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|This article may be expanded with text translated from the corresponding article in the German Wikipedia. (December 2013)|
A tadpole (also called pollywog or porwigle) is the larval stage in the life cycle of an amphibian, particularly that of a frog or toad. They are usually wholly aquatic, though some species have tadpoles that are terrestrial.
Tadpoles are young amphibians that usually live in the water, though some tadpoles may be terrestrial. During the tadpole stage of the amphibian life cycle, most respire by means of autonomous external or internal gills. They do not usually have arms or legs until the transition to adulthood, and typically have a large, flattened tail with which they swim by lateral undulation, similar to most fish.
As a tadpole matures, it most commonly metamorphosizes by gradually growing limbs (usually the legs first, followed by the arms) and then (most commonly in the case of frogs) outwardly absorbing its tail by apoptosis. Lungs develop around the time of leg development, and tadpoles late in development will often be found near the surface of the water, where they breathe air. During the final stages of external metamorphosis, the tadpole's mouth changes from a small, enclosed mouth at the front of the head to a large mouth the same width as the head. The intestines shorten to accommodate the new diet. Most tadpoles are herbivorous, subsisting on algae and plants. Some species are omnivorous, eating detritus and when available, smaller tadpoles.
Tadpoles vary greatly in size, both during their development and between species. For example, in a single family, Megophryidae, length of late-stage tadpoles varies between 33 mm (1.3 in) and 106 mm (4.2 in). The tadpoles of Pseudis paradoxa grow to 25 cm (9.8 in), the largest of any frog.
The name "tadpole" is from Middle English taddepol, made up of the elements tadde, "toad", and pol, "head" (modern English "poll"). Similarly, "polliwog" is from Middle English polwygle, made up of the same pol, "head" and wiglen, "to wiggle".
Despite their soft-bodied nature and lack of mineralised hard parts, fossil tadpoles (around 10 cm in length) have been recovered from Upper Miocene strata. They are preserved by virtue of biofilms, with more robust structures (the jaw and bones) preserved as a carbon film. In Miocene fossils from Libros, Spain, the brain case is preserved in calcium carbonate, and the nerve cord in calcium phosphate. Other parts of the tadpoles' bodies exist as organic remains and bacterial biofilms, with sedimentary detritus present in the gut. Tadpole remains with telltale external gills are also known from several of the Labyrinthodont groups.
Tadpoles of megophryid frog Oreolalax rhodostigmatus are particularly large, more than 10 cm (3.9 in) in length, and are collected for human consumption in China. In Peru at least Telmatobius mayoloi tadpoles are collected for food and medicine.