It some sort of silicon i think. It is reasonably strong but crazy stretchy and wiggly. They look like a worm has just fallen from the sky and landed in the water. There are lots of small aquatic red worms in the water and around the banks. Just about everything eats them. This is a great little bait to mimic worms or small baitfish. The material can be found at fly shops or online. Look up squirmy wormy.
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