The technique is called pool feeding, or telmophagy. The female tabanid's mouthparts include a pair of mandibles that work like small blades, opening a wound in the skin rather than entering a vessel. Saliva containing anticoagulants and vasodilators is introduced so the blood keeps flowing, and the fly takes it up through a sponging labellum. The wound is larger and slower to close than a mosquito puncture, which is why the bite is painful and often continues to bleed.
Males lack the cutting mandibles entirely. They feed on nectar and honeydew, as do females between blood meals, and are frequent flower visitors.
Tabanids are strong fliers, and this has generated one of the most repeated numbers in entomology: a claimed speed of about 145 kilometres per hour for a male deer fly pursuing a lure. That figure comes from a single mid-twentieth-century observation and has never been confirmed by instrumented measurement.