Long-legged Flies

Happy “Fly Day Friday,” everyone. Long-legged flies in the family Dolichopodidae include those little, bright metallic green flies you see darting over leaves in sunny spots; but they also include flies that run up and down tree trunks, or skate across the surface of puddles. The diversity of this family is almost as staggering as the Syrphidae, Bombyliidae, and other fly families that are more familiar.

The Condylostylus species imaged above, from Skokie, Illinois, is the typical dolichopodid we normally see in the garden, along forest edges, and similar habitats. Like all longlegged flies they are predatory on other small insects. It can be maddeningly difficult to get a good image of one because the flies aren’t that large to begin with, hardly ever stop moving, and can be sensitive to the pre-flash on your camera such that you wind up with either an empty frame, or an out-of-focus fly in mid-air as it departs.

Condylostylus sp. with prey

Dolichopodids range in size from barely one millimeter to “giants” of ten millimeters. Most are five millimeters or less. They are usually metallic green, copper, or bronze, though many species are muted rather than brilliant.

Hydrophorus sp. pair, Colorado Springs

What is truly astounding is the diversity of this family. There are nearly sixty (60) genera in North America north of Mexico, with about 1,300 species. It is difficult enough to identify a given specimen to genus, but species ID generally requires examining the male genitalia. Not that the male’s “man parts” are not obvious. His claspers and other, um, paraphernalia are easily seen as forward-protruding appendages at the tip of the abdomen (curled beneath the abdomen).

The front “feet” (tarsi) of males of most Dolichopus species are adorned with special scales that may resemble tiny flags. These ornaments are used to display to females in complex courtship dances.

Hydrophorus sp., Pueblo Reservoir, Colorado

Besides looking for these flies on foliage, keep an eye out for them on the water, too. Hydrophorus species skate across the surface of puddles, intermittent streams, and near the shores of lakes and reservoirs where they behave much like water striders. Members of the shorter-legged genus Dolichopus can also be seen on the surface film. The adults of these flies are even adept at preying on mosquito larvae, nabbing the “wrigglers” when they ascend for air.

Liancalus genualis, South Deerfield, Massachusetts

Longlegged flies do not have beak-like mouthparts like robber flies or dance flies, but instead have a lower “lip” modified into an extendable appendage and equipped with a pair of flaring, opposable lobes that can crush a victim or tear it to pieces. Hard to believe these diminutive, delicate insects are really quite vicious predators.

The “woodpecker” flies in the genus Medetera are commonly seen on the trunks of trees, literally standing on tiptoe in a seemingly awkward posture. They still have the agility of their cousins, I assure you.

Medetera sp., Redondo Beach, California

As far as is known, dolichopodid maggots are also predatory. Most are aquatic or semi-aquatic, but the larvae of at least some Medetera species roam the galleries of bark beetles, feeding on the beetle grubs. The larvae of most species, however, remain a mystery.

It is impossible to do justice to these remarkable flies in a blog post, or even a book, but I encourage you to do a double-take the next time you see an unfamiliar little fly. It may well be a dolichopodid, and what you observe could easily yield new knowledge.

Tachytrechus angustipennis(?), Colorado Springs

Sources: Bartlett, Troy, et al. 2004. “Family Dolichopodidae – Longlegged Flies,” Bugguide.net
Marshall, Stephen A. 2006. Insects: Their Natural History and Diversity. Richmond Hill, Ontario: Firefly Books Ltd. 732 pp.
McAlpine, J.F., B.V. Peterson, G.E. Shewell, H.J. Teskey, J.R. Vockeroth, and D.M. Wood (coordinators). 1981. Manual of Nearctic Diptera Volume 1. Ottawa: Agriculture Canada. 674 pp.