What were those flies doing?

Earlier today Matthew put up a post showing a mess of flies sitting, nicely aligned, on a leaf:

What, he asked, were they doing?

Well, we don’t know for sure, but at least one person thinks that this is a case of communal resting, which has been observed in some flies (these ones are in the Chloropidae, or frit flies [not “fruit flies”]). Here’s the tw**t that Matthew put out, which got an answer from Morgan Jackson, a Ph.D. student, specializing in flies at the University of Guelph:

I suspect this is communal resting, just as starlings gather in trees en masse on cold evenings. Of course that’s partly for warmth, which can’t be the case for cold-blooded flies, but both cases could also be a way of reducing predation through “safety in numbers.” (It’s easy for a predator to pick off single flies on leaves, but when you’re with several hundred others, the predator can get just one before everybody else flies off.)

I can’t find any information on nightly communal resting in flies, but if you find some, put a link below.

And as for why they’re all pointed in the same direction, I haven’t the slightest idea!


  1. Torbjörn Larsson
    Posted October 28, 2017 at 2:14 pm | Permalink

    Assuming this behavior helps against predators, perhaps aligning decreases risk for collisions as they take off?

  2. Derek Freyberg
    Posted October 28, 2017 at 2:16 pm | Permalink

    I should have added to my translation on the previous post a translation of the picture caption: 数で勝負 means “competing by numbers”, which would tie in to Jerry’s comment on “safety in numbers”.
    My total guess at their lining up is so that they can take off en masse without colliding – don’t birds do the same thing?

    • Posted October 28, 2017 at 3:00 pm | Permalink

      Why didn’t I think of that?

    • Tim Harris
      Posted October 29, 2017 at 5:52 am | Permalink

      That’s surely a good explanation. If they are facing downwards, it would surely be difficult to explode upwards – which is the most obvious and natural thing to do – without getting in each other’s way and colliding, as Derek Freyberg says.

      • Tim Harris
        Posted October 29, 2017 at 7:51 am | Permalink

        数で勝負 – kazu de syoubu – means not so much ‘competing by numbers’ as ‘gambling on numbers’.

        • Derek Freyberg
          Posted October 29, 2017 at 11:26 am | Permalink

          Thanks Tim – that would convey the sense of the phrase much better. “We must, indeed, all hang together, or assuredly we shall all hang separately” (attributed to Ben Franklin at the signing of the Declaration of Independence).

      • Gregory Kusnick
        Posted October 29, 2017 at 10:50 am | Permalink

        If the goal is predator avoidance, wouldn’t they want to do something other than “the most obvious and natural thing”?

        • Tim Harris
          Posted October 29, 2017 at 7:59 pm | Permalink

          Perhaps I should have said something along the lines of ‘in position for the most efficient escape’. I am not a fly, nor have I ever been in the position of Gregor Samsa, but I imagine it is easier to fly up to safety without barging into one another head-first than arse-first.

          • Tim Harris
            Posted October 29, 2017 at 9:18 pm | Permalink

            Also, if you are all facing downwards it will be more difficult to sense danger, which I imagine is more likely to come from above, or as someone, I think, remarked earlier along the stem of the leaf from the tree.

  3. Posted October 28, 2017 at 2:24 pm | Permalink

    —And as for why they’re all pointed in the same direction, I haven’t the slightest idea!—

    Muslim Chloropidae ready to take off, perhaps?

    • Diane G.
      Posted October 29, 2017 at 3:41 am | Permalink


      I was gonna suggest that maybe they were facing Mecca…

      • Posted October 29, 2017 at 5:51 am | Permalink

        Or awaiting the arrival of the Lord of the Flies (h/t to W Golding)

  4. John R. Vokey
    Posted October 28, 2017 at 2:28 pm | Permalink

    An explanation of the flies all pointing in the same direction: Given it is in the evening, is it not just a fly by night organization?

  5. dougeast
    Posted October 28, 2017 at 2:40 pm | Permalink

    No doubt, it’s flypaper cleverly disguised as a large green leaf…

  6. W.Benson
    Posted October 28, 2017 at 2:41 pm | Permalink

    Why parallel orientation? Each fly will have the wings of the fly in front smack in ‘his’ face. If the one in front flies away, it gets smacked in the face and takes off too. This may not happen for flies that are head to head.

