These traps were originally invented by a Swedish entomologist interested in sampling warble flies which parasitise reindeer. They are passive interception traps and work on the simple principle that on encountering an obstacle most insects try and go over rather than around. The trap is a tent-like structure with a central net pegged close to the ground and a sloping roof leading up to the collecting bottle. Insects hitting the vertical net move up the net to the roof. Continuing upwards along the roof they eventually find their way into the collecting bottle.