Pioneering molecular work on chelicerate visual system development in the horseshoe crabLimulus polyphemussurprised with the possibility that this process may not depend on the deeply conserved retinal determination function ofPax6transcription factors. Genomic, transcriptomic, and developmental studies in spiders now reveal that the arthropodPax6homologseyelessandtwin of eyelessact as ancestral determinants of the ocular head segment in chelicerates, which clarifies deep gene regulatory and structural homologies and recommends more unified terminologies in the comparison of arthropod visual systems. Following this phylotypic stage, chelicerate visual (...) system development differs fundamentally from other arthropods during the compartmentalization of the ocular segment in that eye and optic neuropil primordia originate independently from each other. Comparative analyses of this phase identified further gene regulatory homologies but also major differences, most notably the possibly compensatory replacement ofPax6byPax2in lateral eye specification. Also see the video abstract here:https://youtu.be/Hdfr3z5kEXU. (shrink)