Fly Paper Salvia

I noticed these flies hanging on a stem of Salvia discolor, and thought, Whoa why are they doing that? On closer inspection it was apparent the flies had no choice in the matter: they were stuck on the sticky exudate covering the stem.

Most of the plant stems are covered in white fuzz or are hardened; the sticky parts are the flower stalk, from the last leaf axil out. May be a defense against aphids, and these flies (they kind of look like march flies or some other sort of gnat) got caught in the cross fire.