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Flies that 'just fly around' may fly into the tent. When they hit the surface opposite of the entrance, most of them will start moving up towards the ceiling. As long as they stay near the ceiling, you can come in later and collect the with an aspirator. If you really want to make things automatic you'd need to create some kind of automatic collection device in it, just like in a regular Malaise trap. Still, it is useful to mention that that part of the Malaise trap is usually the part that increases it costs.