When studying insects deep in the Peruvian Amazon, sometimes all you can say is “wait- this is weird, right?” That was my expert opinion – “weird” – that I declared to our TechKnow producer and production crew while watching this obscure group of insects known as barklice (not related to head lice) move in almost perfect coordination.





At first I couldn’t put a finger on why this six-legged dance looked so odd- until I realized that when one moves, so do the others, and when one pauses, so do the others- but all in different directions. It’s a cohesive, start-and-stop movement that I haven’t seen before in any animal. So I wondered, had anyone documented this before? The answer, it appears, is no. How does a one millimeter insect inches away from another know when to start and stop? And why pause in coordination when they could get to their destination faster if they just kept walking?





My first guess for the how was that they could ‘feel’ each other’s movements through the silk they live within; other animals like spiders are known to send messages through vibrations in silk so why not barklice too? Back from the remote rainforest and into civilization, I tried searching through scientific literature for anyone else who had seen this. Nothing. It was time to call in the experts to get to the bottom of this. I started with Dr. Alfonso N. García Aldrete, researcher at the Universidad Nacional Autonomo de Mexico who has published extensively on this group in Latin America. His thoughts? “No idea about that behavior,” but he did help me identify this group as the genus Archipsocus. Next up, Dr. Glauco Machado of Universidad de Sao Paulo in Brazil, neighboring Peru. Again, hadn’t seen this before, but did guess that it could be an altruistic behavior and by moving together a predator could “perhaps mistake the whole aggregation for a single organism.” So the people that work with this group hadn’t seen this type of movement, but what about in other groups of animals- could there be fish or birds that move like this, or maybe this movement an entirely new thing. Enter Dr. Iain Couzin, arguably the world’s top expert in cohesive animal behavior who has a lab at Princeton and recently moved to be the Director of the Department of Collective Behavior at the Max Planck Institute for Ornithology in Germany. Couzin studies “how animals coordinate their motion and make collective decisions regarding when, and where, to move” and works with species ranging from “swarming locusts to schooling fish and even human crowds.” So if anyone were to have heard of a start-stop, mass movement like the barklice do, it would be Couzin. But he hadn’t, and he was amazed. “In all honesty I don’t know why they would move like this,” he told me, but wagered what seems an excellent guess. Some species are known to stop in order to detect predators around them. It’s hard to hear a predator if you’re making noise, or detect their movement if you’re moving yourself. “So the group may be going through a repeated process of move together - check for external cues - move together, and so on.” “There seems to be evidence of rapid social transmission across the group allowing them to behave almost as a single entity. My other thought, of course, was ‘wouldn’t this be fantastic to study - where can I find some?’” Both he and his grad students are eager to get to the bottom of this, and watching their discussions on potential mechanisms and origins of the behavior unfold over email even further fascinated me.