Tonight I caught a decently large horsefly (compare it to the pen) under a drinking glass and took some good pictures of it.
Several times, the fly tried to crawl up the sides of the inverted glass, and when it got the base, fell straight down on its back. For some reason it didn’t consider using its wings to right itself and flailed helplessly for a minute or so. Then I got nervous about it escaping and put it back under the glass, but it did the same thing a few more times.
Since the fly was so energetic, I cheated in order to get a good picture; I put the glass in the refrigerator until it calmed down.
In this shot, if not for a slight motion blur, you can see the mouth in detail. The 2 mouthparts are about the same size as the tip of the pen, which is ironic because female horseflies suck mammal blood just like a fountain pen — in reverse. The jaws are small blades designed to pierce the skin, allowing the blood to flow up the channel between them while an anti-clotting chemical in the fly’s saliva keeps the blood flowing (even after the fly is done drinking).
Fortunately, the pain of the initial bite on a human usually gets the fly killed before it can cause any serious bleeding, but horses (and other livestock) are not so able to defend themselves. Hence the name horsefly.