1.
swarm of mosquitoes; column of gnats
A swirling column of small flying insects, typically midges or mosquitoes, that cluster together in the air. Named because the swarm resembles a pillar.
蚊柱 literally means "mosquito pillar" (蚊 = mosquito + 柱 = pillar), describing the column-like shape of a swirling insect swarm.
Watch out
Despite the name, the insects forming a 蚊柱 are usually midges (ユスリカ) rather than actual mosquitoes. These midges do not bite and are harmless, though walking through a swarm is unpleasant.
Common collocations
Cultural note
蚊柱 are commonly seen on warm summer evenings, especially near rivers, ponds, and parks. In Japanese folk belief, 蚊柱が立つと雨が降る (when a swarm appears, rain is coming) is a traditional weather saying.