Spider Habitat Fly Trap

I really liked this design... My son had a large jar of spiders. On occasion he would go on fly hunts to feed the spiders. I had a great idea to improve his habitat and hopefully rid our yard of flies. I printed a new lid for the jar with an inverted funnel (a common fly trap design). I also added a top so he could control when flies could enter. I figured that if we had the right bait the flies would voluntarily feed my son's spiders.

During our test it caught a few flies. Perhaps the flies