Water pennies are common insects in the White Mountains of Arizona and they are fabulous! They are the larvae of a terrestrial insect, so this species does not spend its entire life underwater. Most people wouldn’t even notice they were there, or if they did see them, they probably wouldn’t think they were alive. If you pick up a rock in a White Mountain stream, say the Black River, the water pennies are the little dark brown discs suctioned onto the surface of the rock. They looks like little debris clumps or mats of algae and are generally very well camouflaged. Sometimes they’ll move around on the rock a bit when you get it out of the water, but most of the time they just sit there. Even I didn’t realize they were insects the first time I found them, and I knew what I was looking for! I wouldn’t have noticed them at all if one hadn’t moved on a rock I’d picked up out of the stream. Once you’ve seen one, though, you’ll see how many of them there are in many of the White Mountain streams. They are everywhere! The White Mountain water pennies (Psephenus montanus, in case you’re interested) look like this:
Pretty cute, huh?
Water pennies are one of many flow-adapted insects, meaning they have characteristics that enable them to live in very fast flowing streams where they are at constant risk of being swept downstream. Being swept downstream is bad for aquatic insects and they have evolved many mechanisms to help keep them in place. Water pennies have evolved a body shape that helps them stay attached to the rocks, but also lessens the force of the water hitting their bodies. Their disc-like shape accomplishes both things. As you can see in the bottom view of the water penny above, they have a nice curved space hollowed out on their underside. This works just like a suction cup, the same as the suction cups you use to attach things onto glass. Water pennies crawl onto a rock, the weight of the water pushes them down a bit, and they suction onto whatever surface they’re on.
The suction cup effect isn’t that strong though – these bugs are easy to pull off the rocks with tweezers. The suction cup shape helps keep them on the rock, but the dome shape of the upper body, the part that is exposed to the flowing water, assists. Here’s how it works. Imagine sitting on a bench on a very windy day, one of those days where the wind is so strong you are worried you’ll be blown over while you’re walking. You are sitting on the bench with the wind hitting you in the back. Which body position is most protected from the wind?:
1) Sitting striaght up, shoulders back, head high, arms outstretched, or
2) Curving your body forward, head down, with your arms tucked into your body
If you answered number 2, you’re right! Water pennies use the same sort of technique to help lower the force of the flowing water against their bodies.There is a lot of great physics happening where water pennies live! Because they are dome shaped, flowing water tends to move up and over the top of their bodies (like the wind in example 2) rather than hitting them against a broad, flat surface (like the wind in example 1). But water pennies are also very flat and their whole bodies tend to be very close to the rock. Because they are so flat, they can live in what’s called the boundary layer of the rock.
A boundary layer occurs when a fluid (in this case, water, but air works the same way) moves over an object. Let’s make the object a rock. Imagine the flowing water moves at a set speed a foot above the rock. Closer to the rock, friction between the rock and the water molecules moving past it slows the flow of the water . The closer to the rock, the slower the flow. The water speed right above the rock, say within 2-3 millimeters of the rock’s surface, flows slower than the water higher above the rock. Organisms that are very flat can hide out in the boundary layer where the flow of water is less strong and less likely to sweep them downstream. Water pennies are a classic example of this type of animal. By being very flat and dome shaped, they are able to live in the boundary layer and are not as affected by the flowing water as other, taller insects.
Bugs that are shaped like your body in example number 1 above are not flow adapted because the water hits a broad, flat surface. Most insects that live in fast flowing water tend to be very flat or have some sort of structure that helps keep them anchored to the bottom of the stream. My next entry will talk about another type of flat insect, a mayfly that lives in the same streams as the water pennies and has similar adaptations to the flow.