Deer flies are blood-sucking insects that can cause painful bites to humans and animals. These flies are also known as yellow flies or summer flies due to their appearance during the hottest months of the year. Other names for deer flies include clegs, breeze flies, and horse flies. They are distinguishable by their large, dark eyes and their erratic flying patterns. Deer flies prefer to live near water and marshy areas, as this is where their larvae develop. While these flies are notoriously difficult to swat and avoid, proper insect repellent and protective clothing can help keep them at bay.