When active at night Hyla robertmertensi is pale yellow above with a white dorsolateral line and pale brown lateral stripe; the dorsal markings are faint. By day the dorsum is yellowish tan with brown markings. The dorsolateral stripe is creamy white, and the lateral stripe is dark brown ([Pl. 14]). The venter is white, and the iris is dull bronze. In breeding males the vocal sac is yellow.
Remarks.—Although this species superficially resembles Hyla microcephala microcephala, the latter is easily distinguished by the narrow brown lateral stripe, as compared with the much wider stripe in H. robertmertensi. No other hylids in northern Central America and southern México can be confused with this species.
Distribution.—Hyla robertmertensi inhabits the Pacific slopes (to elevations of 700 meters) and lowlands from eastern Oaxaca (east of the Plains of Tehuantepec) southeastward to central El Salvador. The species also occurs in the Cintalapa Valley (Atlantic drainage) in southwestern Chiapas ([Fig. 2.]) The distribution seems to be limited on the northwest and southeast by arid environments. The region in which Hyla robertmertensi lives is characterized by higher rainfall and more luxuriant vegetation than occur on the Plains of Tehuantepec or on the Pacific lowlands of eastern El Salvador and southern Honduras. In addition to the localities listed below, Mertens (1952:30) recorded the species from Hacienda Cuyan-Cuya, Depto. Sonsonate, El Salvador.
Fig. 2. Map showing locality records for Hyla robertmertensi.
Specimens examined.—490, as follows: Mexico: Chiapas: Acacoyagua, USNM 114754-61; *2 km. W Acacoyagua, UMMZ 87843 (28), 87844 (50), 87845 (50), 87846 (45), 87847 (27), 87848 (3); 32 km. N Arriaga, KU 57619-24, 59917-8 (skeletons); Asunción, FMNH 100413, 100501-4, UIMNH 26989-90, USNM 134267; *La Esperanza, USNM 114737-48, 114750-3, 17 km. S Las Cruces, KU 57625-49, 59997 (eggs); 8.5 km. N Puerto Madero, UMMZ 119981 (2); *11.7 km. N Puerto Madero, UMMZ 119982; Tapachula, FMNH 100096, UIMNH 26987; *11 km. S Tapachula, KU 57605-18, 59916 (skeleton); Tonolá, FMNH 27073, 100505-10, UIMNH 26988. Oaxaca: Tapanatepec, UMMZ 115245 (2), *1.6 km. E Tapanatepec, UMMZ 115244 (14); *4.3 km. E Tapanatepec, UIMNH 38368-9; *7.5 km. W Tapanatepec, UMMZ 115246 (39); 12.8 km. W Tapanatepec, KU 65007-14; 7.2 km. WNW Zanatepec, UMMZ 115243 (77); *13.6 km. WNW Zanatepec, TNHC 25213-22; 22.7 km. WNW Zanatepec, TNHC 25203-9.
Guatemala: Jutiapa: Jutiapa, UMMZ 106848; La Trinidad, UMMZ 107733 (23). Retalhueleu: Casa Blanca, UMMZ 107732.
El Salvador: La Libertad: 16 km. NW Santa Tecla, KU 44112. San Salvador: 21.9 km. N San Salvador, UMMZ 119983 (6).