Mayflies, Damselflies, and Stoneflies: What’s the Difference?

I haven’t done an identification post for a while, so its high time that I write another one!  I find that a lot of people have a hard time distinguishing the aquatic insect nymphs with tails sticking off the back, the mayflies, the damselflies, and the stoneflies.  They’re easy to tell apart once you learn a few basics!  A lot of people have read my post on how to tell the damselflies and dragonflies apart as nymphs, so let’s start with them.

Behold, the mighty damselfly:

damselfly nymph

Damselfly nymph

There are several things to look for that will let you know this is a damselfly nymph and not a stonefly or mayfly.  However, the mouthpart is a dead giveaway!  If you don’t know about the awesome odonate mouthpart, allow me to enlighten you.  Odonates have highly adapted mouthparts that form a long, hinged structure that they can thrust out toward prey to capture it and draw it back to the chewing mouthparts to be eaten.  There are pictures of this structure available on the post linked above and you can see a little part of it sticking out past the head of the damselfly in the image above.  Odonates are the only insects that have this style of mouthpart, so if you have a nymph with tails sticking off the back-end and you can see a long, folded mouthpart under the head, you’re looking at a damselfly for sure.

But perhaps you’re looking at an insect in the water and you aren’t able (or willing) to pull it out to look at the mouthpart – what then?  Well, take a look at the location and structure of the gills:

Damselfly gills

Damselfly gills

The three damselfly “tails” are really gills that they use to help them breathe and swim!  They are always located at the back-end of the insect and they tend to be broad and leaf-shaped with varying levels of pointy-ness.  As you’ll see in a moment, the stoneflies and the mayflies have gills in other locations and do not have broad, leaf-like tails.  If you see gills that look like the image above, you’re looking at a damselfly nymph!

Let’s move along to the mayflies:

Mayfly

Mayfly

You should notice some differences between the mayfly and the damselfly right away.  First, look at the tails:

mayfly tails

Mayfly tails

Nothing broad and leaf-like about these tails!  Mayflies have long, filamentous tails, often longer than their bodies.  They also usually have three tails like the damselflies, but some groups only have 2.  Clearly, the flat-headed mayfly in the photo falls into the latter category.  This causes some confusion when distinguishing the mayflies from the stoneflies, as you’ll see in a moment.  However, if you see 3 filamentous tails, you’ve got a mayfly on your hands!

Now let’s take a look at the location of mayfly gills:

mayfly gills

Mayfly gills

The gills  are always attached along the sides or the bottom of the abdomen in the mayflies, never on the thorax or sticking off the back. If you see gills in another location, you’re not looking at a mayfly.  Mayfly gills tend to be broad and leaf-like as in the damselflies, though they may be fringed or sharply pointed in some groups.  They usually have a pair of gills on nearly every abdominal segment, though the exact placement on the abdomen varies by group.

Now we’re left with the stoneflies:

stonefly

Stonefly

Stoneflies and mayflies look a lot alike in most cases.  The mayfly in my photos above is a specialized species adapted for living in fast flowing water, but a lot of mayflies are shaped more like the stonefly depicted here.  How do you tell them apart when the body shapes are similar?  Let’s look at the tails first:

Stonefly "tails"

Stonefly "tails"

Stoneflies always have two tails.  Like the mayflies, they’re long and filamentous.  In some species, these tails are very long.  In others, they’re shorter than the length of the abdomen.  They’re never leaf-like.

Let’s check out the location of the gills too.

Stonefly armpit gills

Stonefly gill location

Unlike the damselflies and mayflies, stonefly gill placement is quite variable.  Many species don’t have gills.  Some species that do have gills don’t get them until they’ve matured to some specific point.  Some species have gills on the abdomen, but if they do they’re located only on the first few abdominal segments and never further down.  (This helps distinguish them from the mayflies, which almost always have gills on the 3rd-6th abdominal segments.)  But in most stoneflies with gills, you’ll find them in their armpits, as indicated in the photo.  Stonefly gills are very different from the broad, flattened gills of damselflies and mayflies.  They typically have a round main stalk with multiple branches.  These are called “finger-like” gills for some reason, but I think the structure is rather similar to the boojum tree, just on a smaller scale:

Boojum Tree

Boojum Tree. Photo by Bernard Gagnon, from http://commons.wikimedia.org/ wiki/File:Boojum_Tree.jpg.

I find that people have the most trouble telling the mayflies and stoneflies apart.  If the mayfly has three tails, no problem!  It’s a mayfly for sure.  However, you have to remember those pesky two-tailed mayflies that throw a wrench in the whole system.  Plus, mayflies are notorious for losing their gills.  If you’re working with preserved specimens, sometimes it’s hard to figure out where the gills did or did not attach.  How then do you tell a two-tailed mayfly with no gills apart from a similarly shaped stonefly with no gills?  It’s easy!  Look at the claws on the legs.  Mayflies have one claw on every foot.  Stoneflies have two.  It couldn’t be simpler.

As with any identification, the more animals you see, the easier this gets.  For those of you who have little experience collecting and identifying insects, getting a specimen IDed to order can be a challenge at times!  Remembering the characteristics of tons of insects can be hard too.  I thus present this handy-dandy chart that summarizes the information I covered above:

Mayfly Damselfly Stonefly
Location of Gills abdomen end of abdomen when present, thorax, base of abdomen
Shape of Gills leaf-like, plate-like, or fringed leaf-like finger-like
Style of Mouthparts chewing chewing + hinged segment folded under head chewing
Number of Tails 2-3 3 2
Shape of Tails filamentous leaf-like filamentous
Number of Claws 1 2 2

If you forget the characteristics of the mayflies, damselflies, and stoneflies, use this chart as a quick reminder of what to look for!

Next up: another thrilling edition of Friday 5!  This week’s will feature 5 places I’ve found a particular type of tiny insect in my home.  Check it out to discover where these little beasts may be lurking!

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Unless otherwise stated, all text, images, and video are copyright © 2011 DragonflyWoman.wordpress.com

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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