The above photograph is of a robber fly feeding on a wasp.
Robber flies are large hunting flies usually around 20 to 25mm long and some flies can be up to 75mm long. Robber flies are active predators and are known to catch other insects whilst in flight before settling down to devour their prey.
When I read the detailed description of how a robber fly feeds, by injecting its prey with saliva containing neurotoxic and proteolytic enzymes which very rapidly paralyze the victim and liquefy the insides of the prey which the robber fly then sucks up through its proboscis, it made me feel sorry for the fate of its prey in the above photograph.
The Asilidae are the robber fly family, also called assassin flies. They are powerfully built, bristly flies with a short, stout proboscis enclosing the sharp, sucking hypopharynx. The name "robber flies" reflects their notoriously aggressive predatory habits; they feed mainly or exclusively on other insects and as a rule they wait in ambush and catch their prey in flight.
Robber flies generally have stout, spiny legs and they have three simple eyes (ocelli) in a characteristic depression between their two large compound eyes. They also have a usually dense moustache of stiff bristles on the face; this is called the mystax, a term derived from the Greek mystakos meaning "moustache" or "upper lip". The mystax has been suggested to afford some protection for the head and face when the flies deal with struggling prey; various Asilidae prey on formidable species including stinging Hymenoptera, powerful grasshoppers, dragonflies and even other Asilidae, in fact practically anything of a suitable size.
In general the family attacks a very wide range of prey, including other flies, beetles, butterflies and moths, various bees, ants, dragon and damselflies, ichneumon wasps, grasshoppers, and some spiders. They do so apparently irrespective of any repugnatorial chemicals the prey may have at its disposal. Many Asilidae when attacked in turn do not hesitate to defend themselves with their proboscides and may deliver intensely painful bites if handled incautiously.
The fly attacks its prey by stabbing it with its short, strong proboscis injecting the victim with saliva containing neurotoxic and proteolytic enzymes which very rapidly paralyze the victim and soon digest the insides; the fly then sucks the liquefied material through the proboscis.
Their life histories are poorly known. Larvae generally seem to live in soil, rotting wood, leaf mould and similar materials, some being predatory and others detrivorous.