The gathering of the flies

by Matthew Cobb

Here’s a lovely tweet by Japanese macrophotographer @muakbno. Look at all those flies! What are they and what are they doing?

@muakbno posted these pics with some explanation:

Twitter translation: “Chloropidae and flock to the leaves of a dark Companion. Touching leaves and flying in one formation can be distorted so with bated breath, calm approach. No matter how many similar upward along.”

Google translation: “A fellow of the flying fly that gathers one after another on the leaves of Misaki of dusk. Touching the leaves, if one flies, the formation will be disturbed and quietly quiet as you kill yourself. Regardless of the number, they are arranged in almost the same upward direction.”

Any Japanese readers want to improve on the translations? These are Chloropidae, tiny flies that are quite common in Japan, where there are 3 subfamilies, 53 genera and 143 species. More here.

But what are they actually doing? Why are they all grouped together? How could we work it out?


  1. Posted October 28, 2017 at 12:14 pm | Permalink

    I have never seen this before. My speculation is that they aggregate at dusk on a leaf, and spend the night there. Since they are in a large group any disturbance (like a predator) will alert other members of the group who can then fly away. Sort of like bats roosting together.

    • Frank
      Posted October 28, 2017 at 12:48 pm | Permalink

      It would be interesting to see if the sex ratio in the aggregation deviates from 1:1. If it is a strongly male-biased aggregation, some obvious hypotheses about the mating system would arise. If there is a 1:1 sex ratio, then, yes, it might conform to Hamilton’s “selfish herd” hypothesis or some other predator avoidance mechanism.

    • W.Benson
      Posted October 28, 2017 at 2:31 pm | Permalink

      I think Mark has it. These flies are called eye gnats. Predators likely include things like spiders and ants. Flying off when neighbors are disturbed could be adaptive. As expected, each fly seems to snuggle up to the one next to it. If predators can only catch one fly per group, an individual fly may be better off (fitter) joining an already big group rather than a smaller one or resting by itself.

  2. DrBrydon
    Posted October 28, 2017 at 12:16 pm | Permalink

    Well, obviously, it’s the council of the flies deciding who will bell the spider.

  3. GBJames
    Posted October 28, 2017 at 12:49 pm | Permalink

    “quiet as you kill yourself”.

    Yikes, Google!

  4. Derek Feyberg
    Posted October 28, 2017 at 1:59 pm | Permalink

    I’ll try a translation:
    A group of “kimoguribae” (chloropid flies, frit flies, grass flies) that have gathered one after another on the leaves of a “masaki” (euonymus japonicus) at dusk. Since if you touch the leaves and one flies away, the formation will be disturbed, approach them quietly with bated breath. They line up in almost the same direction regardless of their number.

    • Derek Freyberg
      Posted October 28, 2017 at 2:00 pm | Permalink

      Sorry, the above was me, with sloppy typing of my name.

  5. Matthew North
    Posted October 28, 2017 at 6:27 pm | Permalink

    It’s understandable that they all congregate together on the the leaf,(for mutual protection), but to me, what’s more puzzling is why do they all orientate in the same direction?

    • infiniteimprobabilit
      Posted October 28, 2017 at 7:43 pm | Permalink

      (a) It’s the most comfortable position to stand in (head upwards?)
      and (b) it leads to closer packing

      That’d be my amateur’s guess anyway. (Note that I have no biological expertise).


  6. Tim Harris
    Posted October 28, 2017 at 8:02 pm | Permalink

    Well, having just translated it, I see Derek Freyberg has provided an accurate translation above, but here’s mine for the sake of it:

    “A company of ‘kimoguri’ flies who have gathered together, one after another, on a spindle-tree leaf at dusk. If even one of them flies up, the formation is disturbed, so you have to approach quietly and with bated breath. How ever many of them there are, they are all aligned so that they face in almost exactly the same upward direction.”

    ‘moguru’ means to burrow, and ‘ki’ must be tree (katakana is used for plant, animal & bird names often); The genus of ‘moguri-bae’ are flies that burrow into the soft tissue of plants.

    • Tim Harris
      Posted October 28, 2017 at 8:42 pm | Permalink

      Damn, missed a phrase; plus a stronger translation of ‘midarete simau’ 乱れてしまう:

      “If you do something like touch the leaf and even one of them flies up, the formation is thrown into disorder, so you have to approach quietly and with bated breath.

%d bloggers like this: