Cottage Q&A
QUESTIONI would like to know why horseflies attack. They seem to be only on certain parts of the lake and attack certain people (mainly me). Do they like specific types of shoreline or people?
ANSWER
Female (and only female) horseflies attack because they need a meal of blood protein in order 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 another ungulate mammal were handy, the flies might leave you alone. That's probably because these animals are bigger and easier to spot, explains Doug Currie, curator of entomology at the Royal Ontario Museum, and also because they give off more carbon dioxide and moisture than you do.
Most biting flies, including mosquitoes and blackflies, home in on sensory cues such as CO2 and moisture. But the tabanids (a group that includes deerflies and horseflies) are also visual predators with huge eyes and acute eyesight, able to detect movement, contrasts, shiny objects (such as water drops on your skin), and even non-moving silhouettes from great distances. Scientists believe horseflies see a target first, fly towards it, then zero 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 breeding and hanging out 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.
* Published in the April/May 2003 issue of Cottage Life
