The Puppeteer fungus infects flies, invading their nervous system and eating them from the inside out.
In their death throes, the infected flies – like puppets on a string – obligingly climb to a high point and spread their wings, exposing their abdomen and allowing the fungus to shoot its spores as widely as possible to infect new flies.
Ahhh, that makes sense. It had attached to my pants pocket when I walked through some long grass but appeared to be dead when I looked at it. I'm also glad I'm not a fly.