The Oak Hairstreak or Southern Hairstreak (Satyrium favonius) is a butterfly of the Lycaenidae family. It is found from southern New England and the Atlantic Coast south to peninsular Florida and west to central Illinois, south-eastern Colorado and the Gulf Coast.The wingspan is 22–38 mm. There are two tails on each hindwing. The undersides of the hindwings are grey brown. Adults are on wing from March to June in one generation per year.