Make Your Own Horsefly/Deerfly Trap!
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Horse flies and deer flies are bloodsucking insects that can be serious pests to cattle, horses, and humans. Horse flies range in size from 3/4 to 1-1/4 inches long and usually have clear or solid coloured wings, and brightly coloured eyes. Deer flies, which commonly bite humans, are smaller with dark bands across the wings and coloured eyes similar to those of horse flies.

Attack by a few of these persistent flies can make outdoor work and recreation miserable. Female horse flies and deer flies are active during the day. They are visual hunters, and are attracted to such things as movement, shiny surfaces, carbon dioxide, and warmth.

          
Trapping these pests is far more effective than trying to kill them with insecticides. Traps are large, conical and have a collection chamber at the apex of the cone into which the flies rise, become trapped and then die. The trap uses the behaviour of the fly to catch them. A large, black, round ball is suspended so that half of the ball is visible below the netting forming the cone. The flies are attracted to the ball and land to investigate. Once on the ball the flies crawl over it, usually ending up on its upper surface. From here, having decided that there is nothing worth biting, they fly directly upwards. This brings them into the conical netting. The collection chamber at the top of the trap is transparent and so the flies are attracted up into it – when there is light, flies won't go back the way they came, but keep trying to go to the light. The flies enter through a funnel and once inside the chamber they die from dehydration in a day or so.

The construction images below are taken from the following blog post: http://sippicancottage.blogspot.com/2008/07/fie-on-thee-horseflies.html