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:

Flat headed mayfly, bottom view

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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