mayfly
Archaic term for mayfly. In modern Japanese, カゲロウ is the standard word. This form appears mainly in classical literature or poetic contexts.
ephemerality of life
Archaic metaphorical use, likening life's transience to the short-lived mayfly. Found in old poetry and prose.
In classical literature, 蜉蝣 is written as a symbol of fleeting life.
There is an old waka poem that laments the ephemerality of life.