Coordinated group movement in caterpillars

January 2, 2015 • 12:00 pm

Here’s a video from Smarter Every Day that shows coordinated movement in a group of Amazonian caterpillars.  They all start and stop at the same time, which is cool, though the video doesn’t test whether this might be due to the larvae’s having detected the photographer! Regardless, the wave of movement and stasis starts not from the front (as in birds or fish), but from the back. Can you think why this makes sense?

The filmmaker also demonstrates, using Legos, why a heaped-up group of caterpillars might move faster than a singleton, but his theory makes several assumptions that he hasn’t confirmed empirically. Anyway, here’s the video, and, since I’m off to the Khajuraho Temples, I’ll let readers chew it over.

This is a nice phenomenon that demands explanation, but the video suffers from a lack of a more thorough explanation and a test of the filmmaker’s theory.  A bit more observation and explanation is in order.

h/t: David ~

59 thoughts on “Coordinated group movement in caterpillars”

  2. The light is hitting them from the back so maybe they’re trying to reach shade?

    1. Yes, it would be interesting to know where they came from and where they end up. Also, what forces might have caused this as an adaptation? Are they crossing an open area where predation is a risk?

  4. Bees on hives in asia will “do the wave,” flapping their wings in a collective pattern that looks like a rolling wave across the hive. The theory there is that they do it so predators can see that the hive is absolutely covered in bees, and not just a brown object, so its a behavior that helps deter attacks.

    Could the same thing be happening here? Could the patterned motion be some sort of deterrent to attack? AFAIK catepillars don’t bite/sting, but maybe by doing this they look like a bigger, scarier target.

    The other hypothesis I can think of is simple herd defense: when you’re in a group, the chance of you being picked off is lower because you’ve surrounded yourself with so many other juicy targets. And the motion starts from the back because nobody wants to be at the back; that makes you look like an unfit laggard (I’m anthropomorphizing for simplicity; there is probably some hormone that kicks in when the catepillar is in that position that makes it decide to move).

    1. Herd defense and mimicry of a larger, more formidable prey item were the first things that I thought of too.
      It’s possible, I suppose, that there could be multiple benefits to this type behavior. Such as defense and increased efficiency in locomotion, but then again, moving quickly, relatively speaking of course, is a common defense against predators so I guess that fits into the avoiding predation category as well.
      So I can add this to my list of favorite unanswered questions like how do swallows know how to turn in unison when flocked together in massive groups.

      1. Catepillars move so slowly in comparison to birds that I can’t see the improved locomotion as a major defense mechanism. I’d put my money on one of the other explanations. 🙂

        how do swallows know how to turn in unison when flocked together in massive groups.

        IIRC, agent-based simulations have adequately answered that: a basic instinct or code that says “steer close but not too close to the birds in front of you” produces a collective flocking action. The wikipedia entry on flocking actually has some more detail on it.

        2. I don’t think it’s a question of caterpillars outrunning birds, but there could be some benefit to reducing their exposure time by crossing open areas more quickly.

          1. Also, the freeze behavior could be a response to the detection of movement above which might indicate a predator.

  6. This is a pet niggle of mine – some countries call Lego Legos when in plural. Lego is a brand name. I know adding an “s” to brand names is not uncommon, but that doesn’t make it right. You never see the word “Legos” used by the Lego company.

    Ready for the attacks and I don’t care what you think of me. 🙂

    2. It is true that we call them ‘legos’, but that is practical since a single one is no fun to the point of being meaningless. The fun begins when you have lots of legos. We have about 50 lbs of them in our basement.

        1. I’m pulling rank here. ( Is that allowed? ) 🙂

          We just call it lego as in “Are you playing with lego.” The equivalent of lego’s is “legoer” and that ain’t kosher.

          I guess if one your friends is called Lego it might be a bit confusing.

          3. I think as a Dane you’re allowed to pull rank, plus I just looked this up:

            The name ‘LEGO’ is an abbreviation of the two Danish words “leg godt”, meaning “play well”. It’s our name and it’s our ideal.

            Plus my dog is part Great Dane and she says it’s just Lego ( or maybe Let’s go for a walk!). OK, I’ll shut up now…

          5. I may have a solution. It is pronounced Legos but it is spelled Lego. The s is silent. Let there be peace.

        3. Heather is right, as always, but that does not matter. The people who call them Legos are not going to change. Logic has nothing to do with it. I will call them Legos and you can have your ‘s’ back when you pry it out of my mummified, dead hands.

      2. I agree with Heather on this! Lego is singular AND plural. I’ve got a “metric s**tload” of them in the basement, too, waiting for grandkids. My granddogs and grandkitties aren’t too interested…I loved them when I was a kid ( when they were just red or white) and loved playing with them again with my kids. Who knows, I might go down and play with them by myself:-)
        Was it on this site where I saw that you can order individual Lego parts online??

    3. I’m surprised to see language police here, especially for the end result of preserving a brand name. I would like to remind everyone that language is not static. Lego is in the process of evolving into Legos. The Lego police are already on bicycles.

      1. I sense a schism in the horizon.

        Just remember the wise words of the Prophet Legolas: “It is old, very old. So old that I almost feel young again, as I have not felt since I travelled with you children. It is old and full of memory. I could have been happy here, if I had come in days of peace.”

        Sometimes change is just a brick away.

      2. I will continue to tell my son to clean up his legos, lest I say Lego and only a single piece gets picked up. He got the Death Star for Christmas, 3803 Lego pieces…

    4. I know adding an “s” to brand names is not uncommon, but that doesn’t make it right.

      Actually, it does. The only meaningful definition of “right” with regard to spoken language is how people actually speak. If it’s common enough to be acceptable to regular folks in their daily speech, then it’s by definition right.

