The longlegged flies are a large, diverse family of true flies that, though small and slender, often have eye-catching metallic green, copper, bronze, or blue bodies and long legs. The abdomens of many species are banded with a dark color. Their delicate wings, usually about as long as the body, are typically clear and look iridescent in bright light, but many species have dark marks near the wing tips. Depending on species, they may hold their wings at a 45-degree angle from the body, or they may fold them straight down the back.
In addition to the larger, eye-catching metallic species, this group has plenty of drab, easily overlooked members, too. Specialists use patterns of wing venation and characteristics of the legs to delineate the longlegged flies as a group.
Longlegged flies are typically quick, darting fliers, often seen resting briefly on tree leaves, or skimming the surface of water.
A closer look at longlegged flies as a family reveals a number of anatomical details, most of which are beyond the scope of this introductory page. There are usually large bristles on the body. The shinlike leg segments (tibias) usually have noticeable bristles, and males often have special bristles and modified claws on their foreleg feet that are used during courtship. Also, males of many species have enlarged reproductive structures that are folded under the tip of the abdomen.
Globally, there are nearly 7,400 species in this family of true flies. Here are some of the common groups found in Missouri:
- Condylostylus spp. (about 35 species north of Mexico). Some members of this group have plain wings, but several species have two smoky-dark bands on the outer half of the wings that are joined along the leading edge of the wing to form a sort of C shape. The big compound eyes are sometimes reddish, contrasting with the usually emerald-green body. This genus now includes some species that were formerly put in genus Chrysosoma, but the current understanding is that genus Chrysosoma only occurs in the Old World and Oceania.
- Dolichopus spp. (more than 300 species north of Mexico). Males in this genus usually have flat, wide feet on the first pair of legs, which they use in courtship displays (these are often called flags, but they look more like mittens or slippers).
- Pelastoneurus spp. (apparently dozens of species north of Mexico). These are not as brightly colored as other longlegged flies, being generally brownish, gray, or blackish. They characteristically have a swollen-looking face that seems to bulge outward between the two big compound eyes. They typically fold the wings flat down the middle of the back and have yellowish femurs (the thigh-like leg segments, near the body).
- Hydrophorus spp. (apparently dozens of species north of Mexico). These are also fairly drab-colored and hold their wings down their back; they are partially aquatic, as the adults skim along the surface of water much like water striders do, but they also fly quite well.
- Medetera spp. (around 50 species north of Mexico). These are sometimes called woodpecker flies for their general angled appearance and posture as they perch on tree bark. This group of longlegged flies, as larvae, creep beneath the bark of trees in the tunnels made by bark beetles. When a larval woodpecker fly finds a larval bark beetle, it stabs it with its needle-like mouth and sucks the juices.
Similar species: Certain members of several other fly families are generally similar in overall shape and size, including some of the dance flies (family Empididae), stilt-legged flies (Micropezidae), bee flies (family Bombylidae), robber flies (family Asilidae), and flower flies (Syrphidae). Certain mosquitoes (family Culicidae), midges (family Chironomidae), and crane flies (family Tipulidae) might be confused with these, too. In the ants, bees, and wasps order, certain sweat bees (halictids) and solitary wasps (sphecids) may look rather similar, too.
Adult length (not counting appendages): most are less than ¼ inch; depends on species. Ranges from extremely small to about ⅜ inch.
Habitat and Conservation
Longlegged flies, like other flying insects, could appear almost anywhere, but they are most common in light shade or dappled-light places near water, such as near streams, wetlands, lakes, and moist woodlands. They are often seen darting around on the leaves of trees or dancing on leaf surfaces. Some members alight on water and skate around on the surface film, much like water striders.
The larvae most typically live in a range of soil types or in sluggish aquatic habitats. Some species live in other habitats.
Adults are something like miniature robber flies, capturing smaller insects (such as aphids and gnats), mites, and other tiny arthropods. Their mouthparts are knifelike, for piercing their prey.
The larval diet varies by species; some are predatory (much like the adults), while others eat from plants, as leaf miners (tunneling around and eating the inner tissues of leaves), stem borers, or beneath the bark of trees. Some apparently are scavengers, eating decomposing organic matter such as rotting vegetation.
The larvae of some types of longlegged flies, notably those in genus Medetera, live under tree bark, in the burrows of bark beetles. There, they hunt bark beetle larvae, stabbing them with their needlelike mouths and sucking their juices.
Not harmful to people. Many people consider longlegged flies beneficial, since they consume mosquitoes as well as many garden pests.
Like other true flies, longlegged flies have complete metamorphosis, beginning as an egg, progressing through a number of grublike larval stages, pupating, then emerging as a winged, sexually mature adult.
Males of many species have elaborate courtship displays, waving and showing the female their decorated legs, wings, or both.
As predators of smaller insects, longlegged flies are generally considered friends to gardeners, as they consume aphids, spider mites and other mites, thrips, and so on. Some longlegged flies eat mosquitoes, and you may see them flying around and skating on the water’s surface, hunting for them.
Longlegged flies whose larvae prey on bark beetle larvae are of special interest to forest ecologists who are concerned with managing bark beetle damage to forest trees. One prominent entomologist flatly stated that these flies “kill more forest pests than all the world’s chainsaws combined.”
The family name, Dolichopodidae, comes from Greek roots and pretty much means “long-legged”: dolicho means “long,” and pod means “foot.”
People often pay little attention to longlegged flies, but many species shine like jewels.
Longlegged flies consume tinier insects, helping to check the populations of mosquitoes, aphids, mites, bark beetles, and many more. Meanwhile, longlegged flies serve as food for many predators that feed on insects, such as spiders, birds, lizards, fish, mantids, shrews, and more. Remember that although longlegged flies can make a fast getaway as winged adults, they spend much of their lives as grubs. Also, many spiders — such as spined micrathenas — build webs with closely spaced lines, perfect for capturing small flies such as these.