 Hi everybody, I'm Michael and today I want to talk to you about how we can use light signals to perturb collectively-swimming midges, and that's work I've been doing together with Kaspar van der Fad over there and Nicolette at Stanford, and all of this was funded by the AIO. So as you all know and seen several times over the past few days that if we look in nature, we often see many species of animals forming groups or aggregates that often exhibit fascinating spatial and temporally complex patterns and dynamics. And often this behavior is collective in the sense that the statistics of the group as a whole are distinctly different from the aggregate statistics of individuals. And now to make our life as physicists more complicated and exciting, often or typically these groups of animals in nature are subject to an countless number of environmental perturbations, let it be light cues, winds reaping through these bird flocks, water currents driving these fish, predators coming in the environment itself might be changing, and so on and so on and so on. So to investigate the interplay between this collective behavior and environmental perturbation, one could go to the field, set up an experiment, and there's all of you that do fieldwork know that even that is a pretty challenging task. But now to add on that, to also try to estimate or even control the environmental conditions is even a much, much harder task. So we are not going to do that, and we make our life much simpler than that. So we go in our laboratory and we set up an experiment there, and the system we are looking at is the Keronimus Reparius mating swarm. So Keronimus Reparius is a non-biting mid-species, which non-biting parts are especially useful for experiments. And most of this time, this Keronimus Reparius spends a larvae or pupae state and only for the final three or so days of their life, they shed their digestive system and mature, and the only thing they basically care about from that point on is mating. And the way they do that is during dusk and dawn in nature, they typically form mating swarms over visual cues, let it be a branch or a side of a river, and males will come in collectively swarm over that visual cue and wait for females to come into mate. In our lab, we have a black marker in the center of our dysplexic glass tank to trigger the swarming, and we set the swarm on an official day and night cycle, which we can easily do with an overhead lamp, and during our night time, the males of the colony come from the walls, start swarming, wait for females to come in. So we can easily set up an experiment here, and what we do is we set up three cameras, we hardware synchronize them, so we actually can use Lagrangian particle tracking, which allows us to reconstruct three-dimensional positions, accelerations, and velocities for all individual midges in the swarm. We can record that with 100 hertz and then get actual trajectories of the individual midges. So this is how the typical situation in our lab looks, so this is time series of the number of swarming midges in our lab, in this experiment, as a function of time, this is over the curse of a few days, and orange background means it's their daytime, blue background, it's their night time, which we conveniently set to start at 10 o'clock in the morning, so we don't need to get up early, and you see that right after their night kicks in, the number of midges swarming over the smarkas quickly increases, then gradually decreases a bit over the night, and as soon as their daytime kicks in, they see to swarm and wait for the next night to come. And a typical swarm would look like this, so you see at the bottom, the marker over which they swarm, on top, this is a post-processed video from our data, you see the males fly around in a very dynamic and interesting way all across the marker. Well and as said in the beginning, we wanted to find out whether light does influence any of their behavior, and the zero's other thing to do there is that of course, if you think of nature, not every night has the same darkness or brightness, the moon might be up, clouds might be in the sky, so not every night is the same, so there's all a question we could tackle is does ambient light actually influence their behavior in any way, and in the lab we have the convenience to be easily able to do that, so we set up an additional broadband white light LED, which we can tune up, and then we can run the experiment at different lighting conditions, and what we see here is that this actually has no effect at all, so what I'm showing you here is for two different experiments, one is at the standard typical night time, and one experiment where we cranked up the brightness in the room quite a bit, and on top I'm showing you the PDF of the velocities of individual midges on the bottom of the PDFs of exploration, and I'm not quite sure whether you can actually see the colors nicely in the back, so there's one PDF in blue corresponding to the darker case, and one PDF in orange corresponding to the brighter case, and these PDFs virtually coincide, so changes in ambient light don't seem to have any effect on the midges at all, but of course we are in the lab, so we can do whatever we want, and so what we now can do we can hook up our light to a function generator and turn on and off with a square wave versus head frequency, and this is what I'm showing you here, so white background here means this light is turned on, while if the background is black it's turned off, so this is roughly on the three second switching period, and now you can see something really interesting. Every time the light turns on these midges tend to get more exciting and also come come together more closely, and every time the light turns off they seem to diffuse away. Now and of course we can quantify that, so what I'm showing you here is time series of the mean velocity of midges in the swarm, in blue and under that the light signal we are giving it to them, the square wave, on top it's a very fast light switching while it goes down to about switching every five seconds at the bottom, and if you look closely at these time series, especially if you look at the bottom, you can see that with increasing period of us switching the light also the mean signal of this velocity seem to follow one to one, and especially if you look start looking here you can see that these peaks and the brightness also seem to coincide with the peaks in the mean velocity. Now to quantify that a bit more we can look at the Fourier transformation of this signal, and you see on the left this is the amplitude spectrum there that the velocity signal itself has a pretty distinct peak, and if we pick that out we see that this peak of the velocity coincides with the frequency of the driving we are putting onto the swarm. So at the same rate as we switch the light on and off the midges also tend to become faster and slower, and to give you a picture that's corresponding to the first plot I've shown you, here are again the PDFs on top of the velocities of individual midges and at the bottom of acceleration