Flat mayflies!

There are all kinds of flat insects in fast flowing streams.  In the White Mountains of Arizona, you can find a few types of flat mayflies alongside the water pennies on the same submerged rocks.  Take a look at these photos:

heptageniid

Flat headed mayfly, bottom view

heptageniid

Flat headed mayfly, bottom view

This mayfly belongs to the flat headed mayfly group and is REALLY flat!  Bug legs tend to curl up when they’re preserved, like in this specimen, so this mayfly would actually be much flatter than it appears here if it were alive.   Flat headed mayflies have several adaptations to flow that you can see in the images above.  First, take a look at the gills, the plate-like structures sticking off the sides of the back half of the bug.  They stick out from the side of the body rather than up like they do in many mayflies.  This helps them keep their gills close to the surface of the rock and inside the boundary layer .  Second, when these insects are alive, they keep their legs held far away from their bodies and absolutely flat against the rock.  These bugs have enormously long legs, but they are also very flat, so they are able to fit them within the boundary layer too.  Finally, they have big, broad, flat heads.  They keep these pushed against the rock, within the boundary layer as well.  The whole bug is only a few millimeters thick, even though they can be close to an inch long!  These are probably some of the flattest bugs there are.  It is a great adaptation to living in a high flow aquatic habitat.

Flat headed mayflies move in a strange way.  Unlike the water pennies, which keep their legs tucked under their bodies and walk along the rock much like other insects do, flat headed mayflies hold their legs flat against the rock and far away from their body.   This makes it hard to walk.  In fact, they tend to shuffle along the rock rather than walking.  Imagine wandering across the floor on all fours.  This is how most insects walk, with their bodies held far away from the surface they’re walking on.  It’s quick and efficient.  Now imagine lying flat on your belly with your legs behind you and your arms out to your side, then crawling commando-style with your body only an inch above the ground.  It’s a lot harder to do, right?  Flat headed mayflies don’t move very quickly or very gracefully.  However, if they pick their bodies up off the rock, they risk getting caught in the current and being swept downstream.  So, they keep their legs close to the rock and push themselves across the rock by pushing with the legs in the opposite side of the body from the direction they wish to go.  It’s not the most efficient way to get around, but it works for them because it helps keep them safely within the boundary layer of their rock.  There probably aren’t many predators that are going to pick them off of rocks in very fast flowing water either, so moving quickly is not as big of an issue as it is for many other insects.

Other aquatic insects have different adaptations to flowing water.  I’ll discuss some of them in future posts.  Next time, however, I’ll talk about why I call all insects bugs and what a bug really is.

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Text and images copyright © 2009 DragonflyWoman.wordpress.com

Water Pennies

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:

water penny

Water penny, top view

water penny

Water penny, bottom view

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.

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Text and images copyright © 2009 DragonflyWoman.wordpress.com