Head composition and homology of cephalic appendages in Onychophora and Arthropoda. (A) Ventral view of onychophoran "head" showing three pairs of modified appendages: antennae (at), jaws (jw), and slime papillae (sp). Scanning electron micrograph. Scale bar: 500 μm. (B) Diagram of expression pattern of segment polarity gene engrailed in an onychophoran embryo in lateral view (based on fig. 1a from ). Scale bar: 200 μm. Note that there are only three anterior expression domains corresponding to posterior borders of antennal (as), jaw (js), and slime papilla segments (ss), in addition to eight trunk segments (numbered). (C) Alignment and serial homology of anterior appendages in Onychophora and the four major arthropod groups [after ]. Note that the onychophoran eyes (black filled circle) may be homologous to the median ocelli  rather than to the compound eyes of arthropods (checked ovals), although all these ocular structures belong to the same, anterior-most body segment. Abbreviations: as, antennal segment; at, antenna; at1, first antenna; at2, second antenna; ch, chelicera; jw, jaw; js, jaw segment; le, leg; md, mandible; mx, maxilla; pp, pedipalp; sp, slime papilla; ss, slime papilla segment.