01 June 2010

Asilidae - Asilinae sp. - Robber Fly

Click image to enlarge

Robber Fly - Asilinae sp. - Asilidae

Worldwide there are thousands of Robber flies in the Asilidae family and in Australia, the Australian Faunal Directory lists 360 species with other authoritative sources quoting more than twice that number. So whatever the figure, there are a lot of them. Australian Asilidae are divided into 4 subfamilies with Asilinae being the largest. As a general rule, these subfamilies can be determined by the structure of their antennae with Asilinae having a fine terminal projection (antennal style), others are more feather-like, or uniformly thick, or have no projection at all.

Around Esperance I have not noticed many species, which is either just me, or that most of these robber flies prefer forested areas, although the one shown above is reasonably common. Asilidae are amongst the largest flies, with some possessing a 7.5 cm (3”) wingspan. The one above although impressively large, had only a 4.5 cm (a little under 2”) wingspan and a head/body length of nearly 3 cm or a little over 1”. However to most insects it would be terrifyingly large, as being predatory they will attack almost any flying insect, some larger than themselves.

Robber flies will perch and wait for an insect to fly past; it will capture it in flight grasping it in its strong legs. The exterior of the prey will be pierced with its sharp proboscis and injected with powerful enzymes to reduce the insides to a mush, then sucked out by the fly, leaving little more than an exoskeleton. Locally the above robber flies were most active from December to February.