The original opsin set. Tree of arthropods showing the ground-pattern of the opsin set for Onychophora and Euarthropoda: arthropsin, visual r-opsin (V r-opsin), c-opsin and peropsin and what is presently known of the opsin set in each of the groups Onychophora, Chelicerata and Mandibulata. We would like to stress that the suggested loss of peropsin in the Mandibulata is highly uncertain at this point, due to limited sequence availability. Further data on the genomes of additional mandibulates may certainly reveal a peropsin homologue. Sources for opsin numbers are as follows: Onychophora, this investigation and ; Chelicerata, NCBI database search in this investigation and [10, 11]; Mandibulata, NCBI database search in this investigation and .