The adaptive significance of specific sexual dimorphism is well studied. However, the evolutionary history and ontogenic origins of the dimorphism are often unknown. As dimorphism represents two phenotypes generated from relatively similar genotypes, it is of interest to understand both its evolutionary and developmental/genetic underpinnings. Here, we present the first ontogenetic examination of the eyes of philomedid ostracods (Crustacea), which exhibit extremely sexually dimorphic lateral eyes. Adult male philomedids have large compound lateral eyes, whereas females have rudimentary lateral eyes. First, we show that eye dimorphism is unlikely to be due to additional genes present on a male-specific chromosome because karyotype analysis suggests philomedids are XX/XO. We then examine the ontogeny of eye development and find that in at least two species of Euphilomedes, this dimorphism is not generated solely by differences in tissue growth rates, as has been commonly shown for sexually dimorphic characters of other species. Instead, the dimorphism appears to arise during development via tissue duplication, where a single tissue becomes two, perhaps with different developmental potentials. The second eye field is only observed in male Euphilomedes, producing most of the adult eye tissue. We point out that tissue duplication is a developmental process with evolutionary implications because novel characters could evolve via alternative modification of the duplicated fields, analogous to the origin of new genes by gene duplication and alternative modification. Depending on the evolutionary history of the duplicated field, it may have either facilitated or directly caused the observed sexual dimorphism of philomedid ostracods.