of individual midges, and now you see actually see a difference, so every time when the so I'm split this up by periods where this periodic light is turned on and turned off, every time it's turned on it's orange curve, every time it's turned off we are back at the blue curve, and now you see that what you've probably also guessed from the video that when we turn on the light midges tend to become faster and accelerations become higher and every time we turn the light off again both PDFs move to the left. This alone is already pretty interesting, but it's not yet any sign of collectivity just because all midges happen to react to the same signal in the same way it doesn't mean there's any collectivity to that response, but of course we can look at more global features of the swarm, so what I'm showing you here for the same experiment is a PDF of distances of midges to the center of Mars, so basically we at each instant in time we compute each the center of Mars and the distance of each individual midge to that center of Mars and plug that into that PDF, and then you see what was also visible in the video that when we turn on the light the swarm tends to be slightly compressed so this PDF the orange PDF tends to slightly move to the left, meaning the swarm is denser than it was in the case where the light was off. It's not only that the swarm becomes smaller, it's also more tightly bound to its core, so this is a slightly difficult plot, so let me walk through here. So what I'm computing now is the acceleration of each individual midge towards the center of Mars, so so basically if I have a midge which has a certain acceleration at any point, so projection on that onto its vector towards the center of Mars, it is it's that's the condition is the acceleration of that midge towards the center of Mars that component, and now if what I'm showing you here is a conditional average of these projected accelerations of all midget conditioned on the distance of the midge from the center of Mars, and what you see here is again that in the light on case these this conditional acceleration towards the center of Mars is stronger, meaning the midges are bound more tightly, and in contrast to the individual's velocity and acceleration PDFs I've showed you a few minutes or half a minute ago this is actually indicative of a collective response of the whole swarm, so not only all midges become all midges become more excited and fly around faster, they also become denser closer together and more tightly together when the light switches on. Now as we are in the lab and we can basically go to a little bit more extreme cases, so we cannot only play them a little bit of light on or we can really crank up the light by a lot, so we did another set of experiments where we were less careful about not scaring them too much and we created a big really significant difference between the bright and the dark case and this is here for a little bit of a bigger swarm and the same visualization as before when the background light is turned off the background here becomes dark, when the background light is turned on the background becomes light again and now you see on top of the features you've seen before namely that the midges tend to get more excited when the light is turned on and they seem to become closer to each other. You also every time if you pay attention we switch the light there seems to be some coherent direct movement of all midges between so when the light turns off they seem to all quickly go here and then relax and the light turns off they come all up there and relax again and this let's call it scared response is interestingly seems to be independent of the driving frequency. So here what I'm showing you here is our face averaged accelerations so we run this experiment for a long long time at constant frequency then we wrap everything up to produce a face average of the acceleration and then we repeat the whole experiment for various different frequencies what you'll see here is the same effect as seen before the midges seem to be more excited higher high accelerations when the in the phase where the light is turned on and be a little slower acceleration when lights turned off but additionally you see these this big peak every time we switch the light when there's a strong light gradient in the square function and this peak seems no matter how fast we drive is always to be about 0.3 seconds. We actually don't know why they're doing that so if I in the end somebody has a suggestion what's actually happening here we happy to hear that but that got us pretty excited about this and to once again wrap back to the beginning what's happening now you see again here's this ever face average acceleration I've just shown you before and on top of that we did a control experiment we at the same day with the same lighting same same swarm we ran one run of the experiment where we had this bright LED on all the time and this bright LED on off all the time this is blue and orange respectively and now something really interesting can be seen here so apparently what happens is every time our extra LED bright LEDs turned on the swarm behaves as if it would be in this static ambient light case while every time the light goes off the swarm tends appears to get into a state that's typically not reached by this ambient light is constant ambient light experiments and again we don't know why we the swarm goes into the state but we can put this into a thermodynamic framework so we've shown earlier that several of the features of these swarms can be described in a classical thermodynamic sense and especially one can define a pressure like this so this different microscopic definition of pressures essentially a virtual pressure that the swarm would exert on an invisible sphere centered around the center of mass if this fear would be responsible for changes in acceleration and so the dots here the orange as always the orange dots are corresponding to a bright case blue dots are corresponding to the dark case conditional averages of this pressure on the density of the swarm and these big two big dots are the global average over these two phases and it appears to be that all of these points no matter whether it's light and light on or light off fall on something that looks pretty much like isotherm and classical thermodynamic and it seems that with these periodic switching of light we seem to be able to move the swarm along the single isotherm but not be able to like create a completely new state it seems to be the same behavior just moved along in the phase diagram and with that let me come to an end so we've seen that mitches react to perturbations so to light gradients we see a dynamic response to these gradients we all we all see an individual response of mitches as well as a collective response of the whole swarm and we seem to see that we are able to move the swarm along an isotherm in a thermodynamic phase diagram with that I'm happy to have questions and would be really happy to hear suggestion what actually the mechanism of these responses would be if anybody has an idea