Arctiidae, Arctiinae, Trichromia sp.. There are quite a few around of them. Key issue would be: Big dark patch with some distance from the margin and with some little notches. From the phenotype it might be Trichromia metapyrioides:
http://www.boldsystems.org/views/taxbrowser.php?taxon=Trichromia%20metapyrioides
But I am not sure if this is actually found in Costa Rica, but mostly in South America.
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