thick-headed fly

Physocephala furcillata is a medium-sized thick-headed fly. It occurs in the United States from Maine to Maryland, west to Minnesota and Ohio. It occurs across southern Canada from Nova Scotia to eastern British Columbia.

Adults are active from July to August. They feed on the nectar of flowers. Larvae are thought to be parasitoids, living inside the body of a host bee or wasp and eventually killing it.

Adults are ⅜″ to ½″ (10 to 12 mm) in length. The body is black or brownish black (black) with yellowish-white or brownish-yellow (pale) markings, reddish-brown areas, and sometimes red areas. With their narrow-waisted bodies and dark wings they look like wasps, but they also have a superficial resemblance to some hover flies (family Syrphidae).

The head is dark, broad, and a little wider than the thorax. The top of the head (vertex) and the back of the head (occiput) are black. There are two large compound eyes and no simple eyes (ocelli). The compound eyes are situated at the sides of the head and do not meet in the middle on either sex. The forehead (frons) is yellow with a dark, T-shaped mark. At the base of the “T”, two black oblique lines extend down on each side bordering the facial ridge. The face is white to yellowish, and it is prominently grooved. The grooves on the facial ridge are pale. The cheeks (genae) are black. On the upper part of the face, there is a narrow, inverted U-shaped mark curling around the antennae bases. The antennae are mostly black and have three segments. They are straight and long, longer than the head, and are projected forward and up. The first and third segments are reddish below. The third segment is inflated and has a slender, elongated process (style) at the tip. The tube-like protruding mouthpart (proboscis) is black.

The thorax is black. There is a raised bump (“shoulder pad”) near but not at each front corner. There is a short, small, pale, dash-like mark in the shoulder (humeral) area. It may be indistinct or even absent on some individuals. The exoskeletal plate between the abdomen and the thorax (scutellum) is usually entirely black, but sometimes it has a narrow pale bar. The sides of the thorax are at least partially reddish brown or have thin red lines.

The leading (outer) half of each wing is dark and the inner half is clear (hyaline). The thin cell on the leading edge (costal cell) is pale to light brown. Cell r5 is hyaline at the base and for almost the entire length of vein M₁. The discal cell is hyaline.

The legs are slender and mostly reddish to reddish-brown. On each leg, the third segment (femur) is darker toward the base and pale at the tip. The fourth segment (tibia) is pale on the basal half. The last part of the leg (tarsus), corresponding to the foot, has five segments. The last two segments are black

The abdomen is black. Segment 2 and part of segment 3 are very constricted, forming a wasp-like petiole. The petiole is pale on the lower sides, and sometimes the rear is light reddish brown above. Segments 2 through 5 each have a pale band at the rear margin. Segments 4 through 6 have a yellowish-white dusting above. On the female, segment 7 is slender and modified into a pincer-like form. This is thought to be an adaptation that allows the female to pry apart a prey’s abdominal segments and deposit an egg between them.

Total length: X¾″ to X¾″ (X to X mm)

July to August

Bees and wasps

Flower nectar

Sources

24, 29, 30, 82, 83.

Physocephala furcillata - 2020/07/23
Bombus mystax

Location: Lake Shore, MN