expression, noun
the blues; a fit of depression
Idiomatic expression referring to a gloomy mood, based on the folk belief that a 'bug' inside the body causes depression. Often used in phrases like ふさぎの虫が騒ぐ or ふさぎの虫がつく.
See also: 虫
最近ふさぎの虫が騒いで、何もやる気が起きない。
Lately I've been feeling the blues and can't get motivated to do anything.
彼女はふさぎの虫に取り憑かれたように沈んでいる。
She's sunk in gloom as if possessed by a depression bug.
憂鬱 is a formal noun/adjective for 'melancholy' or 'depression', while ふさぎの虫 is a colloquial, idiomatic expression for a temporary bout of the blues.
虫 (bug) appears in many idioms for emotions or dispositions (e.g., 虫がいい, 虫の知らせ). ふさぎの虫 specifically personifies depression as a bug.
From ふさぐ (to be depressed) + の + 虫 (bug), based on the folk belief that a bug inside the body causes low spirits. The exact origin of the phrase is uncertain.