A male Teratohyla spinosa climbs through the trees at Rara Avis Reserve, Costa Rica.
Emerald green jewels barely an inch long with belly skin so transparent you can see the organs and beating heart within – these are the wonderful little frogs in the family Centrolenidae. In 2009 Guayasamín et al. revised the family in a Zootaxa monograph and currently recognizes 12 genera with over 120 species. For the most part glass frogs are tiny fragile frogs that inhabit vegetation surrounding tropical streams. They can be found from southern Mexico through much of northwestern South America and parts of Atlantic Brazil and Argentina. They are nocturnal arboreal creatures with large protruding eyes and long slender limbs. But they are most well known for their partially to completely transparent ventral skin – skin like glass. In some species you can see all of the organs, the beating heart pumping blood, and the colors of their bones.
This ventral view of Hyalinobatrachium sp. shows the white tissue lining the organs. You can make out the heart and the outlines of the eyes.
In other species a membrane covers some of the organs. Their dorsal surface is usually a bright shade of emerald or lime green and most species are flecked or dotted with spots of white, yellow, green, gold, or black. Their coloration allows them to completely blend in with the leaves they are so closely tied to. These cryptic frogs spend much of their time in tree canopies coming lower down to breed when rains swell small streams and rivers. Egg clutches are attached to leaves or rocks overhanging the stream such that when the tadpoles are ready to hatch they drop down into the torrents and attach themselves to rocks and substrate. There they will undergo development to little frogs and climb up into the vegetation to begin their adult lives.
A clutch of Hyalinobatrachium vireovittatum eggs as laid on the underside of a leaf in Costa Rica.
I can tell you this, hunting glass frogs will get you wet. They love the rain. It is a fun but often maddening pursuit. These frogs emit short chirps, trills, clicks, tic-tic-tics, or other similar high-pitched noises at infrequent intervals. Their small size and perfect camouflage make them nearly impossible to spot until you develop a hunting search image for them. So you usually start by tracking down a frog by its call. But glass frog choruses are not like the frog choruses most people are used to hearing. These guys will emit a call that attracts your attention in such a way to tease you that one is around. But, where? You look up and see a wall of little leaves. Then quickly get a giant raindrop smack in the eye.