"Drill" isn't the right word. Really what they do is to cut and slice into one's flesh until blood starts flowing, then they sop up the blood with their sucking mouthparts. You can see a horsefly's mouthparts at the left.

In the lower, right corner of that picture locate the large, black, roundish object, and then notice right above it but below the eyes and antennae the stiff- looking, brownish items, which are sharp and bladelike. They work together with other bladelike items not visible in the picture to cut like scissors (not bite or chew) into an animal's skin and cause bleeding. Once blood is flowing the horsefly extends its black "labium," the black, roundish thing below the scissors-like appendages, to suck up the blood.

Horseflies such as greenheads suck blood for the same reasons mosquitoes do -- the females need the nutritious blood for their egg production. That means that only female horseflies bite us. Male horseflies peacefully feed on flowers. In the first horsefly picture above, notice that the two green compound eyes are completely separated from one another by a blackish band. If that were a male horsefly the eyes would touch at the front of the head, so that's how you tell a male from a female horsefly.

That horsefly's green eyes are typical of some horsefly species in the genus TABANUS and such species are commonly called greenheads. Most horseflies don't have green eyes. The green-eyed, golden-bodied one in the picture slicing into the calf of my leg is a particularly colorful one.

Last weekend at Pipes Lake the deerflies were bad, too. Sometimes they landed on my legs as I peddled mile after scorching mile down the one-lane gravel road, and fed on me as I peddled. Deerflies belong to the same family as horseflies, the Tabanidae, but they occupy a different genus. In general, deerflies are smaller than horseflies, and often their wings bear darkish zones, giving the wings a mottled effect.

Where I was, there wasn't a horse for miles around. I felt sorry for the deer and other mammals, such as myself, the horseflies and deerflies attacked.

from the June 1, 2003 Newsletter, issued from near Natchez, Mississippi:
HORSEFLY ATTACKS
A Newsletter subscriber in McComb, MS writes that she understands what I was talking about a while back when I referred to these as horsefly days. "Walking in the woods is just not a whole lotta fun when you have to flail your arms or swat the air with a sweetgum limb the entire time you're walking," she writes.

She asks if horseflies are attracted to carbon dioxide the way mosquitoes are. This reminded me of a moment last weekend when I spent the night at my future base. As it began getting dark, horseflies swarmed over the white pickup truck that had taken us there, thumping into it again and again. They also hit against the barn's tin roof. The truck and barn roof weren't producing carbon dioxide, so something else was obviously the attraction. Of course the question of what attracts horseflies was made for Googling.

One Web site for fishermen provides all kinds of advice for outsmarting various blood-sucking insects -- except for horseflies. Of them it says, "Horseflies aren't smart enough to be confused. They have a steadfast mindset: find a human and bite its exposed flesh."

Another site in Canada focusing on problems canoeists face claims that "horseflies appear to be attracted visually, especially by bright or glittering surfaces. This may explain why they always appear to do their dirty work when we finish our swim and sit on the rocks, our bodies still covered in a reflective film of water." That would explain their attacking the white pickup truck and the shiny tin roof, but not their thumping into my dark mosquito net in the dim light of dawn and dusk.

Several Web sites selling insect traps using carbon dioxide as an attractant claim that just about every biting insect is attracted by carbon dioxide, including horseflies, but I'm not sure these claims can be trusted. I suspect that horseflies use a variety of cues to find their prey -- sight, odor, maybe carbon dioxide, maybe even infrared vision.

Whatever attracts horseflies, I feel sorry for their victims. A Web site in the UK claims that "50 horseflies feeding on livestock can remove 1/3 litre {quart} of blood per day." This site also points out that horseflies can transmit diseases such as the agents of anthrax (Bacillus anthracis) and tularaemia (Francisella tularensis). Plus, I can tell you that their bites hurt like the dickens!