mountain deer fly

Mountain deer fly is a common, relatively small, deer fly. It occurs in the United States and southern Canada east of the Great Plains.

Mountain deer fly larvae are aquatic and predaceous, feeding on other insect larvae and on worms. Adults are active from June through August in Minnesota. They are found in open woodlands and wetlands, often near the margins of streams and ponds. Males feed on plant nectar. Females feed on the blood of mammals.

Males are relatively rare, and they are difficult to identify to the species level. Female adults are 5⁄16″ to ⅜″ (7.4 to 9.0 mm) in length. The body is stout and brownish yellow with dark markings. The wings have dark markings.

The head is broad, and it is covered with whitish-yellow hairs. The compound eyes are bulging, iridescent, and brightly colored. On males they meet at the top of the head. On females they do not meet. There are three simple eyes (ocelli). The black area around the ocelli (ocellus spot) is broken up into three individual spots. Females have a large, bare, hardened area (frontal callus) on the front of the head between the compound eyes. The frontal callus is completely black and somewhat shiny. The antennae have three segments. The first segment (scape) and second segment (pedicel) are brownish yellow and are covered with short black hairs. The third segment (flagellum) is long, clearly ringed, and black, at least toward the tip. The large, bare, hardened area on the face (facial callus) and the small, bare, hardened area on each cheek (cheek callus) is brownish yellow. The mouthparts are modified for piercing flesh. The finger-like sensory mouthparts (palps) are light yellowish brown.

The thorax is light brownish yellow. On the upper side there are three blackish-brown longitudinal stripes. On each side there is a brown stripe between the base of the wing and the shoulder (humeral) angle.

The abdomen is brownish yellow with 4 blackish-brown longitudinal stripes, 2 in the middle (middorsal stripes) and 1 on each side (lateral stripes). It has 7 segments (tergites). The first segment (T1) has a faint brown spot that appears to be two spots representing the middorsal stripes merged together. On T2 the middorsal stripes are blackish-brown and bold. They merge in the front either broadly or narrowly, creating an inverted V, or they are very narrowly separated in the front. The lateral stripes are either represented by a poorly defined spot on the rear margin or they are missing altogether. On T3 and T4, all of the stripes are bold. On T5 they merge at the front, creating a broad notched black band. T6 and T7 are black with a narrow yellow rear margin.

The wings are clear with dark markings, including a dark leading edge (costa), a broad dark band in the middle (crossband) and a dark spot at the tip (apical band). The area between the crossband and the apical spot (hyaline triangle) is clear. Details of the extent and shape of these markings are important identifying features of species in the genus Chrysops. The dark markings are not broken by pale areas bordering the veins. The hyaline triangle is relatively large, extending beyond cell m1. The tip of the hyaline triangle does not cross vein R₂₊₃. The apical spot is wide, filling more than half of cell r4. Cell br is at least one-third clear.

Total length: 5⁄16″ to ⅜″ (7.4 to 9.0 mm)

Open woodlands, wetlands, the margins of streams and ponds

June through August

Larvae are aquatic.

Other insect larvae and on worms

Males feed on plant nectar. Females feed on the blood on mammals.

Sources

27, 29, 30, 82, 83.

Common

Infraorder
Orthorrhapha was historically one of two infraorders of Brachycera, a suborder of Diptera. However, Orthorrhapha was paraphyletic, meaning that it did not contain all of the descendants of the last common ancestor. It was split into five extant (still existing) and one extinct infraorder. Orthorrhapha is now considered obsolete and has not been used in the last decade, but it persists in printed literature and on some online sources. A recent revision of the order Diptera (Pape, et al., 2011) revived the name Orthorrhapha, but this has not been widely accepted.

Alfredo Colon
6/12/2024

Location: Albany, NY