Noggin's Ultimate Fly Collection over at DAZ has maggots as part of package.  It is from the P6/7 days so it should work in any version of Poser you are using.

The product uses python scripting to create the swarm.  IIRC, it's compiled so it won't work in P8 or better because of the python changes.

Check the free stuff as well.  IIRC, there used to be one there as well.