Several varieties have been described: the typical or European form is ornamented with a narrow black stripe, which, beginning at the nose, extends backwards along the side of the body to the groin, where it generally forms a hook turned upwards. This black colour forms the ventral boundary of the green, and is itself narrowly seamed with white on its upper border.
In the south of France, the Iberian Peninsula, Morocco, and the Canary Islands the black lateral stripe is often absent; this is the var. meridionalis. In Spain and Portugal both forms are found in the same localities.
In the Asiatic, chiefly in the eastern specimens, the lateral stripes tend to break up into irregular spots, vanishing altogether towards the groins; this var. savignyi s. japonica occurs also on most of the Mediterranean islands.
H. arborea can change colour to a great extent, mostly in adaptation to its immediate surroundings, but ill health and moulting may also influence it. The change is slow. The usual colour is green, brightest on bright, sunny hot days, dull when the sky is overcast, or when it is windy and showery. Day and night have no influence upon the colour-changes. The hue of the green agrees mostly with that of the foliage on which the frog happens to take its rest, for instance a field of Indian corn, birch-trees, or oak-trees. I once received a consignment from Saxony. When the box with moss was unpacked, they were of the dullest greenish-grey; they were put into a wired-off corner of the yard and were given the freshly cut branches of a lime-tree to sit upon. On the following morning I at first looked for most of the frogs in vain. The leaves had withered and all those frogs which sat upon the dark brown branches had put on a light brown garb, mottled with darker patches.
Another specimen, one of several which were at liberty in a greenhouse, took to resting on the frame of the window-pane, in a corner where putty, glass, and discoloured white paint met; in the morning it was always of a mottled leaden colour, but during the nocturnal hunting it was green. In the winter, the window-corner being of course cold, the frog remained stationary for several months, but kept the leaden grey colour, until one day in the early spring it was mottled with green, and soon after it joined its green mates.
Liebe observed a half grown tree-frog which he kept in Gera during the winter in a glass with water-cress. While the temperature was near freezing the frog sat in the water, very lethargic, breathing perhaps once every quarter of an hour. Its colour was light green. When the water-cress was cut and removed, the frog darkened and became at last quite a discoloured grey. When the water-cress was put back, the creature reassumed the light yellowish-green colour, remaining in its lethargic condition until it became lively in the spring sunshine.
The European tree-frog spends most of its time in the summer, after the pairing is over, in trees, often in the very crowns; but the neighbourhood of even a small patch of Indian corn has still greater attractions. There are all sorts of green insects to be caught, there are fair chances of coming across the common Cabbage White, a butterfly which the tree-frog loves, and last not least the large luscious leaves afford a firm foothold, and the axillae between stalk and broad-based leaves are just the places for the frog to slip into, where nobody can find it. During the day they mostly sit still, on the keen look-out for passing insects, which, when they settle within reach, are jumped at; otherwise they have first to be stalked. The jump is quite fearless, regardless of the height above ground; there is the leaf upon which the prey sits, and even if this leaf be missed, there are others, and one of them is sure to be struck by some of the discs of either fingers or toes. If the fall is broken by the toes, and the new leaf or branch is very elastic and bends down, then there are some frantic antics to be gone through until the frog has settled itself again. Then the large blue-bottle, or the butterfly, is devoured at leisure, wings and all being poked in with the assistance of the little hands. But the real hunting-time is the night.
During a shower the frog shifts its position to the under side of the leaf, or into a less slippery position, and during continuous wet it descends into the grass, or it takes to the water. Its greatest enemy is the Grass Snake, which prefers it to anything else, not minding the poisonous secretion of the skin, which is sharp enough to produce sneezing or even temporary blindness when incautiously brought into the human eye.
The male has an internal vocal sac, which, when inflated, bulges out the whole throat into a globe, much larger than the head. The voice is a sharp and rapidly-repeated note, something like "epp-epp-epp," or "creck, creck, creck," with more or less of an a sound. It is uttered at any time of the day, more frequently at dusk, and of course chiefly during the pairing season. This tree-frog suffers from the reputation of being a good weather-prophet, and it is for this reason often kept in confinement, the orthodox abode being a muslin-covered glass jar, with a hole to put flies through, water and plants at the bottom, and a little ladder to sit upon. The prophesying is of the usual popular unreliable nature, although the little creature, provided it is a male, often sounds its voice on the approach of a shower, or when there is a thunderstorm in the air. During continuous fine weather it sits on the top of the ladder, or is glued on near the rim of the glass, while on wet and dull days it is less active, and may keep nearer the ground or in the water. There is a German rhyme which well expresses the prophet's reliability by its ambiguity:–
Wenn die Laubfrösche knarren,