Image Credit: S. Rae
Hover-flies (Syrphidae: Syrphinae) – predators. All of the typical banded hornet-mimicking hover-flies and their close relatives (in the subfamily Syrphinae) are predators on soft-bodied insects (i.e., aphids and scale insects). I recognize them as second only to bees in general importance. There are nearly 200 species of this group alone in the PNW. The species with black faces related to Platycheirus can be observed foraging frequently on the flowers of grasses and sedges; it is unknown whether the catkins of wind-pollinated conifer trees are also used as a food resource.
The typical aphid-feeding hoverflies are easy to distinguish as a group, but difficult to identify to genus in the field. Probably 50% of those you will encounter are the mid-sized genera Syrphus and Eulonchus (three species in the top row); Syrphus has a yellow face but Eulonchus has a black midline running down across the “nose”.
Left to right: the very large bumblebee-sized convex Chrysotoxum, the elongate Scaeva and the minute Toxomerus. Chrysotoxum is the only hoverfly genus with more than a stub of an antenna. Scaeva has distinctly U-shaped often whitish abdominal markings and Toxomerus has complex abdominal spots.
Left is the extremely abundant genus Sphaerophoria; tiny, but very elongate and the males are distinctive with their enormous swollen genitalia. Right is Platycheirus, an aphid-feeder, even though it has a black face and body. Both Platycheirus, similar to Sphaerophoria in size, and the more average-sized Xylota, are black with apparent colored quadrangular markings on the abdomen; the markings are due to refractive iridescence only and disappear when viewed from different angles.