I do not recommend flies. Just last week a fly was flying around my house so I slapped it with a fly-swatter while it was flying in the air. It fell to the ground still alive and moving but couldn't fly anymore so I put it in a paper cup to feed to my VFT. About 2 minutes later when i looked at it, it was dead and maggots were crawling out of the rear end of the fly, about 20 so far coming out one by one. I squished it and all the maggots. Good thing I didn't feed it to the plant, I would've had maggots all over the pot! YUCK!
Eeewww thats wierd I thought all flies laid eggs anyway Im sure that the maggots wouldnt have hurt the vft lots of people from this site feed maggots to their plants so it probably would have been a little extra bonus for the plant.
I fed mine some flightless fruitfly maggots once after about 5 days later the traps opened with the pupated maggot a few days later they hatches into the flies and triggered the traps again. After that I dont do it no more.