From Sprout Wings and Fly – documentary about old-time fiddler Tommy Jarrell (1983):

[Pulls a rattlesnake tail out of his fiddle] “…Well, the old folks used to put them in there to keep the spiderwebs out. When you hung them up on the wall before you had them fiddle cases, spiders would web in there and they would den in there and that snake rattle would keep them cut out, you know. Some folks claimed it made the fiddle sound better you put the rattle in there and the snake was a charm in the fiddle. See the fiddle’s called the devil’s instrument anyhow. So I been told. But I never did believe that.

Ever since I was 13 years old I been trying. Well I learned a few tunes from other fellas, you know, but I started out with them.”

That’s the inspiration for the song Snake’s in the Fiddle.