North America’s largest robber fly

Female Microstylum morosum perched on fragrant sumac (Rhus aromatica) bush

Female Microstylum morosum perched on fragrant sumac (Rhus aromatica)

A few days ago, I featured Promachus hinei, one of the so-called “giant robber flies” and a common inhabitant of the glades and grasslands that dot Missouri’s largely forested landscape. That individual was seen at Long Bald Glade Natural Area in Caney Mountain Conservation Area, one of the many limestone glades that are a prominent feature of extreme southwestern Missouri’s White River Hills, as it snacked on a small carpenter bee (Ceratina sp.) and posed obligingly for a series of super close-up photographs. Promachus and its congeners are impressively large; however, I would see an even larger robber fly that day. I didn’t know what it was at the time, but I knew that never before had I seen such a magnificent fly, with its large, shimmering, emerald eyes, streamlined body almost devoid of setae (hairs), and ludicrously large size. These monsters were actually quite common at the glade, so I failed to appreciate the significance of what I was seeing as I chased one after another – more intent on securing photographs than specimens. This was not an easy task – they were extremely wary, rarely allowing me to approach within 12 feet no matter how cautiously and slowly I moved. Not one to back down from such a challenge (remember, I stalk tiger beetles), I persisted, traversing the rough, rock-strewn terrain amidst clumps of big bluestem (Andropogon gerardii), Indian grass (Sorghastrum nutans), and fragrant sumac (Rhus aromatica) until, at last, I got within striking distance of the impressive female shown in these photos. Taking flight before I felt assured of a good shot, I followed her repeated long, loping escape flights until I was able to get another few shots and she disappeared for good.

Same individual as in previous photo after flying to another perch.

Same individual as in previous photo after flying to another perch.

It didn’t take long after I returned home to figure out what I had seen, as there is really nothing that can be mistaken for Microstylum morosum, North America’s largest robber fly (Back 1909)¹. At 35–40 mm of length, this individual didn’t quite match the astounding 50-mm upper body length for the species (that’s 2 inches, folks!). Nonetheless, it was an impressive beast indeed! It is not surprising that North America’s largest robber fly should be a species of Microstylum, as it is this same genus that contains the world’s largest robber fly – the aptly named M. magnum from Madagascar, with a body length of 60 mm and an almost preposterous wingspan of up to 84 mm (that’s over 3 inches folks!). I don’t know if any flies exist that are larger than this, but certainly none can be more imposing.  While I’m happy with the photos that I did obtain, I must confess some disappointment that I wasn’t able to get more than these basic lateral profile shots.  Of the several photographs of this species that can be found on the web, this female, photographed by Greg Lavaty of Houston, Texas, is (in my humble opinion) certainly the most stunning.

¹ Puzzled by the use of the prefix “micro” in the genus name – hardly seeming appropriate for such an enormous fly – I asked Eric Fisher (retired, California Department of Food and Agriculture) about the name’s derivation, to which he replied, “The name refers to the quite small ‘stylus’ of the antenna apex; Macquart specifically mentions this character in his 1838 original description of the genus. (This is not a very helpful diagnostic character, as many asilids share this feature…).”

Even more significant than its size, however, was its very occurrence on this glade. Like Ospriocerus abdominalis, which I had seen just a few weeks earlier in the Loess Hills of extreme northwestern Missouri, M. morosum is a denizen of the Great Plains, and also like that species it has until now not been known from Missouri. That’s right – another new state record!  Unlike O. abdominalis, however, the Missouri occurrence of M. morosus represents a significant northeastern extension of its known range.  The species was long considered a Texas endemic until Beckemeyer and Charlton (2000) confirmed its occurrence in southeastern Arizona and documented significant range extensions into Oklahoma, Kansas, extreme southeastern Colorado, and extreme northeastern New Mexico.  Its eastern distributional limit was thought to occur along a north-south line from Douglas County, Kansas to Mayes County, Oklahoma to Brazoria County, Texas; however, Warriner (2004) documented its occurrence some 200 miles east of this line in the blackland prairies of southwestern Arkansas.  The occurrence of M. morosum in the White River Hills of Missouri represents yet another significant eastern extension of its known range – Long Bald Glade lies 185 miles NNE of the collection site in Arkansas and 155 miles ENE of the nearest known record in Mayes County, Oklahoma (Locust Grove), making it the easternmost known locality for this species.

As in Arkansas, where the collection site represents one of the highest quality blackland prairie remants in the state, Long Bald Glade represents a high quality remnant of the limestone glades that once occurrred much more extensively within Missouri’s White River Hills.  Like the blackland prairie of Arkansas, the limestone glades of the White River Hills have been dramatically reduced since EuroAmerican settlement due to land use conversion, and fire suppression and overgrazing of the remaining tracts have resulted in significant woody encroachment – chiefly by eastern red-cedar (Juniperus virginiana) – and loss of vegetational diversity. This has caused dramatic reductions in populations of the many Great Plains plant and animal species that are found here and nowhere else in the state.  Considering the overall distribution of M. morosum, it is unlikely that it occurs more extensively within Missouri than the White River Hills, emphasizing the importance of continued conservation and restoration activities in this unique part of Missouri.  However, since the White River Hills extend into northwestern Arkansas, M. morosum may occur in that part of Arkansas as well as the southwestern part of the state.

I thank Eric Fisher and Herschel Raney for confirming the identity of this species and its status as a new record for Missouri.

Photo details: Canon 100mm macro lens on Canon EOS 50D (manual mode), ISO-100, 1/250 sec, f/10-11, MT-24EX flash 1/2 power through diffuser caps.


Back, E. A. 1909. The robberflies of America, north of Mexico, belonging to the subfamilies Leptograstrinae and Dasypogoninae. Transactions of the American Entomological Society 35:137–400.

