And now, a press note from the American Chemical Society:

Water striders’ ability to walk and jump on the surfaces of ponds and lakes has long amazed curious observers — and inspired robot designers who want to mimic the bugs’ talent. Now, scientists have measured for the first time key parameters that allow them to walk on water — by studying their leg shadows. The findings, reported in the ACS journal Langmuir, could contribute to designs for water-skimming robots.

More than 2,000 years ago, Greek scientist Archimedes explained flotation, stating that the upward, floating force on an object in water equals the weight (or downward force) of the water displaced. The principle has informed the building of ships, submarines and other aquatic vehicles. But for tiny water striders, water isn’t displaced. It is expelled by the insect’s hairy legs. The updated Archimedes principle predicts that the weight of the expelled water should equal the floating force. But confirming this prediction experimentally is a challenge. Because water striders are so light, they are almost impossible to weigh using conventional techniques. So Yu Tian and colleagues used an unconventional method — analyzing the shadows cast by the insects’ legs.

The researchers placed a white sheet of paper at the bottom of a lab aquarium housing water striders and installed a light source above the water. The insects’ stick-straight legs cast shadows that were rounded, representing the curvature of the water and the expelled water volume from which the floating force and weight can be calculated, the researchers say. Also, from these measurements, the striders’ slightest shifts in weight and body angle could be detected for the first time.

The authors acknowledge funding from the National Natural Science Foundation of China.

The abstract from the original publication:

Forces acted on legs of water-walking arthropods with weights in dynes are of great interest for entomologist, physicists, and engineers. While their floating mechanism has been recognized, the in vivo leg forces stationary have not yet been simultaneously achieved. In this study, their elegant bright-edged leg shadows are used to make the tiny forces visible and measurable based on the updated Archimedes’ principle. The force was approximately proportional to the shadow area with a resolution from nanonewton to piconewton/pixel. The sum of leg forces agreed well with the body weight measured with an accurate electronic balance, which verified updated Archimedes’ principle at the arthropod level. The slight changes of vertical body weight focus position and the body pitch angle have also been revealed for the first time. The visualization of tiny force by shadow is cost-effective and very sensitive and could be used in many other applications.

Citation: Yelong Zheng, Hongyu Lu, Wei Yin, Dashuai Tao, Lichun Shi, and Yu Tian. 2016. Elegant Shadow Making Tiny Force Visible for Water-Walking Arthropods and Updated Archimedes’ Principle. Langmuir 2016 32 (41), 10522-10528. DOI: 10.1021/acs.langmuir.6b02922

Now, I’ve been all over Africa, and I’ve sat down in Nigeria, Kenya, Lesotho, Botswana. I admit having had a hard time finding a place to sit down in Namibia but that’s because I’ve only been in places with no chairs but there were a lot of rocks. I once sat for a long time on a curb in Rwanda. I’ve never been to Egypt but I’m pretty sure they may have invented the chair there. South Africa is, of course, all about sitting down. Lots of places to do it there.

Having said that, the question of where to sit down is an interesting one when certain things are true. For example, if you go into the deep forest to hang out with the Efe Pygmies siting down can get a little dicey. We can talk about that later. But what she really meant, I think, is that there is no place to sit where there will not be a bug or a spider or something either where you want to sit, under where you want to sit, or flying around where you want to sit.

It isn’t really true though. When I first went to the Ituri some people quite thoughtlessly (i.e., they did not put any thought into what they were saying) advised me to bring bug spray, because the place would be thick with mosquitoes and such. So I brought a couple of small cans of bug spray, and after I arrived, I found the big basket hanging from the roof of the supply hut that contained dozens of unused containers of bug spray that various researchers had brought there over the previous five years or so, only to discover as I had that there was no use for such a thing. “Maybe we’ll have a garage sale someday” I thought as I added my bug spray to the rest.

