Expression of insect, centipede and onychophoran six3 and otx genes. In the fly Drosophila (a-c), the beetle Tribolium (d, e), the centipede Strigamia (f, g), the onychophoran Euperipatoides (h-i'), six3/optix orthologues (a-f, h) are expressed in an anterior-median location, while otx/orthodenticle orthologues (a-e, g, i) are expressed more posterior-laterally. Single (f-i') or two-colour (a, b, d, e) whole-mount in situ hybridisations. (a, b) Drosophila stage 6 (a) and 11 (b). (c) Schematics of six3 (blue) and otx (red) neuroectodermal expression in the left head hemisphere of a stage 11 Drosophila; expression of both genes is also detected in the underlying brain neuroblasts . (d, e) Tribolium germ rudiment (d) and early elongating germband (e) stages. (f, g) Strigamia early segmentation stages. (h-i') Euperipatoides mid-segmentation stages. (h', i'): nuclear SYBRGreen stain of embryos in (h, i) for better visualization of the mouth opening. Dotted line in (e): Anterior labral border. Blue arrows in (b, e): six3+ neuroblasts. Dashed/dotted lines in (f, g): anterior germband margin. Yellow arrowheads in (h-i'): mouth opening. Abbreviations: a = anterior, AN = antennal segment, CL = clypeolabrum, d = dorsal, DC = deutocerebrum, FG = foregut, Lr = labrum, MD = mandibular segment, p = posterior, PC = protocerebrum, TC = tritocerebrum, v = ventral. Thin dashed line in (c): midline; thick dotted lines in (c): posterior borders of the protocerebrum, deuterocerebrum and tritocerebrum. (a): Lateral view. (b-g): Ventral views. (h-i'): Ventro-lateral views. All embryos with anterior to top except a: anterior to left.