four-lined hornet fly

Fairly common

Early July to mid-October

Wetlands in deciduous forests, sometimes on hilltops

Four-lined hornet fly is a medium-sized, wasp-mimicking, flower fly. It occurs in northern United States from Maine to Minnesota, and in southern Canada from New Brunswick to Saskatchewan. In Minnesota, where it is at the western extent of its range, it has been recorded in the northeast, north-central, and metro regions. Adults are found from early July to mid-October hovering around flowers in wetlands, sometimes on hilltops. Larvae are found in deep rotholes in trees. A rothole is a cavity formed at the site of a wound as the result of fungal and bacterial action. A deep rothole is a rothole that does not heal. The water inside is mostly from the vascular system of the tree, not from rain, and is reddish-brown, not clear.

Four-lined hornet fly mimics vespid wasps (Family Vespidae) in both appearance and behavior, but unlike wasps it cannot sting. The face is yellow and straight, not concave. The lateral parts of the head (gena), equivalent to the cheeks, are yellow. A black line separates the face and gena. The antennae are are black and have three segments. They are longer than most members of this genus. The basal segment (scape) is long, at least 1.5 times as long as the second segment. The second segment (basoflagellomere) is elongated, much longer than wide. The third segment is longest and has a large, brownish-yellow bristle (arista) on the upper side. The structure of protruding mouthparts (proboscis) is short and fleshy. There are two large compound eyes on the sides of the head and three small simple eyes (ocelli) in a triangle on the top of the head. The compound eyes are dark brown with an obvious yellow pigmented pattern that helps to disguise the large eyes. On the male the compound eyes meet at the top of the head. On the female they do not meet, and the black lines between the face and gena taper and converge approaching the ocelli, becoming a single thin brown line at the front ocellus.

The thorax is black with yellow markings. The large exoskeletal plate (scutum) covering the middle segment of the thorax has two pairs of yellow spots on the front margin, a yellow line on each side from the rear margin that bends inward at the transverse suture, and a pair of oblique yellow lines at the rear margin that form an inverted V. The small plate (scutellum) between the thorax and the abdomen is entirely black.

The abdomen has four segments, and each segment is covered by a hardened exoskeletal plate (tergite). Tergite 1 is entirely black. Tergites 2 and 3 are black with a single yellow band. The band on tergite 2 is complete but may be shallowly notched in the middle. The band on tergite 3 is deeply notched or slightly separated in the middle. There are two bands on tergite 4. The front band is noticeably separated in the middle.

The wings are mostly clear but the front third is darkened. The small, knob-like structures on each side of the thorax (halteres) used for balancing in flight are yellow. There is a spurious vein between the radius (R) and media (M) veins. The cross-vein that runs between the R and M veins is strongly oblique. The anal cell is long and is closed near the wing margin. The R₅ and M₂ cells are also closed. The radial sector vein (Rs) has 2 branches.

The legs are mostly yellow. The third segment (femur) of the hind leg is at least half black and has an unbranched spur near but not at the tip. The fourth segment (tibia) of the front leg is mostly black, yellow just at the base. The last part of the leg, corresponding to the foot, has 5 segments. It is black on the front legs, yellow or brownish-yellow on the middle and hind legs.

Pollen and nectar of goldenrods, flat-topped aster, pearly everlasting, and other plants whose flowers produce large amounts of pollen.

Sources: 24, 29, 30, 82, 83.

Outstanding eyes

Location: Woodbury, Minnesota

outstanding eyes