During a recent holiday in Gran Canaria I was fortunate to encounter a robber fly (Promachus latitarsatus). Initially it was sat motionless on a rock before launching vicious attacks on passing insects. I was mesmerised by the explosive nature of its pursuit for prey, not to mention its bravery, attacking insects more than twice its size. I was glad to find out that despite their fearsome nature, they are harmless to humans!

Promachus latitarsatus sitting in wait for a passing insect

Robber flies are fascinating. Their name is a reflection of their aggressive predatory nature, ambushing and capturing arthropods as they pass. The Asilidae are a family in the order Diptera (true flies), and there are over 7,500 described species. Species within this family attack a wide range of prey species, including dragonflies, wasps and grasshoppers; they possess a dense moustache of bristles on their face (‘mystax’) which is thought to provide protection from struggling prey items.

Robber flies capture their prey using their tarsi and then inject it with saliva, this serves two purposes; the neurotoxins cause paralysis and proteolytic enzymes break up and liquefy flesh!  After a short period of time the prey is consumed by drinking the internal fluids.

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Asilidae typically frequent habitats that are open, sunny and dry occurring in all zoogeographical regions except Antarctica. There are 27 species of robber flies in Great Britain, with the Hornet Robberfly (Asilus crabroniformis) probably being the largest at around 25mm in length…keep an eye out for these fascinating flies when you are next in suitable habitat.