Celebrating Robber flies – big, beautiful venomous assassins | Curator of Diptera

Deadly predators. Venomous machines. Stealth assassins. Yes, it’s the robber flies, or assassin flies if you’re from across the pond. These beautiful, lethal creatures are, to my mind, some of the most amazing species on the planet, with not only some amazing adaptations to their predatory lifestyle but also exhibiting a great morphological variety – including some of the most hirsute insects on the planet.

Robber flies are in the family Asilidae, which translates literally as gad fly. This initially struck me as odd, as it is the horse flies that have the common name of gad flies, but further investigation reveals the meaning comes from the Middle English word gadling, meaning wanderer or vagabond. Nowadays the meaning is closer to flitting around in search of pleasure. I wonder what Linnaeus had in mind when he named them so – was it their amazing aerial abilities he homed in on?

Asilidae were described by Linnaeus in his magnum opus Systema Naturea… in 1758 (although the family name came along much later in 1812) with the type species for the genus Asilus being Asilus crabroniformis – one of the UK’s most charismatic animals.

Fig 1: Linnaeus’ original Latin description of the ‘horny beak bone’

Linnaeus’ original Latin description describes them as Os Rostro corneo literal translation ‘horny beak bone’ – with reference to their magnificent proboscis, a prominent feature of this group. These specimens, from which Linnaeus made his – rather short – descriptions, are now kept safe in the vaults of the Linnaean Society (though I have had the pleasure of seeing them!)

Yet within the collections of the NHM, there are even older specimens of Asilus crabroniformis. These specimens may be flatter than usual, lying as they do within the pages of a book, but they have preserved their colour marvellously, despite being caught as long ago as 1680, in the gardens of Hampton Court Palace. These may be the earliest examples of this species, and the data we have on them is all thanks to the royal gardener Leonard Plukenet.

Fig 2: A rather squashed fly, flattened by time

From the original 11 species Linnaeus initially described, there are now more than 7,500 species. This is impressive, when you consider that this is much larger than the number of described species of mammal, at around 5,400. They are one of the most species-rich dipteran families, though this may be due to a sampling bias because of their obvious appeal to Dipterists – we are not immune to being taken in by these large and charismatic creatures. This is reflected in the diversity of species and the number of specimens of the Natural History Collections. Held within 339 drawers, there are nearly 3,000 species of these beasties, and 2,300 type specimens.

Fig 3: The giant yellow robber fly Blepharotes coriarius on my hand

#WorldRobberflyday is a day in celebration of these flies, aiming to highlight their rich diversity – with over 7,500 they are one of the larger families of flies. And it’s not just the species richness that is large – they themselves can reach quite impressive sizes. Several species of the Eastern Australian genus Blephorates are notably large, with giant yellow robber fly Blepharotes coriarius, measuring over 4.5cm in length (Fig. 3) and the wonderfully named B. splendidissimus attaining wingspans of over 4cm (Fig 4).

Fig 4: The rather large and beautifully named Blepharotes splendidissimus

They have some amazing life histories, from their predatory feeding strategies, of which we have many examples of predator and prey in the collection (Fig 5), to some amazing courtship displays (Fig 6).

Fig 5: Fighting flies. A Mallophora ruficauda immortalized forever as the victor, albeit short lived in this case!
Fig 6: Loving flies. Promachus latitarsatus male hovering above the feeding female, with his legs stretched apart in an attempt to woo her. (Grimm-Gellar & Geisthardt 1996)

Since this day was launched three years ago, our knowledge of these species has grown in terms of taxonomic descriptions – both morphological and molecular; in terms of phylogenetic resolutions (read the work done by Rebecca Dikow et al; their ecology; their behaviour, including their courtship displays; and now we are beginning to understand their physiology, including their venoms.

Only this January was a paper published on the venoms of Asilidae, wherein they told us that the venoms were not like the other venomous diptera with 6 toxin compounds, which they named Asilidin, matching no other venom proteins. Not only was the paper detailing the venom composition but there was some smart imagery using micro-computered tomography of the internal structure of a robber fly’s head (Fig 7).

Fig 7: A 3D reconstruction of the gland and head morphology in E. rufibarbis. (From Drukewitz et al 2018)

However, we still have huge gaps in our knowledge of these predators, including establishing where most of the larvae stages develop and what they feed on.

This day is not purely a for showing off pretty pictures (although I do love the images), but one that will hopefully highlight species that are not only an important part of the ecosystem, but ones that can help us understand other issues such as the intricacies of vision and flight, both very useful subjects to study to help in the field of aeronautical engineering for example.

Fig 8: A tweet from last years #worldrobberflyday. A beautiful fly with a bad reputation.

Robber flies, in conclusion are some of the most interesting creatures on the planet and for those aesthetically minded, some of the most attractive!


One Reply to “Celebrating Robber flies – big, beautiful venomous assassins | Curator of Diptera”

  1. On the issue of the name – Linnaeus was using a name that Virgil had used – but Virgil describes Asilus as being the same fly as the Greeks called Oestrus – flies that in swarms pursued terrified cattle through the woods

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