Spy craft leech

My first (shamefully late) days on the water this year were fishing with a friend on a local lake last week. Water temperatures are running about a month ahead of schedule. Or at least they were until it snowed on Wednesday. We did well on scuds and chironomids. One of my fish had this gift balanced leech stuck in its jaw.
I always look at these flies as a sort of spy craft-surreptitiously seeing what another angler used to trick a fish.