What's 火垂る and why is it so rare?

Question

I've stumbled upon the word 火垂る, meaning firefly, in the Japanese title for Grave of the Fireflies. The thing is, there's already a kanji (蛍), pronounced the same way. The other weirded thing is that I can't find 火垂る in most dictionaries. On Jisho.org there's an entry but it's for the whole title, 火垂るの墓. There's even another, obsolete, kanji for ホタル, 螢, so I don't think it's merely an old writing.

What's the deal with these kanji? Why do every occurrence seem to be linked with the movie? Is it a novel, play-on-word writing or an older one? And why is it so hard to find in dictionaries?

Answer 1

The word "火垂る" in "火垂るの墓" is just an ateji.

The meaning is somewhat clear from the movie itself, which is a visualization of the Kobe Air Raid during World War II. In the film, there is a scene where the main characters, a boy and a girl, are watching the bombs of the Kobe air raid burning beyond the window of a train. This is the "火垂る". The word "火垂る" comes from bombs(=things that creates fire:火), dripping(垂[れ]る) down from bomber.

That's all it means. As far as I know, it's not actually used in real life to describe fireflies.

Edit: Looks like it wasn't complete creation of the author, and was actually used to describe fireflies during Edo period.

It seems that the light of the fireflies was compared to fire, and the way the fireflies were floating was described as "fire dripping" - "火垂る". It has a very literary feel to it.