Beckemeyer, R. J. and R. E. Carlton.  2000.  Distribution of Microstylum morosum and M. galactoides (Diptera: Asilidae): significant extensions to previously reported ranges.  Entomological News 111(2):84–96.

Warriner, M. D.  2004.  First Arkansas record of the robber fly Microstylum morosum (Diptera: Asilidae).  The Southwestern Naturalist 49(1):83–84.

Copyright © Ted C. MacRae 2009

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20 thoughts on “North America’s largest robber fly

  1. That is one huge fly! The eyes look striking, and it would be great to see a closer view of the head.

    I wish I would come across more robber flies in my wanderings, they seem almost as exciting as tiger beetles!

  2. Hey Ted,

    Great series on robber flies. You’ve impressed me with Missouri’s insect fauna and, no disrespect to you, John Acorn, or the rest of the cicindellaphiliacs, but I’d go Adrian one better and put my vote with the asilids. I appreciate their lurking around long enough to get a good view of them and their snacks – and I bet their larval habits are even more mysterious.

    Haven’t seen any robber flies as impressive here, but there is a good bumblebee mimicking Laphria at our Moose Pasture that is always worth a few minutes of contemplation.

    • I did see a bumble-bee in our garden that was ‘bee’-having in a very un-bumblebee like manner. It would fly from flower to flower without actually feeding there, and its flight was much more direct than the bumbling of the actual bee. That was probably a Laphria (he said, after Googling for images…)

      Pity, I did not manage to get a photograph.

      • Laphria is one possibility, Mallophora is another – both of these genera are very effective bumble bee mimics (though not particularly closely related to each other). Mallophora are hairier and have a thin terminal segment on the antennae (Laphria has thick antennae). I’ve seen them in some of my visits to prairie remnants in western Missouri.

        Keying in on “odd” behaviors is a good way to find new things. I found my first Cicindela lengi when I was watching what I took to be C. formosa – a very similar but much more common species – and noticed something “odd” about the way it flew. I only netted it because of that oddness, and only after I had it in the hand did I realize what I had.

    • Thanks, Dave – and no disrespect taken, as it seems impossible to me that anyone could not become enamored with robbers once they’ve seen a few. They certainly have the behavioral charisma to go along with their impressive appearances.

      I’ve been hoping to get some shots of one of the Laphria spp. – I don’t think there is a more realistic bumble bee mimic!

  3. Nice persistence in getting your photos! You’ve really got the hang of your camera system- the photos and composition are stunning. I think I saw one of these the other day, snacking on one of my honeybees!

  4. She’s very impressive! Great shots. I laughed at the visual of your chase scene, Ted. It sounds all too familiar.

    And a state record to boot. Congrats! You can’t complain about that.

    I do wonder if finding one this far out of the accepted range means only that it’s been overlooked until now (low population numbers at the edge of its territory) or that its range has dramatically changed due to environmental pressures or opportunities. Or something else entirely.

    • Thanks, Jason.

      I’m inclined to believe the species has always been here but has simply been overlooked until now. The robber flies of Missouri have not, to my knowledge, been studied extensively, and the White River Hills region of the state is rather remote. There are many other floral and faunal elements in this area that, like this species, are more typical of the southern Great Plains and that appear to be at or near their northeastern limit of distribution here. All seem restricted to remnant limestone glades, and their disjunct distributions in the area are probably a result of the interdigitation between the eastern forest and western grassland biomes that occurs along the western edge of Missouri. It seems likely that this and other species unique to this part of Missouri occurred more commonly across southwestern Missouri in the past but have become more restricted since EuroAmerican settlement and their (our) land use practices (chiefly fire suppression and overgrazing) that have promoted largescale conversion of grassland habitats into more wooded habitats.

  5. Hi Ted

    A very strange fly with a very small head! Any records of these US species catching jewel beetles like the big Phellus do over here?

    Its great to know the USA is still not well explored as yet for most insects. The scope for field observatons must be still great.

    Best regards, Dr T

    • Hi Trevor,

      Yes, the head looks very small because the body is so enormous.

      Most of our robbers will eat just about anything they can capture. There is a photograph of a robber with a buprestid on this page – I can’t tell for sure what the buprestid is, but possibilities include Acmaeodera, Acmaeoderoides, and Lepismadora.

      New state records are nothing here in Missouri – I’m still discovering new species! I’ve already described one cerambycid (Purpuricenus paraxillaris) and one buprestid (Agrilus betulanigrae) as new species from the state and am currently working on a new species of Taphrocerus. People don’t realize how little we still know about insects, even in supposedly well-collected places like the eastern U.S.

  6. Good find! I’ve often heard, but I can’t find any specific citations, that the largest fly in terms of length is Gauromydas heros, a mydid from Brazil. I’m pretty sure it is over 6 cm, it more or less looks like a giant version of the Mydas spp. you’d catch in the US. I’d think that the heaviest fly is a species of the Neotropical wood-boring Pantophthalmus, maybe P. gigas but that group needs revision; several species are really massive. Here’s a photo of a *small* one
    A giant wood-boring or timber fly (Pantophthalmus sp, Pantophthalmidae) in my hand, BCI, Panama

    • My gosh, that is a gorgeous fly irrespective of its impressive size. It doesn’t look at all related to the Asilomorphs, more like a stratiomyid?

      Not at all an attempt to one-up, but Bromophila caffra, though much more pedestrian in size, is certainly among the most striking flies I’ve ever seen.

  7. Pingback: Giant Robber Fly of the Great Plains – Microstylum morosum « I C U Nature

  8. Pingback: Bumelia borer on white « Beetles In The Bush


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