Complex sentence: The blue-tail fly of the song is probably a Southern variant of the horse-fly, which feeds on the blood of animals such as horses and cattle, as well as humans, and thus constitutes a prevalent pest in agricultural regions.
Split into simpler sentences: The blue-tail fly mentioned in the song is probably a Southern variant of the horse-fly. As it feeds on the blood of animals such as horses and cattle, as well as humans, it constitutes a prevalent pest in agricultural regions.