Spotting Robber Flies
Two tools are essential. First, close-focusing binoculars (binoculars with a minimum focus of eight feet or preferably less). Most birdwatching binoculars, that sometimes don't focus closer than twenty feet, are useless for studying insects. Quite often you can get within five or six feet of an insect, and then with 8-power binoculars, you can see, for instance, the fine details of leg color that might be necessary to separate one species from another. Next, a digital camera with macro capability. A robber fly may only stay within view for a few seconds, not giving you time enough to take in every detail, but if you get a close-up picture you can study the blown-up image of that fly at leisure.
Quite often when we see an insect in the field that is so small we can't even make it out with our binoculars, we take a close-up picture of it, and blow up that image on the viewing screen to give ourselves an idea of what we are looking at. When you are first learning the robbers, it is probably a good idea to try to take a picture of every species. Frankly, it is so much fun, and Robber Flies are so photogenic, you'll quickly get addicted.
A tip: The most helpful picture for identification purposes is generally from the side, but from slightly above, and slightly from the front. Thus you can get in one picture such important features as shape of antennae, color and shape of mystax (the hair between the eyes), color and marking of the legs, color and marking of the thorax, color, shape, and marking of the abdomen, and length of the wings
Picking out robber flies from other kinds of insects is easy with a little practice. If you are generally interested in nature, you tend to notice big showy insects like butterflies or dragonflies, and we are programmed to notice the orange or red or yellow and black warning colors of bees and wasps, because they might be painful to get too close to. But once you get your binoculars out and start really looking, you will find that flies dominate the world of day-flying insects. These aren't just house flies and bluebottles, the ones we think of as pests. The majority seem to be Tachinids (mainly ordinary looking flies with lots of bristles), and flower flies (the majority mimic bees and wasps in their color patterns). Both of these huge groups have larvae that are parasitic or predacious on other insects, and they are vital controls for population balance. The adults drink nectar, and we usually see them on or about flowers, and this is the first tip: Robber Flies don't drink nectar and they are almost never on flowers.
If you see a fly sitting in the middle of a broad leaf, or sitting on the tip of a bare twig, look again: it might very well be a robber. If it flies up, then returns to the same perch, it is almost certainly a robber.
Many flies land with their wings facing apart from each other on their backs; Robber Flies land with their wings folded together over their backs.
Many flies have their two eyes touching each other: Robber Flies have their eyes widely separated from each other. Moreover, they always have some, usually a lot of, hair between their eyes.
They are bulky and heavy with muscles, so if you see a fly at a distance from you land on a leaf, and that leaf noticeably bends down with the weight, that might be a robber.
Robbers usually land either on bare ground, or near the top of vegetation, where they have clear air space all around them, to see approaching prey, and to instantly fly up after it with no obstacles in the way. For that reason, they are often in plain sight, and you can spot them from some distance away.
Robber Flies are such voracious and efficient predators that very often you will see them carrying prey, so if you see a fly holding and feeding on another insect it will probably be a robber. But check carefully: Empids and some other flies are also predatory.
When you flush a large robber from nearby, you will often hear a loud buzz.
Robber Flies protect the sharp talons at the ends of their feet, with which they capture their prey. Therefore, Robber Flies fly from place to place, even a short distance away, and under most circumstances never walk. If a possible robber lands on a leaf, then begins walking actively about, it is a wasp, or some other kind of fly. (An exception is when a Robber Fly walks along the bare dirt with its abdomen hanging limply to the ground: It is feeling for an area of soft earth in which to bury its eggs.)
With any of these clues, look more closely, and if you see hair between the separated eyes, and a beak (bees and wasps, which robbers often mimic, have biting mouthparts; a few other kinds of flies have beaks, but most have spatulate tongues), then you have a robber.
Places to look for robbers: In spring and early summer the most productive places on the Ridge are closed canopy woods with a dense understory of poison ivy. (Many woodland insects are most active in early spring, before emerging leaves have blocked all the light.) Look for Laphria, Echthodopa, Machimus and other species sitting on the poison ivy leaves, usually where dappled sun is coming through.
Dirt trails through woodland that are wide enough to get some sun are good. Look for robbers on the bare dirt, or on broad leaves, or bare twig-tips alongside the trail.
Sandy beaches, abandoned quarries with sandy soil, and in fact any sandy area, vegetated or not, is good.
In mid summer, meadows with calf- to knee-high grass, especially with bare dirt patches, are good for Diogmites.
Sunny openings in the woods.
Bottomland overflow areas after they have dried out in the summer.
Large well rotted hardwood or pine logs, especially in woodland with a fairly clear understory. Many species of Laphriinae come to these logs to lay their eggs, and males wait there for the females to come.