That's because almost all have left for a while. The few that have remained behind, do so because they are too old to be interested in anything other than just buzzing about the bayou. And that is because at this time of year is the mating season for fireflies. So up they went and flew off, to another part of the bayou, a much deeper part, where only natives and lost souls know the way. Here, in what is arguably the most mystical area of the ol bayou, resides the almost magical like dance of the fire flies. Where they dance and show their bright colors to prospective mates.
Don't worry though, they will return soon enough to the part of the bayou, where they enjoy listening to the strumming of the old man's banjo.