A pair of bristles that usually arise from the ocellar triangle near the anterior ocellus but that are sometimes displaced, usually outside the ocellar triangle near the anterior ocellus.
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http://upload.wikimedia.org/wikipedia/commons/c/cf/Head-diptera-muscoidfly.gif
Tried to attach an image to a forum post. jpg, 32kB, 72dpi, no blanks, ... File name is correctly displayed, but when I click "Preview Thread" it just vanishes. Help!