any typically arboreal frog belonging to one of several families of the order Anura. Of these, the hylid, or “true,” tree frogs from the family Hylidae are the most numerous. Hylids are usually slender, less than 10 cm (4 inches) in length, and long-legged, and they possess enlarged adhesive disks on the tips of the fingers and toes that aid in climbing. They often possess jewel-like eyes that glint with flecks of gold or copper. Their lustrous skin varies widely in colour and commonly has a metallic sheen.

Most species deposit their eggs in water, although it is also common for eggs to be laid on vegetation overhanging water (see video-->). After hatching, the tadpoles drop through the air into pools or streams below. Some, such as the blacksmith frog (Hyla faber), build mud nests at the water's edge; others lay their eggs in the water that collects between the leaves of plants. (See rainforest ecosystem sidebar, “Life in a Bromeliad Pool.”) In the marsupial frogs (Gastrotheca) of South America, the young hatch and develop from eggs into tadpoles inside a brood pouch that forms on the back of the female. Some hylids do not climb well and live in the water, on land, or in burrows.

In the family Hylidae there are more than 700 species belonging to 40 or so genera. They are found primarily in the New World tropics but are also present in Europe, Australia, and across much of nontropical Asia. The genus Hyla includes hundreds of species; better-known representatives include the barking tree frog (H. gratiosa), the European green tree frog (H. arborea), whose range extends across Asia and into Japan, the gray tree frog (H. versicolor), the green frog (H. cinerea), and the Pacific tree frog (H. regilla). The smallest is the little grass frog (Pseudacris, or Limnoaedus, ocularis), which does not exceed 1.75 cm (0.69 inch) in length and is found in cypress swamps in the United States from Virginia to Florida and Alabama. Nonhylid tree frogs include members of the families Centrolenidae (the transparent “glass frogs,” see video), Hyperoliidae, Rhacophoridae (an Old World family similar to the hylids), and Microhylidae.