  7. Ok, I suspect (am not sure) that the caterpillars are actually sawfly larvae . These are ‘primitive’, stingless wasps whose larvae look a lot like caterpillars. They are commonly very gregarious, and these have the ‘look’ of sawfly larvae. The easy way to tell is to count their prolegs, but I cannot quite make that out in the video.

      1. Ironically, the two subjects, Legos and larvae, are kinda similar in appearance.

  8. Why do they tend to stop from the rear first? I do not know, but a seemingly simple reason is that this operates on two rules.
    a) Stop if you feel your neighbors stop.
    b) Be more inclined to stop if you are in the rear.

  9. I just find the video an unconvincing illustration of the hypothesis. The movement seems to me to begin in the middle of the group. My hypothesis based on that observation is that the caterpillar at the bottom of the middle of the group gets pissed off being sat on.

  10. It kind of looks to me like the pack is moving like the tracks of a tank or bulldozer. The ones in the rear follow the caterpillar instinct to climb upwards, and then they move faster than the ones below because they can. The Lego demonstration makes the effect more pronounced because they are easy to tell apart. So it could also be that the caterpillar who makes it to the front stops when he sees daylight (or smells it, since his sibling’s butt is no longer in his face). Then the one in the back starts the cycle when he sees a chance to move up. I don’t think my impression is any more likely than any other, and I may just be saying the same thing as others in different words. I hope I hear the results of a real investigation some day – it’s a mesmerizing little behavior!

    1. In order to understand the function of swarm locomotion in any particular species, it would be useful to compare related species and isolate rules and parameters in which they differ at the level of individual larvae. Variation may affect how effectively the swarm detects potential predators, how quickly it moves, and what sort of visual/auditory signal the swarm presents.
      The ‘spitfire‘ is an Australian sawfly with similar collective locomotion but different in details, e.g. head-raising (mostly in the front rows) and tail-tapping (at the rear), and tendency to form a flat sheet rather than multi-layer stack.
      Standing still and raising the head are useful if you want to visually detect motion of foreground objects vs. background, and most effective if there’s no part of the larva-swarm in front of you. If you see tail-tapping up ahead it’s probably a good idea to get a wriggle on, or you’ll get left behind (spitfires can be manipulated into a futile circle, but eventually break out).
      Presumably a complete set of rules for larvae would describe behaviour when feeding, and decisions to stay with a food source or set out for a new one…

  11. To me, it looks like the ones in contact with the surface and are weighted from above stand still, and the ones which have yielding surfaces under their feet continue forward. When one has something above which isn’t moving, like another caterpillar stopped with limbs on the rear portion of the lower one, then the lower one stops its forward motion as well.

    In that situation, the only ones which would be free to initiate forward motion would be the ones at the back edge, and if those at the back edge stop moving forward, the ones below its legs would also stop moving forward.

    The behavior of not pulling forward when there is unmoving pressure from above could arise from an evolved freeze, with caterpillars which continued moving after applied pressure being more likely to be eaten by the predator doing the pinning, and those caterpillars playing dead being more likely to have offspring which inherit the freeze in response to pressure.

    Was I there? No, but that’s my guess, based on the demonstrated behavior.

  13. Then caterpillar phalanx is not dissimilar to the massing and bridging of ants confronting or crossing a body of water. Could it be that the caterpillars are motivated to ‘take turns’, moving from bottom to top, because the sand is ickyto them (too warm or too sandy perhaps?)

          1. Very cool fossils and great animation in the show. Wasn’t Shark Bay mentioned in Jerry’s book ( or maybe it was Shubin’s?)

          3. You’re probably right, Mark. I read both of Shubin’s around the same time I read WEIT.
            Great shots of stromatolites(?) in the NOVA show.

    2. My inclination is it is spun from a group defense. Sawfly larvae (and I suspect that is what they are) congregate to put more punch in their chemical odor defense. They normally do the group defense while hanging out together on their host plant, but how do they defend themselves when on the move? They make a delightful mass ‘o larvae, and move together! At least these do that.
      I just loves the wiley wiggly wisdom of nature!

    3. That’s what I thought. It seemed like a way of moving across something that they didn’t like. Would be easy to test–just see if they only engage in this behavior on certain surfaces.

    4. If the sand is excessively loose and shifts undergrub (as it were), the extra weight on top will help with that. This, for the whole mass of larvae as a unit, would be akin to a snake sidewinding, restricting area of body contact with ground to increase traction. Of course mass behaviour is driven by individual actions that may well be mediated (is motivated too strong a word?) by what stinky, toxic sawfly larvae find ‘icky’. But also by the angular momentum of the rolling mass that throws them forward onto the icky sand again.

  14. When I was in Ecuador a couple of years ago, I videotaped something similar with another species of larva. They traveled as heaped-up, writhing mass. The movement of individuals seemed to be more random and less coordinated than the movement of these caterpillars, yet the mass stayed together slowly traveled in one direction.

  15. The increased speed of the lego movement is a “trick” that wouldn’t apply to caterpillars. Don’t watch the blue lego, as suggested. Instead watch the front of the collection of legos. Every time a lego block on the second layer extends it’s entire “body length” over the lego below, it drops down, increasing the distance by four clicks at once (instead of one). This would only work with caterpillars if the top layer of caterpillars were extending their entire body lengths over those below, then dropping down. As you’ll see from observing the video, this doesn’t happen.

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