Subdivision of arthropod brain and alternative possibilities for the position of neuronal cell bodies innervating the third pair of cephalic appendages in Onychophora. (A) Position of protocerebral (yellow), deutocerebral (red) and tritocerebral structures (green) in the brain of the fruitfly Drosophila melanogaster [modified from ]. (B) A position within the brain of neuronal cell bodies innervating the onychophoran slime papillae, as shown in this diagram, would support the existence of a region of the onychophoran brain equivalent to the arthropod tritocerebrum. (C) A position outside the brain of neuronal cell bodies innervating the onychophoran slime papillae, as shown in this diagram, would speak against the existence of a tritocerebrum in the onychophoran brain. Abbreviations: al, antennal lobe; an, antennal nerve; br, cerebral ganglion or brain; cc, central complex; jn, jaw nerve; lb, labral nerve; mb, mushroom body; nc, nerve cord; oc, ocellar nerve; ol, optic lobe; sn, slime papilla nerves.