Fig. 36.–Map showing distribution of Hylidae. The vertically shaded countries are inhabited by Hyla and by other genera of Hylidae; the horizontally shaded countries only by Hyla.
Fam. 4. Hylidae (Tree-frogs).–The upper jaw–in Amphignathodon the lower jaw also–and the vomers carry teeth; Triprion and Diaglena alone have teeth on the parasphenoid also, and the latter genus is further distinguished by possessing palatine teeth. The vertebrae are procoelous and have no ribs; the sacral diapophyses are dilated. The omo- and meta-sternum are cartilaginous, the latter forming a plate with scarcely any basal or style-shaped constriction. The terminal phalanges are invariably claw-shaped and swollen at the base, and carry a flattened, roundish, adhesive cushion. The tympanic disc is variable in appearance, being either free, or more or less hidden by the skin. The tongue is also variable in its shape and in the extent to which it can be protruded.
Most, if not all, Hylidae are climbers, and many lead an arboreal life, but it does not follow that all the "Tree-frogs" are green.
Their distribution is very remarkable. To say that this family is cosmopolitan with the exception of the African region, is literally true, but very misleading. There are in all about 150 species, and of these 100 are Notogaean; one-half of the whole number, or 75, being Neotropical; 23 are Central American, 7 Antillean, and about 18 are found in North America. One species, Hyla arborea, extends over nearly the whole Palaearctic sub-region, and two closely allied forms occur in Northern India and Southern China. Consequently, with this exception of three closely allied species, the Hylidae are either American or Australian. We conclude that their original home was Notogaea, and that they have spread northwards through Central and into North America. The enormous moist and steamy forests of South America naturally suggest themselves as a paradise for tree-frogs, and it is in this country, especially in the Andesian and the adjoining Central American sub-regions, that the greatest diversity of generic and specific forms has been produced. It is all the more remarkable that similar forest-regions, like those of Borneo and other Malay islands, are absolutely devoid of Hylidae (while there are about a dozen species in Papuasia), whose place has however been taken for all practical purposes by correspondingly modified Ranidae, notably the genus Rhacophorus. Lastly, the fact that tropical evergreen forests of Africa and Madagascar possess no Hylidae, but are inhabited by several kinds of tree-climbing Rhacophorus, points with certainty to the conclusion that the origin of this large and flourishing family of Hylidae was not in Arctogaea.
The versatility and the wide distribution of the Hylidae has naturally produced cases of convergent analogy, and the various species of one "genus" may be in reality a heterogeneous assembly. Such an instance is probably the genus Hylella, of which four species live in the Andesian and Central American provinces, while the two others occur in New Guinea and Australia.
The two North American genera Chorophilus and Acris, and the Brazilian Thoropa, connect the Hylidae with the Cystignathidae, in so far as their finger-discs are very small, or even absent, and their sacral diapophyses are only slightly dilated. On the other hand, it has to be emphasised that the possession of adhesive discs on the fingers and toes does not necessarily constitute a member of the Hylidae. That requires the further combination of an arciferous sternum, with dilated sacral diapophyses and teeth in the upper jaw. Finger-discs are easily developed, and still more easily lost. Those of the typical Hylidae are constructed as follows. The terminal phalanx is elongated, claw-shaped, swollen at its base. Between it and the penultimate phalanx lies an interphalangeal cartilaginous disc which projects ventrally below the end-phalanx, thus assisting the formation of the ventral pad, and the turning upwards of the whole disc-like phalanx like the claw of a cat. This peculiar motion can be well observed in Tree-frogs which are at rest upon a horizontal leaf, or, better still, upon a rough stone, when the creatures take good care to adjust their discs into a safe and easy position. The pad or disc itself is furnished with unstriped, smooth muscular fibres, the contraction of which produces one or more longitudinal furrows on the under side. When the disc is in action or adhering, being flattened to a smooth surface, the end-phalanx sinks into the cushion; when not in action, the cushion swells and the phalanx appears as a slight dorsal ridge. The disc is rich in lymph-spaces, and its surface contains mucilaginous glands.
Various suggestions have been made to explain the function of these discs. Suction, adhesion, and glueing-on have been resorted to. Suction, through production of a vacuum, is quite imaginary and does not exist. The question has been thoroughly studied by Schuberg.[[84]] Adhesion is due to the molecular attraction of two closely appressed bodies. The less air remains between them the stronger it is. Consequently it can be increased by the interference of a thin layer of fluid, which as everyday observation shows, possesses both adhesion and cohesion. The more sticky the fluid, the more effective it is, as shown experimentally by Schuberg, who moistened the under surface of a glass plate, and pressed it against a little disc of glass from which was suspended a weight. A disc of 16 square millimetres, approximately equal to the aggregate surface of the 18 discs of a European tree-frog of 4 grammes in weight, carried with water-adhesion no less than 14 grammes, with glycerine-solution 20 grammes,–more than sufficient to suspend the frog. The sticky secretion of its glands greatly enhances the adhesive power. Tree-frogs, when hopping on to a vertical plane of clean glass, slide down a little, probably until the secretion stiffens, or dries into greater consistency. After a few days I find the glass-walls of their recently cleaned cage quite dirty, covered everywhere with their finger-marks. On the other hand, wet leaves or moist glass-walls afford no hold. The adhesion of these frogs is assisted in most cases by their soft and moist bellies, just as a dead frog will stick to a pane of glass.
All Hylidae have a voice, often very loud, and enhanced by vocal sacs, which are either internal, swelling out the throat, or external, paired or unpaired.
The various Hylidae resort to all kinds of modes of rearing their broods. Most of them lay many eggs, up to one thousand, in the water, not coherent in strings but in clumps; others lay only a few, attach them to various parts of the body, or, as in the genus Nototrema, the female receives them in a dorsal pouch. These modifications will be described in connexion with the different species.
Sub-Fam. 1. Amphignathodontinae.–Both upper and lower jaw with teeth.