In 2014, 14 new species of "dancing frogs" were discovered in the Western Ghats, a wildlife-rich mountainous region along the west coast of Peninsular India.

These Indian dancing frogs are named for the antics males get up to when they want to grab the attention of a female.

Perched on a wet rock in a noisy, fast-flowing stream, a male sings and waves to the females. He lifts and stretches a leg to flag his white, webbed toe.

Any rival males on the territory are literally kicked out by the singing-and-dancing male.

It now turns out that this foot-waving, which was observed in nine species of the genus Micrixalus, is not the only bizarre trait that runs in the family.

For the first time, researchers have found the tadpoles of a dancing frog, specifically the Kallar dancing frog (Micrixalus herrei).

Unlike most tadpoles, which live in water, they live underground until they develop into froglets.

They have several adaptations to deal with a burrowing lifestyle.

First, they can swim in sand. Their bodies are muscular and flattened, like eels. They shift sand grains by moving their heads sideways, while their bodies and tails propel them forward.

The tadpoles' eyes are covered with skin to shield against the wear and tear of swimming in harsh gravel. Their internal organs are protected by ribs.   

They breathe mainly through the skin, thanks to a network of capillaries just below the skin – which also means they are a tad red on the underside.

Their underground lifestyle helps explain why nobody has ever found the tadpoles of a dancing frog before, even though the first Micrixalus species was described in 1882.

The Indian dancing frogs are the last of the 54 frog and toad families in the world to have their tadpoles discovered.

Most frogs lead a "biphasic" life: the first "tadpole" phase is aquatic and the second "adult" phase is terrestrial, says S. D. Biju of the University of Delhi, who also led the 2014 discovery of 14 new dancing frogs. But the Kallar dancing frog's tadpoles are "fossorial", meaning they always live under the soil.

In earlier expeditions, Biju never saw dancing frog tadpoles swimming or resting in the water. So he decided to look for them in the sandy beds of the streams where females bury their eggs.

He found the tadpoles at depths of 4 to 16 inches (10-40cm) in the sandy earth. When he exposed them they swam back into the sand.

Biju has published his findings in the journal PLOS ONE.

The tadpoles do not just live in sand. They also nibble on it, drawing nutrients from the organic matter present among sand particles.

Their long, coiled guts are chock-full of sand. Thick, serrated lips around each tadpole's toothless mouth keep the larger sand particles from entering.

"It is an interesting finding, predominantly because it was an entire family of frogs where we previously had very little indication of what the tadpoles were like," says amphibian biologist Jodi Rowley of the Australian Museum Research Institute, who was not involved in the study.

"Members of at least five other families are known to have fossorial tadpoles, so it's not unique among the frog world, but it is relatively rare," she adds.

"These tadpoles are also interesting because they are very similar to other fossorial tadpoles," says Rowley. "Clearly there's a particular body shape, etc. that works when you need to 'swim' underground."