Members of the plant genus Selaginella are often considered living fossils, as extant taxa are virtually indistinguishable from 300 Ma fossils. In contrast, the brush-footed butterflies are a relatively recent radiation, having diversified primarily within the last 60 Ma. Satyrs are among the most derived of these butterflies, likely radiated ~35 Ma and are known primarily for their high diversity and propensity to feed on grasses and sedges. In contrast to its close relatives, the Neotropical

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... s, the Neotropical satyr genus Euptychia also feeds on Selaginella, which is thought to be nutrient poor. Using no choice feeding experiments, I compared growth rates Costa Rican E. westwoodi that were offered two species of Selaginella, to those that were offered Lasiascis rusifolia, a grass commonly fed upon by close relatives. I discovered E. westwoodi larvae fed on two species of Selaginella and there was no difference in mass gained between the two species. However, larvae refused to feed on L. ruscifolia and lost mass over the course of the trial and expired unless they were transferred to Selaginella. To the best of my knowledge, these are the first data to report larval performance of the butterfly genus Euptychia or any Selaginella feeding insect. . Though far from conclusive, these results support the proposition that Euptychia have lost the ability to feed on other host plants and are now specialized on grasses.