The vibrating skin plays a very important role in producing erhu sound. Its quality and the skill of the Erhu maker determine the quality and the characteristic of the sound.
Most of huqin's vibrating skin is made of Python skin. How to differentiate a good skin from a bad one? Despite of just using words to explain, I would like to show you 4 pictures of the snake skin.
This one looks tidy, smooth and the sizes of every piece of scale are almost even, and it shows certain degree of yellowness. Such properties shows that the skin is mature and quite good for Erhu making.
This one looks a bit messy, rough, slightly worn, the scales are uneven and small (from small snake), and its colour is dull. Such skin is bad.
This one.... well, just look at it, it has wrinkles and the colour is dark, this shows that it's from an old Python. Some players like it as the sound is much sadder or deeper(not in terms of pitch) than the young python skin. And most of them are more expensive. For me, I prefer mature but not old one.
This one looks ugly and almost distorted. So, bad skin it is.
Although the original vibrating skin of huqin is made of Python skin, but due to the protest of environmentalists and law enforcement, more and more artificial vibrating skins for Erhu begins to emerge on the market. The photo below shows the artificial vibrating skin, design and manufactured by Hong Kong Chinese Orchestra Research and Development Department.
Usually the skin is made of some kind of plastic. The advantage is that it's louder and eco-friendly....so called, I'm not sure, as it involves chemical process, which will normally leave some toxic residue. The disadvantage, is that the sound characteristic become so awkward that it's not Erhu sound anymore.
So... the arguments continue among the professionals. In my opinion, Erhu should stay what it is, because that's what makes it an Erhu. Otherwise, let's call the evolved Erhu as Terra! Haha!