Muddler Minnow trout fly is a general pattern
that although does not seem to closely resemble any particular insect
or fish works well. It is essentially a wet fly or streamer but is
often fished static on the the surface where it is effectively becomes
a dry fly.
Fish either wet as any other streamer fly or try
it late in the evening when trout are selectively feeding on something
and refusing all other offerings you present. By stripping a mudddler
minnow across the surface film causing a wake to form will very often
induce a savage take when all else has failed.