Pass the earthworm, please.

Pollenia rudis is known as the attic or cluster fly because of its tendency to overwinter en masse in human structures. P. rudis belongs to the family Calliphoridae, and most calliphorids feed on carrion (or occasionally living flesh) as larvae. {that's not completely true, some Australian Calliphoridae:Rhiniiae are myrmecophiles}  Taking the road less traveled, P. rudis larvae seek out earthworms. For the first two instars, they feed internally, although apparently larvae may leave an unsuitable host and find a new worm to consume. Third instar maggots are larger and have been known to feed externally on worms. How this life strategy was adopted from feeding on carrion seems to be a big mystery. Gradualist (classic) views of evolution are often hard-pressed to answer these questions, which is why I'm a big fan of hopeful monsters (think bulldog or chihuahua) and punctuated equilibrium. These ideas theoretically allow for a small number of individuals to experience a rapid change in form and function....which makes it easier to understand increasingly common situations such as this lizard population which evolved new digestive tract valves in less than 40 years.

P. rudis image from: http://bugguide.net/node/view/362910/bgimage