Bob Engelhardt wrote in message ...
Dave wrote: ...
I have always wondered though whether the trap was really catching as
many flies as it seemed or whether they were just reproducing, living,
and dying inside the trap. ...

My understanding is that flies reproduce by laying their eggs on/in
something that the larvae (i.e., maggots) can eat. E.g., a carcass.
There would be nothing like that in the trap. Bob

The bait in the trap is a foul broth, the stench of which attracts the
flies, and maggots can always be seen living in it.