Plural of deerfly; small bloodsucking insects (family Tabanidae) that often attack deer and humans.
From 'deer' + 'flies' (plural of 'fly', Old English 'fleoge'). Named for commonly feeding on deer, though they're agricultural and human pests.
Deerflies are so aggressive that deer will run into water to escape them—this ancient pest-prey relationship shaped how deer behave, and they still do it instinctively.
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