PROC. ENTOMOL. SOC. WASH. 109(2), 2007, pp. 416^26 NEW SPECIES OF LEPTOHYPHIDAE (EPHEMEROPTERA) FROM COSTA RICA David E. Baumgardner Department of Entomology, Texas A&M University, College Station, TX 77843-2475, U.S.A. (e-mail: [email protected]) Abstract. — Three new species of leptohyphid mayflies (Ephemeroptera: Leptohy-phidae), Leptohyphes mandibiilus, Tricoiythodes kirki, and Tricorythodes primus, are described from Costa Rica based upon the larval stages. Leptohyphes mandibuhies can be distinguished from other species in the genus by its unique mandibular morphology and color patterns of the operaulate gill. Tricorythodes kirk larvae are distinctive due to their contrasting abdominal coloration. Tricorythodes primus is distinguished from other speices of Tricorythodes in Central America due to the presence of a distinctive and well developed frontal shelf and genal projections. Key Words: Ephemeroptera, Leptohyphidae, Leptohyphes, Tricorythodes, new species The mayfly family Leptohyphidae is a diverse assemblage of more than 125 species endemic to the Western Hemi-sphere. The family is most diverse in South America, with fewer species known from Central and North Amer-ica. The family has received critical attention recently in both South Amer-ica (Molineri 1999, 2001a,b, 2002, 2003a,b, 2004; Molineri and Zufiiga 2004), and North and Central America (Baumgardner and McCafferty 2000; Wiersema and McCafferty 2000, 2003, 2005; Wiersema et al. 2001; Baumgard-ner 2003; Baumgardner et al. 2003, 2006; Baumgardner and Avila 2006). Additional descriptions and revisions of species are still required before generic boundaries and relationships can be fully analyzed. Examination of collec-tions of mayflies made in 2001 from Costa Rica revealed three distinctive new species of leptohyphid mayflies, as described below. Materials and Methods Larvae were collected from streams using forceps and by disturbing the sediment using kick nets, then pre-served in 70% ethyl alcohol. Legs, gills, and mouthparts were removed and mounted on slides for detailed study. Figures were drawn using a camera lucida. Collection sites are given in longitude/latitude coordi-nates as degrees, minutes, seconds, and were determined using a hand-held global positioning system unit. Setal descriptive terminology follows Baumgardner and Avila (2006). Gill formula follows Molineri (2003b), and indicates the number of membranous lamellae on abdominal segments two through seven. Collections (and their acronyms) housing materials used in this study include Florida A&M Uni-versity, Tallahassee (FAMU) and Texas A&M University, College Station (TAMU).