Williston's Wasp Fly

Size Between 11–14 mm (0.43–0.55 in).

In overall appearance this species mimics a wasp with the darkened forewing, elongate antennae, black and yellow markings on the thorax and banded yellow markings on a thin-wasted abdomen.

Head

The face is long, flat, black with varying yellow marks.It is yellow in the middle with a large black arrowhead shaped spot and the broadly rounded apex rests on the oral margin and its base and is narrowly connected with the broad black field below the antennae. This black field extends transversely to the orbit of the eye, separating the yellow of the face and from a small yellow somewhat triangular spot situated at the orbit of the eye and opposite the base of antennae. From the antennal process to the middle of the arrowhead shaped spot there is a yellowish line, very narrow above, widening below. Some have either a few yellow spots or a transverse stripe below antenna base. Above the base either a yellow V-shaped stripe (usually on male), or another series of 4 spots (usually on female). The front is black, broadly connected with the black below the antennae. A little below the middle are two small, narrowly separated, reddish brown, transverse spots. The opaque portion of the front is thinly white pollinose, and can be seen only in certain lights, most conspicuously at the orbit of the eye. …