A robber fly (also known as an assassin fly) is one of any of the nearly 4,000 species of fly within the family Asilidae, which can be found in grasslands throughout the world. Robber flies are distinctive for their hunting tactics: Catching other flying insects in mid-air. Once the robber fly grasps its prey with its legs, it stabs a proboscis (a tube in place of its mouth) into its victim, injecting the captured insect with a paralyzing, toxic saliva. Inside the robber fly's prey, the saliva begins to liquefy the internal organs as the fly carries its meal back to a plant to perch on. Once there, the fly begins sucking the liquefied innards of its prey through the proboscis. Yum.
The robber fly will prey on most flying insects, though it seems to prefer bees and wasps. The fly is capable of preying upon insects larger than itself (robber flies range in size from 12 to 25 millimetres). Robber flies are considered acceptable to have around by some people as they eat pesky insects, similar to ladybugs (which eat aphids, which eat plants like roses, which people generally don't want eaten). On the other hand, some people dislike the robber fly, as it preys on beneficial bees in some areas. Unlike ladybugs, however, robber flies aren't generally regarded as cute. The robber fly is covered with short, rough hairs. Between the fly's two compound eyes there is a depression. The fly's abdomen is elongated and its legs are noteably lengthy. Some species' hair has a grey and yellow colouration, making them appear similar to bees or wasps.
As with all species in the order Diptera (flies, mosquitoes, etc.), the robber fly has two pairs of wings, only the larger of which functions in an actual wing-like manner. The second pair of wings (known as halteres), located directly behind the first, are used to stabilize the insects during flight. The robber fly also has a suctoral probiscis, as described above, which it uses to deliver its toxins to and suck the juices from its prey. If a robber fly bites a human (which will pretty much only happen if the bitten human was handling the fly at the time), the bite can be painful. Human innards won't liquefy though.
Robber flies can be observed hunting usually at the edge of a meadow, preferably one with a lot of flowers (so more prey will frequent the area). The fly will perch itself onto a plant and watch other insects fly by. The robbery fly's compound eyes have excellent vision, giving the fly the ability to determine the speed and distance of objects moving in front of it. When the fly spots something that may be an acceptable meal, it flies close to its target to investigate. Robber flies have been observed seemingly chasing other insects (they're actually inspecting their targets at this point, as they can catch their targets without the need for much pursuit) and inspecting leaves falling near their location. The robber fly doesn't attack every insect it flies out to inspect. After it does feed on an insect, the robber fly will drop the shell that remains onto the ground and return to watching the insects flying around it. Some robber flies will move from plant to plant (which, combined with its colouration being similar to that of wasps and bees, makes them look all the more like a wasp or bee) in search of prey, rather than waiting in one spot.
Robber flies mate in the early summer, shortly after becoming adults, in a ritual that involves aerial displays and abnormal buzzing sounds. Once a female robber fly has been fertilized, she will lay her eggs in a small case in the soil. In the winter, the eggs hatch and the larvae will squirm around beneath the surface, feeding on tiny soil-inhabiting insects and other insect larvae. The larvae will still be living underground when pupation occurs but once that stage is complete, the adult fly will emerge from beneath the surface in late spring or early summer. Robber flies live about a year.
Sources:
http://oror.essortment.com/robberflyinsec_rbky.htm
http://www.insects.org/entophiles/diptera/dipt_004.html
http://rbcm1.rbcm.gov.bc.ca/end_species/species/robfly.html
http://www.inhs.uiuc.edu/chf/outreach/good/card15.htm