World’s smallest fly found, and oy, is it small!

I don’t consider myself a “geek” (indeed, I hate that word, since it’s basically anti-science), but I have to admit that the publication of this paper, which I learned about from Brian Brown—one of its authors who posted about it on his website, flyobsession—got my juices flowing a bit faster.  In a paper in the Annals of the Entomological Society of America (reference below), Brown describes the world’s smallest fly, a phorid.

Phroids are weird flies (I’ve posted about them twice, here and here): often parasitic, tiny, and wingless, and assuming bizarre forms. The picture below, taken from Brown’s paper, shows some other phorids, and check out my two posts in the last sentence. Remember, these are adult flies!:

Fig 1 from the paper: Variety of body forms in adult female Phoridae. Clockwise from upper left: Thaumatoxena sp., a termitoxeniinae,
and Vestigipoda sp.

One female specimen of the new species, named Euryplatea nanaknihali, was found in Thailand. This is what it looks like (like many phorids, its wings are tiny); the photo is from Brown’s website:

Okay, so how big is it? Brown’s paper says (my emphasis):

Of all the contenders for the title of world’s smallest species of fly, it seems that a species of Phoridae described here is the champion. At a body length of 0.40 mm, it is smaller than all of the tiny gnats, mosquitoes, and other flies so far described.

Four-tenths of a millimeter is 0.016 inch. In other words, you’d have to line up 63 of these little guys to make an inch.  Here’s a line that is about 1 mm long, so the fly is less than half this size:

Here’s how big it is compared to a housefly (Musca domestica); the figure is again from Brown’s paper:

Although only one specimen was found, Brown hypothesizes, almost certainly correctly, that this is parasitic on small ants.  Brown’s website notes:

The smallest fly in the world is a member of the family Phoridae, and is one of the “ant decapitating flies”. Adult females lay an egg in the body of an ant, and the resulting larva feeds in the ants head, eventually causing the decapitation of its host. Some of these flies are being used to attempt biological control on imported fire ants, and were even featured on an episode of the popular television show “King of the Hill”.

Because these flies usually develop in the head of their host ant, they are smaller than their hosts. One would think that the smallest ants would be therefore immune to these nasty parasites, as their heads are vanishingly small. But the world’s smallest fly is one of these ant killers, and at the astoundingly small body length of 0.4 mm, these flies can probably decapitate ants with heads as small as 0.5 mm. That is pretty close to the smallest size that ants can get!

So is this the world’s smallest insect? Nope. According to Brown’s paper, “the smallest known insects are reputedly mymarids that are only ≈0.14 mm in length, also making them the smallest animals in the world.” That’s a third the size of this fly, and 0.0055 inches long (you’d have to line up 182 of them to encompass an inch). Mymarids, which are minute wasps, are so small because they parasitize the eggs of other insects. And within these flies, and those wasps, are tiny, tiny brains that have enough information to control their complex behaviors.

Ain’t nature wonderful?


Brown, B.V. 2012. Small size no protection for acrobat ants: world’s smallest fly Is a parasitic pphorid (Diptera: Phoridae). Ann. Entomol. Soc. Am. 105(4): 550-554.

45 thoughts on “World’s smallest fly found, and oy, is it small!

  1. One point of clarification, Jerry: only one specimen *was described in the paper* but Brown told me that he has since found several more.

    1. I was reading about this in Ed’s bit yesterday. The Thysanoptera that the Mymarids feast on are pretty tiny themselves. All the pictures in my parents’ house always had lots of Thysanoptera that crawled through tiny cracks behind the glass only to expire. There are sooooo many parasites – I guess every beetle has one which would answer the argument as to which insect type is most numerous…

  2. As always interesting post, but I really take objection to your geek is “basically anti-science” comment – I don’t think it is a generalization that can be applied to all geeks, some are, but all? Yes, my “geek” friends and I went to see Prometheus, but we spent the next 30 minutes criticizing it’s horribly inaccurate depiction of science and scientists.

    1. Not speaking for Jerry, but I know a number of older, um, nerds (for lack of a better word) who hate the word “geek” because of its origins as a name that was used to pick on kids who were smarter than average or more interested in science than average. (The word was originally used to describe side-show freaks at a carnival, after all).

      The word was later “reclaimed” (actually ‘claimed’ to be technical about it) by the folks who were bullied by it, and calling yourself a geek these days is a way of actually putting yourself INTO a particular group rather than being used to bully people OUT of a social group. But not everyone thinks its shaken off its negative connotations.

      1. Fair enough, I can see that geek was a serious pejorative. However, in my world it has been replaced by the more positive ‘expert’ or ‘enthousiast’ meaning… alas, I hang my head in shame for using the wrong *its* in my first post

      1. The point being what Jer points out about the word being embraced by some as a badge of hono(u)r.

  3. I wouldn’t worry about the word geek, it’s been owned (like the word gay) . Use it proudly

  4. “the smallest known insects are reputedly mymarids that are only ≈0.14 mm in length, also making them the smallest animals in the world.”

    – NOPE, afraid not ! The smallest rotifers and tardigrades are down to 0.05 mm long ! Both are animals.

    1. I also note a contradiction on the fairyfly wikipedia article

      “Like most chalcid wasps, fairyflies are very tiny insects, averaging at only 0.5 to 1.0 mm (0.020 to 0.039 in) long. They include the world’s smallest known insect, with a body length of only 0.139 mm”

      – So if the smallest is 0.139 mm long, how can they describe the various species as averaging 0.5 – 1 mm long ?!
      – As it happens, I have previously looked into this and the smallest is indeed in agreeance with the paper and the smallest @ 0.139 mm. NOT “0.5 to 1.0 mm” as stated on wikipedia.
      – So the smallest rotifers and tardigrades @ 0.05 mm long are, to my knowledge the smallest animals.

