noun
aquatic insect larva; river insect
Refers to the larval stage of insects found in rivers, especially caddisflies and mayflies. Often used in fishing contexts as bait.
釣りに使う川虫を探しに行った。
I went looking for river insects to use for fishing.
川虫はきれいな水にしか住めない。
River insects can only live in clean water.