From Nookipedia, the Animal Crossing wiki
The Firefly (ホタル, Hotaru) is a very common insect in the Animal Crossing series. It can be found between 7 p.m. and 4 a.m. in June, flying near fresh water (rivers and ponds). It is easy to find, emitting a glow that fades and strengthens. In Wild World, fireflies are represented by bright yellow 'blobs', the same color as the light given off by buildings. In City Folk, fireflies appear more realistic.
Donating to the Museum
In Wild World
"I've heard tales of children unable to buy candles... Yes, these unfortunate souls are said to have studied by firefly light! Ugh. The mere thought of having a bug that close... Blech! So very vile... I'd rather read in darkness!"
The Firefly can be found hovering over the pond in the first room of the Insect exhibit.
In City Folk
"Many theories exist as to why fireflies can glow, but we've yet to find a clear answer, eh wot? The answer is not nearly so relevant as the fact they are bugs, and therefore wretched."
It can be found hovering over the small pond to the bottom-left in the bottom terrace, occasionally.
In New Leaf
After being donated to the museum, the Firefly can be found periodically hovering over a small pond in the first room of the insect exhibit. The exhibit says the following:
"Both male and female fireflies are capable of producing light from their lower abdomens. This light isn't the same as the light from a lightbulb, though, and isn't hot when you touch it. Fireflies can actually glow when in the egg and larval stages, but that light is comparatively weaker."
The lampyridae, colloquially known as the firefly or lightning bug, is an insect most notable for its bioluminescence. In this regard it is similar to the jellyfish. Oddly enough, there are many species that do not produce light at all.
Names in other languages