      1. Oops ! I just reread my second comment and noticed the silly reading error I made, ignore it altogether !

    2. DJ,

      I rushed to the comments to make the same point you did, but of course I am always late to these parties. Curse you, time zones!

  5. As a layperson, I have to say that I love these biology posts. I had never even really considered the idea that insects might have other insects that were parasites living in them. Predator-prey relationships sure, but not parasitic ones. It make sense, but I’d never even given it much thought.

    1. Doesn’t everybody know this bit of Swift?

      So, naturalists observe, a flea
      Has smaller fleas that on him prey;
      And these have smaller still to bite ’em;
      And so proceed ad infinitum.

      Of course he was writing as a satirist and critic (it’s called “On Poetry: A Rhapsody”, 1733) rather than an entomologist, but it’s pretty accurate (have we found the smallest bacteria and viruses yet?).

    1. Many times I have found tiny wasps outdoors, but it is impossible to identify them without killing them & getting them under a microscope, & I wouldn’t want to kill them just to satisfy my curiosity. Besides which, even the quite good insect identifier for the British Isles fails me frequently & never gives enough information or further sources.

      1. Determinating these parasitoid wasps (especially the tiny ones ) is allways hard specialists work

        Maybe these two are good starting websites =
        (I have a copy of the last one which is hard to connect with … click on the attachment at the
        bottom of the evodisku article )
        attachment = Ichneumonidae_subfamily_key[1].pdf

  6. There is no way of proving that God did no specially create iny brain-desroying phorids as a judgement against tiny acrobat ants!

  7. All creatures great and small; the Lord God made them all.

    That’s some pretty sick God to design a decapitating parasite.

    1. We already know he was sick. It’s just the extent of His psycopathic sadism that never ceases to appal me.

  8. Physics works rather differently at these scales, including things like fluid dynamics. I wonder how different, say, the vascular system is in these thing compared with, say, a sphinx moth or a large dragonfly or a bird-eating spider?

    What’s the physical limit to how small the arthropod body plan can be shrunk?


  9. Phroids are weird flies (I’ve posted about them twice, here and here): . . .”

    Hmmm, is that what they call a Phroidian slip? If so this is one for the textbooks!

  10. I vaguely remember inches. Weren’t there 21 to the Guinea and 144 to the gross? Or am I an order of magnitude out, 12 to the shilling and 4 to the bushell? Radical idea, lets go decimal…

    1. Except for air temperature, for which I’ll yield to Dan Fahrenheit. 0 is really cold, 100 is really hot. 180 degrees between water solid and water vapor. What’s not to like? Not saying it should be in any way official, mind you.

      1. I agree. Fahrenheit is definitely a better scale for everyday use as it nicely places the range of typical human experience on a scale from 0 to 100. In Celsius that’s -18 to 38. Arg. 😛

        And Celsius is hardly any better for science, as far as I can tell. Does anybody really use to their advantage the fact that in the Celsius scale one particular substance (water) has a range from freezing to boiling of 100 degrees at standard atmospheric pressure? Seems pretty dubious. I realize that a several other units have been based on Celsius (e.g. the calorie) so it’s too late to change, but still.

        Kelvin, on the other hand, at least has the mathematically attractive property of having its zero at absolute zero. 🙂

          1. Kelvin and Celsius degrees are the same, but they aren’t the same scale.

            And if you want absolute zero = zero, there’s also Rankine.

            1. That’s precisely what I said: they are the same scale with different origins. In particular, a difference of 1 C = a difference of 1 K. Rankine, similarly, is the same scale as Fahrenheit with the origin shifted, but you probably don’t want to be caught using it :).

              1. My understanding of the term “temperature scale”, wikipedia’s use of the term, and I suspect the general use of the term, is that it does not refer to just the temperature difference that a degree represents, but includes the offset.

              2. Well, it is abundantly clear what each one of us is trying to say, and I don’t understand your objection. The original poster to whom I replied made the point that we couldn’t change from celsius to kelvin because some energy unit (calorie) was defined in terms of the amount of energy needed to increase the temperature of water (under certain conditions) by one Celsius. I was just making the point that in this context, it does not matter whether we use Celsius or Kelvin, since, as I will assert again, they are the same scale with different origins.
                Your point about the terms Celsius and Kelvin themselves including the offset seems to me, therefore, to be completely out of context in relation to the above point.

  11. Nature is indeed wonderful and amazing.
    Couldn’t help but notice (perhaps someone has?) that the image of the Thaumatoxena reveals a interesting face (a squinting, smiley face with fuzzy nostrils). At first I thought, “how nice, a very small smiling fly!”, but then realized I was viewing its hind end. I just hate it when facial recognition gets in the way of more pertinent data.

  12. When I read that 0.14mm figure, it reminded me that I had read somewhere that the largest bacteria can be ~0.5mm. And that’s actually true: Thiomargarita namibiensis is known to have a mean size of about 0.3mm with specimens of length about 0.7mm having been observed.

    I am not a biologist, but still find it embarrassing that for such a long time, I held the completely wrong implicit belief that bacteria are all smaller than insects. 😀

  13. “And within these flies, and those wasps, are tiny, tiny brains that have enough information to control their complex behaviors.” This last sentence really brings home for me how amazing this is.

  14. You can’t really do “a millimeter is this long” sort of things on the Web. There’s nothing in HTML that allows for fixed length.

    1. Actually, the CSS standard does provide for absolute lengths. I am not sure if WordPress allows CSS styles though (they probably do).

      However, I would expect compliance with this part of the standard to be rather poor. pt though is a really common unit for specifying font sizes (both in LaTeX and HTML/CSS), so that might be well supported.

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