I have photographed Small-headed flies, Eulonchus tristis near our house—about 2,000 feet elevation, five miles south of Port Angeles, WA.—in June or July for several years. The female is a little over 1 cm in length.
Small-headed flies, family Acroceridae, may feed on flowers as adults, or may not feed at all as adults. They lay eggs on leaves, which hatch into tiny, flat larvae that then burrow into a passing spider, later emerging to pupate on the web.
Thanks to Eric Eaton and Dennis Haines at bugguide.net for identification.