Micro Flex
Micro-Flex is a smaller, thinner version of materials like Super Floss or Life Flex. This thin, super flexible material is the way to go for legs on smaller nymphs and dry flies. Micro Flex is super durable, incredibly stretchy, and lasts way longer than rubber when exposed to UV light and years in a fly box. Also use it for antennae, wrap bodies on chironomids/midge pupa, nymph bodies, ribbing material, chop it up and put it in a dubbing loop, and many other possibilities.
For larger sized flies, see Super Floss, which is the same as Micro-Flex but in a thicker size, perfect for legs on large flies like stonefly nymphs and dry flies.