noun
hellgrammite; dobsonfly larva
Colloquial name for the larva of the dobsonfly species Protohermes grandis. Often used as fishing bait.
See also: 蛇蜻蛉
川で孫太郎虫を捕まえて釣りに使った。
I caught some hellgrammites in the river and used them for fishing.
蛇蜻蛉 is the adult dobsonfly, while 孫太郎虫 refers specifically to its larva.
The origin of the name 孫太郎虫 is uncertain, but it is a traditional Japanese colloquial term for this insect larva.