Lord of the flies!

I happened upon a rather interesting scene last week in a soybean field in northern Argentina (Chaco Province). This assassin bug (family Reduviidae) had captured and was feeding on an adult stink bug of the species Piezodorus guildinii—an important pest of soybean in Argentina and Brazil (where it is known by the common names “chinche de la alfalfa” and “chinche verde pequeño”, respectively). Assassin bug predation is always interesting enough itself, but what made this scene especially fascinating was the large congregation of flies surrounding and even crawling upon the predator and its prey. I had not witnessed something like this before, but it seemed clear to me that the flies were engaging in kleptoparasitism—i.e, stealing food. I’ve gotten into the habit of keeping a full set of extension tubes mounted on the camera with my 100mm macro lens—this not only provides the most useful (for me) range of magnification but also serves as a convenient and easy-to-use field microscope. Through the viewfinder I could see that there were at least two markedly different types of flies involved—more abundant, small, brown flies that I presumed (incorrectly, as it turns out) to be some type of drosophilid (vinegar fly), and a few larger, black flies that were completely unfamiliar to me. The flies were apparently feeding on fluids from the stink bug prey but also crawled all over the assassin bug as it fed. The assassin bug seem unencumbered in its feeding by the presence of the flies, but periodically it would slowly wipe its forelegs over its head to dislodge flies that had settled onto it. Just as quickly as they flew away, however, they crawled back.

The assassin bug, on the other hand, I recognized as very likely a species of Apiomerus—a large, exclusively New World genus known in North America as “bee killers” for their habit of sitting on flowers and ambushing visiting bees for prey. The prey selection behaviors of these insects, however, are more generalist than the name implies, as can be seen by these photographs. To verify my generic ID and possibly obtain a species ID, I sent some of these photos to Dimitri Forero at the Heteropteran Systematics Lab at University of California-Riverside. Dimitri is revising portions of Apiomerus (e.g., Berniker et al. 2011) and working on a general phylogenetic hypotheses for the genus. In the past he has been quite helpful in fielding questions from me about these bugs, and within a few hours Dimitri replied to inform me that the assassin bug was, indeed, a member of the genus Apiomerus, likely representing the common, widespread species A. lanipes (ranging from Panama to Argentina), based on its coloration, locality, and relative size. Update 12 March, 3:07 pm—After seeing the last photo in this post (which I did not send to him initially), Dimitri wrote to say the ventral abdominal pattern was not characteristic of A. lanipes. He asked about its size, to which I replied that it was about the same length but maybe a little less robust than A. crassipes (eastern North America). He later added, “I now think that this is A. flavipennis Herrich-Schaeffer, 1848. It is very similar to A. lanipes, but a lot smaller (lanipes is really robust), and with the abdomen with black and white patches, whereas in lanipes the abdomen is always black. I checked some series of specimens that I have here and, I am pretty sure now of the ID. I have material from Argentina as well. In some specimens that coloration of the corium varies, but the original description says it is yellow with a “hairy” pronotum, which fits very nicely your photos.” Apiomerus flavipennis is known from Argentina and Southern Brazil only.

Quite unexpectedly, Dimitri also noted that at least some of the flies could belong to the family Milichiidae. He first became aware of these flies after seeing a photograph of Apiomerus showing something similar and suggested Milichiidae online as a possible source for more information. This remarkably informative  website by milichiid expert Irina Blake, who dubs species in the family as “freeloader flies”, is a model for how websites dealing with obscure insect taxa should be organized and populated (and features on the home page a great photo of ant-mugging flies taken by our favorite myrmecophile). At any rate, I forwarded my photos to Irina and within minutes received her response that the bigger black flies most probably represent the cosmopolitan Milichiella lacteipennis and the smaller flies a species of the family Chloropidae (of “dog pecker gnat” fame) in the subfamily Oscinellinae, noting that she has seen similar (or the same?) chloropids in other photos as well engaging in kleptoparasitism.

Not long after receiving the first reply from Dimitri, I got another message from him with a link to a very interesting paper by Eisner and colleagues (1991), who recorded freeloader flies in Florida preferentially attracted to stink bugs and leaf-footed bugs (family Coreidae) being preyed upon by the orb-weaving spider Nephila clavipes. Olfactory stimuli were already suspected to be involved in attraction of milichiids and also chloropids (Sivinski 1985); however, Eisner et al. (1991) experimentally demonstrated that milichiid attraction was tied to specific components of defensive sprays in several pentatomid and coreid species (including P. guildenii, the prey species in this series of photographs). The defensive sprays of the bugs were generally ineffective at preventing predation by the spiders (and apparently this is the case for A. lanipes and other reduviids as well), thus serving as a signal to milichiids and chloropids not only of the presence of a food source but perhaps also assisting search for mates in a density dependent fashion (Sivinsky 1985). Milichiid attraction to hymenopteran prey, richly endowed with integumental glands themselves, has also been documented; the Eisner study raises the question whether these types of prey are also detected from chemical cues.


Berniker, L., S. Szerlip, D. Forero and C. Weirauch. 2011. Revision of the crassipes and pictipes species groups of Apiomerus Hahn (Hemiptera: Reduviidae: Harpactorinae). Zootaxa 2949:1–113.

