Robber flies, known scientifically as asilidae, are bristly flies with a short, stout proboscis enclosing a sharp, sucking hypopharynx, and spiny legs. These traits, and their powerful build, give them a decided advantage in clashes with other insects their own size and larger. Accordingly, they are notoriously aggressive predators which feed mainly or exclusively on other insects. They often wait in ambush and catch their prey in flight.
The Asilidae are a family in the order Diptera, the true flies. They sport three simple eyes (ocelli) in a characteristic depression on the top of their head between their two large compound eyes. Thus, these creatures have exceptional vision, and that keen sight plays an essential role in the detection of prey and their capture.
The head is free and mobile and dichoptic in both genders. The occipital region has one or more rows of bristles aligned behind the posterior margin of the eye, and the facial region has a convex profile with a characteristic dense bundle of bristles, called the mystax, a term derived from the Greek mystakos meaning “mustache” or “upper lip.” The mystax affords some protection for the head and face when the fly deals with struggling prey.
The antennae are typically composed of five segments but sometimes as few as three, depending on the structure of the stylus.
The mouthparts are short and modified for piercing and sucking. They consist of a strongly sclerotized proboscis which includes the labium and maxillae which form a food canal,  and a piercing organ, the hypopharynx. The proboscis is rounded in cross section or laterally compressed and is usually stout, and straight and sometimes able to penetrate through the hardened exteriors of other insects they must deal with.
The thorax is robust and compact. It bears long bristles. The abdomen consists of 6-8 visible segments preceding the genitalia in males, but the eighth segment is sometimes entirely or partially concealed, and terminal forming the ovipositor.
The legs are relatively long and strong, with many spikes performing a raptatorial function. The wings are well developed, often relatively narrow for speedy flight
Asilidae prey on all order of other insects, including other flies, beetles, butterflies and moths, various damselflies, and some spiders. They do so apparently irrespective of any repugnatorial chemicals their prey may have at its disposal and exhibit little reluctance to take on insects which feature the ability to fight back or have impressive weapons of their own, such as powerful grasshoppers, dragonflies, stinging Hymenoptera (including bees, ants and ichneumon wasps) and even other Asilidae.
They station themselves at strategic points to ambush prey. The fly attacks its prey, catching it with the tarsi and thereupon stabbing it with its short, strong proboscis in preferential points of least resistance as the eyes, the membranous area of transition between the head and thorax (neck) or between the thorax and abdomen, or between the last urotergiti. In doing this, it injects the victim with saliva containing neurotoxic and proteolytic enzymes which very rapidly paralyze the victim and soon digest the insides, leading to the breakup and liquefaction of internal tissues. The fly then sucks the liquefied material through the proboscis and into the alimentary canal.
When attacked, robber flies defend themselves with their proboscides and may deliver intensely painful bites if handled incautiously.
Adults are generally medium to large in size, with an average body width of 1 to 1.5 cm but with a range of 3 cm to more than 5 cm in length. The shape is generally elongated, due to the conformation of the long tapering abdomen. However there are also compact species with broad abdomens. The integument is covered with thick hair, especially on the head and thorax and liveries are often showy, with colors ranging from brown to black to grey, sometimes in contrast with other colors such as red and yellow. Frequently they are aposematic, that is, they have warning coloration, an antipredator adaptation in which a warning signal, in this case its coloration that mimics the Hymenoptera, which possess stingers. This signals to would-be predators the likely unprofitability of seeking to attack a robber fly.
The egg is hyaline or pigmented and of variable shape from spherical to oval and up to 2 mm in length. The surface is smooth or bears microsculptures which are generally polygonal and visible only through an electron microscope.
The larva is apodous, cylindrical and elongated, more or less flattened and tapered at the cephalic and caudal ends. The color is white or yellowish. The head is small, rugged, dark pigmented and hypognathous, the abdomen is composed of 8 apparent urites, with the last two often fused and more or less reduced. The respiratory system is amphineustic, with two pairs of spiracles, one thoracic and one abdominal. There are also rudimentary and nonfunctional stigmas in other abdominal segments.
The pupa is naked, as in the majority of Orthorrhapha, exarate and therefore able to move.
The Asilidae are predators, both in the juvenile stage and the adult stage. They are aggressive to the point of earning the common name, in English, of “robber flies.” They are sometimes called “assassin flies.”
The life cycle takes place in 1 to 3 years. The postembryonic development consists of four larval stages (instars) and one pupa. The larvae of most known asilids live in the soil or in the case of some taxonomic groups, in rotting organic material, usually wood and the bark of dead trees. During postembryonic development, the larvae of the first instar does not feed on insects, those of the second instar feed on secretions by larvae of beetles, while the larvae of the third and fourth instars actually behave like predators.
In general, the activity of predation of adults is concentrated in the hottest hours in open, sunny spaces, while at night they take refuge in dense vegetation. The Asilidae are excellent flyers and, in most of the family, capture prey in flight.
Egg-laying takes place, according to the species, with three different behaviors which relate to the structure and the morphology of the abdomen. Females with an undifferentiated ovipositor release eggs randomly and independently from the substrate. In other cases, however, the abdomen bears a differentiated, specialized ovipositor to lay eggs in the soil or sand, or lay them in cavities within plant tissues.
Asilidae for the most part occur in habitats that are open, sunny, and dry, even arid. They favour open or scattered vegetation, and some species even frequent bare ground.     From Wikipedia.