Why are horseflies more attracted to certain parts of lakes and to certain people?

Female (and only female) horseflies attack because they need a meal of blood protein to hatch their eggs. When you’re feeling like an all-too-attractive target, it may help to remember that you’re not really their preferred victim. If a cow, a moose, or one of the other ungulate mammals were handy, the flies might leave you alone. That’s probably because these animals are bigger and easier to spot. They also give off more carbon dioxide and moisture, which are sensory cues for most biting flies, including mosquitoes and blackflies. Another reason for the flies’ preference for ungulates could be a longer shared evolutionary history.

The tabanids (the group that includes deerflies and horseflies) are also visual predators with huge eyes and acute eyesight, able to detect movement, contrasts, shiny objects, and non-moving silhouettes from great distances. Scientists believe horseflies see a target, fly towards it, then home in on the sensory cues as they get close.

Ontario is well endowed with 48 of the 75 known species of horseflies in Canada. Like most biting flies, they’re found in moist areas, such as ponds, marshes, and bogs. Keeping to drier ground won’t protect you because they’re such strong, fast fliers, but it would help to stand beside somebody a lot bigger than you.