Eisner, T., M. Eisner & M. Deyrup. 1991. Chemical attraction of kleptoparasitic flies to heteropteran insects caught by orb-weaving spiders. Proceedings of the National Academy of Sciences of the United States of America 88:8194–8197.

Sivinski, J. 1985. Mating by kleptoparasitic flies (Diptera: Chloropidae) on a spider host. Florida Entomologist 68(1):216–222.

Copyright © Ted C. MacRae 2012

22 thoughts on “Lord of the flies!

  1. Wow, I see flies like these in my photos occasionally, but I’ve never seen so many at once.

    I often find them associated with spider prey for whatever reason, and not just true bugs. Here’s an example.

    One thing I’ve noticed recently though is that these flies like to hang out on the side-view mirror of my car. On just about any warm day I’ll notice them crawling around whenever I’m stopped. My theory is that insects are colliding with the side-view mirror while the car is under way, and whatever chemicals the flies are attracted to accumulate there. I was confused why I would see them crawling around on the mirror surface though (opposite where collisions occur). After watching them for awhile it seems they’re just flying all around the front and back (probably confused as to where the corpse is), and I just notice them when they land on my side.

    Since you’re doing so much driving around in Argentina, check and see if you find them on your side-view mirror when you’re stopped.

    • That’s a great observation. It seems that any situation that facilitates locating an available food source might be exploitable by these flies – hanging around spider webs, sniffing the air for stink/leaf-footed bug defensive spray, congregating around dog… well, you get the idea.

      I love your photo of the fly on the approach!

  2. “periodically it would slowly wipe its forelegs over its head to dislodge flies that had settled onto it ”

    So it sounds like even other insects are plagued by pesky flies buzzing around their heads! Well, at least we can take comfort from fact that the ones that bother humans aren’t nearly the size of our heads themselves.

    • This post is a perfect example of why I love blogging! I see something in the field I’ve never seen before, stop to take photographs, send them to experts around the world who so graciously offer their opinion and perhaps some helpful literature, and before I know it I’ve learned a really good lesson in some aspect of natural history. Ten years ago I would have said, “Hmm, that’s interesting.” and moved on – no smarter than I was before I saw it.

  3. The chloropid flies look like the genus Olcella (as far as I can make out from the photos). It’s a New World genus with high diversity in South America (including Argentina; the type species is from Mendoza). In the Nearctic, they’re becoming more frequently documented as kleptoparasites (thanks mostly to keen-eyed photographers). Several of the species have a long geniculate proboscis that probably makes the dine’n’dash easier. They are also attracted to damaged insects, not just prey, and they’ll sometimes converge on a sweep net if things like stink bugs have been rattling around inside it. There are a few chloropid genera that can be collected that way without even swinging the net, probably because of the chemical attraction to damaged insects inside.

    • A possible genus ID – thanks, Terry!

      And how is it that I’m just now finding out about the Lyman Museum Blog? I’ve shortened my blogroll recently, narrowing its scope only to my favorite insect blogs – you’re now on it.

  4. Matthias Buck suggests the chloropids belong to the genus Olcella.

    I see that Steve Marshall has a paper on a kleptoparasitic Olcella in Canada:


    and there is a paper on chemical attractants for Olcella parva:


    I think I’m going to be crushing a few stinkbugs this summer, just to see what happens.

    • Thanks – I always appreciate such useful references. I wouldn’t be surprised if chemical-mediated attraction is rampant across diverse families – the tough part is figuring out what the components actually are.

      I’m going to the field tomorrow – think I’ll crush me a stink bug or two! 🙂

  5. “The stink bug seem unencumbered in its feeding by the presence of the flies…” shouldn’t “stink” be “assassin”?

  6. Hi Ted,
    I am South African and here some of us call such flies (Milichiidae in particular) “Jackal flies”. Personally I am not at all fond of common names, but this particular subset of kleptoparasitism is so characteristic and common that it deserves some such name. It is very common with crab spiders, so common that in some species (such as in the genus Thomisus) the attractant actually seems to play a role in sexual attraction as well, as the tiny male tends to mount a female where she cannot get at him, though she tries to brush him off, but he is likely to sit there till she makes a kill and has her mouth full. Then he does his thing in safety, though he might fight off another male or two. If flies (Most often Milichiidae, but also Chloropidae and VERY often Ceratopogonidae, some of which actually specialise in insect blood) show up, then he might help himself to one on his way out. I have no idea how likely he is to mate again in future; he doesn’t sacrifice his pedipalp in the process, so it seems possible.
    Anyway, this happens mainly with prey that gives off a strident smell when killed and that take a long time to eat, such as a Hemipteran or bee caught by a Thomisid, Asilid, Reduviid, or sometimes a mantid. Such scenes are full of interest and well worth watching for, all ecology, physiology, sex and violence; lion kills pale into boredom in comparison. For specialist jackal species you often see females eating and males waiting to mate. Females often fill up till their abdominal sclerites stand out separately on clear skin, then they sit and suspend globes of clear prey blood to concentrate it, possible so that they can go back for seconds, or maybe to fly more easily.
    But there is no short-cut; sometimes you can (must?) sit for hours shooting and speculating, and of course, what I see in South Africa need not correspond to what you see in the Americas!
    All the best and thanks for publishing.

  7. Pingback: Kleptoparasitic flies - Colin Purrington


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