    I have an alternative explanation. In the foto, their heads are directed toward the leaf petiole, the side an ant or spider is likely to come from. Most of the sensory equipment of the fly is on its head.

    Both of these ideas could be tested, the first using high speed video and the second (assuming that the flies really do align to face the petiole) recording predator success on flies attacked from different angles.

    • DiscoveredJoys
      Posted October 29, 2017 at 4:37 am | Permalink

      I was thinking that there could be two instincts at work. As an individual a single fly could orientate towards the most likely approach of a non flying predator/best take-off direction/upwind etc. But once a few flies orientate themselves that way a second ‘flocking’ function could see them self-organising in one direction to achieve optimum packing density for the area available.

      My guess is that if the species was most likely to be predated by flying predators then they would ‘flock’ in random directions to maximise the collective detection capability (whether optical or vibration etc).

      Short of gluing flies onto leaves…

  7. Randall Schenck
    Posted October 28, 2017 at 2:44 pm | Permalink

    If other groups of flies are also lined up in the same direction it could be the magnetic direction instinct in animals?

  8. Gregory Kusnick
    Posted October 28, 2017 at 2:51 pm | Permalink

    It’s good to know that this behavior is called “communal resting”, but (meaning no disrespect to Dr. Jackson) I’m still no wiser about why they do it.

    • Posted October 28, 2017 at 3:02 pm | Permalink

      Why they rest or why they do it communally. If you mean the latter, I suspect it’s because the chance of an individual surviving a predator attack is greater if it rests with a bunch of its mates than if it rests alone. That’s individual selection that will eventually lead individuals to aggregate since each gets a fitness benefit.

  9. Posted October 28, 2017 at 2:59 pm | Permalink

    Their shared alignment can be for one a combination of simple reasons. Suppose this species simply prefers to bed down pointing upward. Then the next one to settle in would also be pointing upward, and so on. As for their rather precise alignment, well, as new ones settle in their neighbors, who are slightly jostled by the new arrival, would adjust their spacing and angle slightly to be equidistant to each other all around. The best solution to be equidistant all around is to point in the same direction (so my gut tells me).

  10. Michael Fisher
    Posted October 28, 2017 at 3:40 pm | Permalink

    Why are these chloropidae facing the same direction? I can imagine a frit fly drive-in cinema & that’s what we’re seeing here – they’re watching “night of the living dead maggots” on big screen & having a helluva time.

    After that they’ll be watching YOU, before feeding off your luscious, salty tears – they love to do thst

  11. MKray
    Posted October 28, 2017 at 3:51 pm | Permalink

    Surely the efficient way of packing the most into a space is if they all point in one direction. The question then is, what is in common between the orientations of different instances. Is it just crystalline beats amorphous?

    • Gregory Kusnick
      Posted October 28, 2017 at 8:08 pm | Permalink

      Efficient packing depends on shape. If they’re wedge-shaped, the most efficient packing would be alternating up and down.

      • Mkray
        Posted October 29, 2017 at 3:59 am | Permalink

        Of course, but they are not triangular.

        • Gregory Kusnick
          Posted October 29, 2017 at 10:48 am | Permalink

          OK, let’s model them as rectangles then, or ellipses. It’s still not obvious why they all need to face the same direction. They could pack just as densely if they faced randomly up or down, but not one of them is facing down.

          It’s not clear why they should care about optimal packing anyway. There are a lot of flies on that leaf; the adaptive value of fitting in one or two more would seem to be near nil for the flies already there. And in fact we see several gaps in the formation; apparently the flies adjacent to those gaps aren’t particularly concerned about dense packing.

  12. DrBrydon
    Posted October 28, 2017 at 5:32 pm | Permalink

    I still like my ‘bell the spider’ explanation. Doesn’t anyone read La Fontaine, anymore?

  13. John Scanlon
    Posted October 28, 2017 at 8:01 pm | Permalink

    Does this demonstrate that chloropids are bosons, not fermions?

  14. Posted October 28, 2017 at 8:59 pm | Permalink

    Wind mitigation?

    • Posted October 28, 2017 at 9:02 pm | Permalink

      ….. in addition to the other comments about organized take-off.

  15. Posted October 28, 2017 at 9:38 pm | Permalink

    Steady steady, don’t break, hold, hold… the humans are guessing what’s going on here. It works on confusing predators and the inquisitive it seems…
    WE! my fellow flies have expanded our fitness trait.

    however, when they go we can get back to our line dancing class.

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