Figure 5.

Evolutionary transformation of the head region in arthropods after Remane (1). (A) An annelid-like ancestor is assumed in which the archicerebrum (darkest shading; top) is connected with the eyes, and the prosocerebrum is located behind the stomodaeum (indicated by a black triangle). (B) More advanced bauplan in which the prosocerebrum has moved anterior to the stomodaeum and has fused with the archicerebrum. The deutocerebrum also has moved to fuse partially with the prosocerebrum but has retained a post-oral commissure. Remane (1) originally proposed this as a hypothetical intermediate step toward the crustacean/insect bauplan whereas our present results suggest that this is the situation in spiders. (C) The crustacean/insect bauplan in which the deutocerebrum has fused fully with the prosocerebrum and the tritocerebrum has fused partly with the deutocerebrum, retaining a post-oral commissure. The position of the tritocerebral ganglion with its commissure and its transformations is shown darkly enhanced in each stage.