An Efe camp is usually out in the the forest somewhere, and that is a good place to sample the invertebrate life in that habitat. There are no clouds of mosquitoes or flies in the rain forest, or at least not in this rain forest. Why? There are too many bugs! If any insect tried out the strategy of being in a horde some other insect would come up with the strategy of eating the entire horde, and said strategist would simply wait round, in numbers, under wet leaves somewhere, for the next horde to come along. Really, clouds of insects, like the mosquitoes or lake flies or black flies we get in the northern states and provinces of North America exist because there is a winter, from which the landscape emerges, and into which swarming hordes of insects swarm, one after another, until deadly winter returns again. A set of evolutionary stable strategies resulting in this pattern have developed in the colder regions. If you got rid of the winters (though you could not get rid of seasonality) there would be fewer swarms of flying insects in a highly species rich forest environments. Swarming insects are more likely to be found in habitats with a winter, in low species diversity forests, and grasslands (including marshes and swamps), or areas a more distinct dry and wet season. Not so much in tropical rain forests.

But that does not mean there are not a lot of insects. There are plenty, and even just sitting in a camp is a great way to discover new ones. One day as I sat on my Efe-made “chair” (we can discuss those another time as well) a whopping big slow moving thingie came along and started to climb up the chair leg. I managed to guess that it was some kind of cricket … bear in mind, though, that crickets in the African rain forest are as much like our temperate crickets as an elephant is like a hyrax. I asked the nearest Efe what it was.

He looked. Shrugged his shoulders. “No idea.”

I was surprised. Normally the Efe knew the name of anything I pointed to (and yes, I did verify their knowledge using various techniques … they weren’t usually making stuff up, though that could happen now and then). We kept an eye on the slow moving creature as it explored around on my chair and the nearby ground, and everybody who came along got asked.

“What is that thing?”

“Huh. No idea. Strange looking.”

Eventually, an older man picked the thing up with a stick and moved it several hundred feet into the rain forest and let it go. Why do this instead of ignore it or squishing it? Well, the Efe don’t squish an insect or other invertebrate unless they know what it is. With good reason.

The reason the Efe won’t normally kill an insect …

…that has wandered into their camp if they don’t know anything about it a priori is … according to what they told me …

Many, though certainly not all, insects are linked to important things in life. This is true of many things that are not insects as well. For instance, one does not walk to the right of a young male Canarium tree in the afternoon, because he who shall not be named could be sitting in the tree waiting to put a curse on you, and then you’re screwed. Or, one should not handle the fetus of an antelope if you are a fertile female or if any females in your family are planning on getting pregnant soon. For many insects, killing them is bad because that may affect fertility of someone related to the one who kills the insect.

Generally speaking, this culture is very uptight about babies and fertility issues. Some of this is spillover from the village-dwelling horticultural Lese with whom the foraging Efe share a culture. The Lese have a repressed fertility owing to a number of causes. When a fertility rule is broken, a great deal of effort may be expended to fix it. As the reproductive ecologist Peter Ellison once said, “The Lese and Efe are constantly afraid of overdrawing on the bank of fertility.” (I paraphrase.) One of the most dangerous things you can do is to accidentally have twins. That’s like going to an ATM machine to get 100 bucks and the machine gives you 200 bucks. What do you do with the extra money? Will you get caught? When you check your bank account later, will there be 100 or 200 bucks taken out? Will there be a fee? A fine?

An insect that you don’t know about might be an insect linked to something important like fertility, or if not fertility, something else. Better to just leave it alone and let it go on its own way.

Oh, and there is probably a lot of heterogeneity across the cultural landscape in the detailed beliefs. It is not at all unlikely that an Efe visiting a distant settlement will discover that those people have a different set of beliefs about various insects or other things. The ethnography certainly shows different things happening across time and space, rather dynamically. The Efe do not generally look at beliefs of other people with disdain. Rather, they figure that those beliefs might be valid as well, and try to incorporate them in their routine.

So it makes sense that Efe would assume that an insect they’ve never seen before … and in this very species rich rain forest that is not as unlikely as it sounds, though it is certainly not a daily occurrence … has an importance of which they are simply unaware.

Want to read more about insects in the Congo? Click here: “We live in little houses made of beans.”

A modified repost; stay tuned for more on the Ituri Forest and Insects.