gnat ogre

Holcocephala calva is a small gnat ogre. It occurs in the United States east of the Great Plains, and in southern Canada in Quebec and Ontario. Adults are found from early July to early September in damp, grassy, open areas near woodlands.

Adults are light brownish-yellow and ¼″ to ⅜″ (7 to 9 mm) in length.

The head is brown in front, ash gray behind. It is much broader than the thorax, and it is strongly compressed from back to front. There are two large compound eyes on the side of the head and three small simple eyes (ocelli) in a triangle on top of the head. The top of the head is depressed (“hollowed out”) between the compound eyes. The compound eyes are relatively large and very widely spread apart. This feature distinguishes the genus Holcocephala from all other fly genera in the United States. Each compound eye is composed of numerous visual elements (ommatidia) with a hexagonal outer face (facet). The facets on the front of the eye are very large but gradually decrease in size approaching the upper, outer, and lower margins. The facets are black but have bright bronze reflections. The antennae are short, slender, and brown. They have three segments. The first two segments are very short and rounded. The first segment is spindle-shaped, distinctly broader in the middle. The third segment is two times as long as the first two segments together. At the tip of the third segment there is a long extension (style). The style is well developed, nearly as wide as the third segment at the base and tapering to a sharp point at the tip.

The thorax is stout, ash gray, and strongly arched. It is densely covered with short, velvety hairs that completely hide the underlying color. It has no bristles. There are three segments. Each segment has four principal exoskeletal plates, one above, one below, and one on each side. The upper (dorsal) plates, from front to rear, are the prescutum, scutum, and scutellum. The prescutum and scutum have scattered, longer, rearward-curved, bristle-like hairs, but no long stiff bristles. There is a broad, brown, longitudinal stripe in the middle and another stripe on each side. The lateral stripes are interrupted at the groove (suture) between the prescutum and scutum, broader and dark on the prescutum, more diffuse and sometimes much lighter on the scutum. The scutellum is unmarked.

The abdomen is broad at the base and tapered toward the tip. It is brown above, with a grayish band between the segments. The lateral margins are grayish, and the underside is ash gray. It has no bristles.

The wings are slender and long, longer than the abdomen. They are evenly smoky brown. They are held over the body when at rest. Every cell near the wing tip is broadly open.

The legs are reddish and are sparsely covered with fine hairs and numerous, weak bristles. The fourth segment (tibia) is light brown at the tip. On the front leg there is no claw-like spur at the tip. On the hind leg it is swollen at the tip, about 35% wider than the femur. The last part of the leg (tarsus), corresponding to the foot, has five segments. They are mostly light brown, black just at the tip. The first segment is at least twice as long as the second segment. The last segment has a pair of black claws at the tip. On the hind leg of the male, the tibia and the first tarsal segment are only slightly broader, about 15% broader, than the femur.

Total length: ¼″ to ⅜″ (7 to 9 mm)

Damp, grassy, open areas near woodlands.

Early July to early September

Adults perch at the tip or on the stalk of a grass blade when hunting.

Gnats and other small flying insects

Sources

27, 29, 30, 82, 83.

Most sources place the genus Holcocephala in the tribe Damalini. However, a handful of sources reject that name as a junior synonym of Xenomyzini, following Article 40.1 of the ICZN rules. The name Trigonomimini is older and therefore has precedence. The short list of sources that place Holcocephala in the tribe Trigonomimini includes Robber Flies of the World, AsilidaeData, The National Museum of Natural History at the Smithsonian Institution, and iNaturalist.

This species has no common name. The common name for the genus Holcocephala is gnat ogres, and it is applied here for convenience.

Holcocephala calva gnat ogre robber fly

Holcocephala calva, gnat ogre robber flies, were found around the seasonal pond at Barnes Prairie, Racine Co., WI. The best time I've found to see these big-eyed flies is right after sunrise, or late afternoon. They're tiny, and like to perch on the tips of leaves, or in some of my photos, the tops of unopened aster flowers. They catch gnats, mosquitos and other small flying insects.

Babette Kis
10/14/2022

Location: Barnes Prairie, Racine Co., WI

Holcocephala calva, gnat ogre robber flies, were found around the seasonal pond at Barnes Prairie, Racine Co., WI. The best time I've found to see these big-eyed flies is right after sunrise, or late afternoon. They're tiny, and like to perch on the tips of leaves, or in some of my photos, the tops of unopened aster flowers. They catch gnats, mosquitos and other small flying insects.