Horse flies are insects that suck the blood of humans, horses and cattle. A typical horsefly has a length of one inch, but some horse flies are a bit longer. Horse flies can be identified by their brightly-colored eyes and solid-colored wings. It's unfortunate that even a small amount of horse flies can make outdoor activity undesirable. Year to year, the level of intensity of a horsefly assault varies. When cattle sustain a large number of bites from horse flies, it's common for dairy production to stall. The horsefly is always active during daytime, and it's attracted to warmth, carbon dioxide, shiny surfaces and movement. Horse flies have sharp mouth parts that are like knives, and they use these parts to slice the skin of humans and animals.