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:

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:

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:

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

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:

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:

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.

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:

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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