opisthosomal expression of Al-Ubx shown in F. Ch, chelicerae; DAPI, 4′,6-diamidino-2-phenylindole; L1-L3, first-third walking legs; O1-O2, first and second opisthosomal segment; Pp, pedipalps; Te, telson.