Other contributors have said "Lifespan of a horsefly?" is the same question as "Why is a horsefly called a horsefly?" If you believe that these are not asking the same thing and should be answered differently, click here
Well first think of a spider. if a horsefly were to land in its web the spider would eat it. so 'I'm thinking it only serves as food for other things like frogs and spiders etc. Which now leads us to...