To Know a Fly

For last week’s Well-Nigh Wordless Wednesday, I shared a photo of a fly that a lot of people in my city think is a bee.  I can see why people think it is a bee because it’s got that bee-like coloration, but to me the insect all but screams “I’m a fly!”  Nature can be tricky sometimes, especially among the insects.  Thousands of insects look a lot like other insects, many mimicking stinging, biting, or poisonous insects for protection.  Today I’m going to go through the things you should look for to be sure an insect is actually a fly.  This is a fly:

fly in house

A fly, specifically a cactus fly (Copestylum isabellina)

There are several things to look for to determine whether something is a fly or not.  Let’s first consider the name of the order that the flies belong to: Diptera.  As is so frequently the case in biology, the name tells you a lot about the insects within the order.  The prefix di- means two and -ptera refers to wings.  Thus the order Diptera contains insects with two wings, the defining characteristic of the group.  Most insects (with a few non-fly exceptions, including the bizarre order Strepsiptera and some scale insects) have four wings in the winged stage.  Some insects, like the bees and some butterflies, have special structures that hold the fore and hind wings together so that they can look like they only have two wings at first glance (tricky!), but they have four if you look closely.  The flies don’t have hind wings at all!  Instead, they have these little knobby things:

Crane fly halteres

Crane fly

I talked about these structures, called halteres, in a previous post so I won’t say too much about them here.  Briefly, the halteres are remnants of the hind wings in flies and act as gyroscopic organs to tell flies how they are positioned in the air as they fly.  The halteres are likely the reason flies have such amazing control over their flight.  All flies have halteres, though sometimes they’re hidden under the wings and hard to see.

So, adult flies have two wings and two halteres.  Most other adult insects have four wings or no wings and no halteres.  Easy, right?

Not always!  Sometimes it’s hard to get a good look at the wings so that you can count them.  Luckily, the eyes can often provide a clue to whether an insect is a fly or something else.  Flies tend to have very large eyes that wrap around the front and/or sides of their heads, sometimes even meeting in the middle:

fly eyes

Fly eyes

You’ve tried swatting flies, right?  It’s pretty hard to do!  Not only are flies expert fliers, but they also have those giant eyes.  They can see you coming at them with a fly swatter and move out of the way before you squish them.  Not all flies have giant eyes like the fly in the photo, but most of them do.  Bees and wasps tend to have smaller eyes than flies, which can help you distinguish the two groups.

Flies often have strange antennae too.  These are called aristate antennae:

aristate antennae

Aristate antennae

For the most part, if you see an insect with large eyes and antennae like this (with a large, pouch-like structure with a bristle coming off it), you’re looking at a fly and not a bee or a wasp.  This is a rather typical fly with large eyes and aristate antennae:

fly aristate antennae

A fly with large eyes and aristate antennae

Not all flies have aristate antennae though.  Many of them, such as the crane fly, have longer antennae.  Flies tend to have short antennae compared to other insects, however, and they are often very complex in structure.  Bees and wasps have longer, simpler antennae than flies, making the two groups easy to tell apart.

Finally, flies generally have mouthparts designed for sucking liquid food.  The variation in mouhtparts among the flies warrants its own post though, so I’m not going to go into detail here.  If you’ve ever been bitten by a mosquito or a horse fly or watched a house fly lap up food off a dirty plate, you have an idea of how some of the fly mouthparts work.

I want to end this post with a bit of trivia.  Ever wonder why a crane fly, a flesh fly, or a hover fly is considered a true fly while a mayfly, a dragonfly, or a stonefly is not?  They all fly, but they’re not all flies.  Take a close look at how I spelled those names for a hint!  According to traditional entomological naming practices, a true fly in the order Diptera has the “fly” part of its common name separated from the rest of the name while things that are not true flies have the “fly” part tacked on to the end.  Thus, a crane fly is a true fly while a dragonfly is a flying insect belonging to an order other than Diptera.  This distinction is muddled a bit these days with people changing how the names are spelled here and there, but for the most part this trend still holds.  Next time you see the word “fly” separate from the rest of a name, you can be pretty sure that the author is referring to something belonging to the order Diptera and not some random flying insect with four wings.  Consider my favorite quote from Shrek:

“You mighta seen a house fly, maybe even a superfly, but I bet you ain’t never seen a DONKEY FLY!”

If you follow the traditional entomological naming format, one would have to assume from this quote that Donkey is a true fly, NOT a donkey!  To indicate that Donkey is not a fly and is in fact some other flying creature, he should technically be a Donkeyfly.  :)

For more information about flies, I highly recommend Morgan Jackson’s wonderful blog, Biodiversity in Focus.  He takes much better fly photos than I do and his love for flies oozes out of his writing.  Be sure to check it out!


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Predaceous diving beetles and water scavenger beetles – What’s the difference?

It’s been a while since I’ve done an identification post, so it’s time for a new one!  Today I’m going to focus on two aquatic beetle groups that a lot of people have a hard time telling apart from one another: the predaceous diving beetles (family Dytiscidae, the dytiscids) and the water scavenger beetles (family Hydrophilidae, the hydrophilids).  Once you know exactly which parts you should be looking for it’s easy, so let’s jump right in!

For these two groups of beetles, you really need to look at some body structures to properly ID them.  This means it’s a whole lot easier to ID them if you take them out of the water for a close look.  However, the body shape may give you a clue.  Let’s look at the dytiscid first:

Dytiscid lateral

Dytiscid, side view

The dytiscids are extremely streamlined and smooth.  Notice how the top of the beetle is rather domed?  And how the bottom is rounded?  These are characteristics of most of the dytiscids that allow them to swim very efficiently.  If you cut one in half across the middle of the abdomen, the shape of the cross-section would be nearly oval.  Now compare this to the hydrophilid:

Hydrophilid lateral

Hydrophilid, side view

The hydrophilids are also streamlined and smooth, but they are a different shape.  Notice how the underside of the beetle is mostly flat, compared to the broadly rounded belly of the dytiscid.  Also, you can’t see it in this view, but the beetle is shaped like an inverted V  or U along the abdomen so that the eltyra slope down away from the center line of the bug like the keel of a boat.  If you cut one of these beetles in half, it would be roughly triangular of semi-circular in cross-section.  So, the dytiscids tend to be very curvy while the hydrophilids are more angular and have some flat edges.  This doesn’t hold true for every member of either group, but it is a general trend.  With practice, you’ll start to notice general body shapes that will let you identify them without taking them out of the water.

Now let’s flip the beetles over for a moment.  First, I’ll draw your attention this structure:

Hydrophilid spike

Hydrophilid spine

Many hydrophilids have a long, sharp spine that runs down the center of the thorax and over the base of the abdomen.  It can be thick and heavy like this one or long and slender.  Not all hydrophilids have these spines, but if you see the spine, you can be sure that it is a hydrophilid and not a dytiscid.  Dead giveaway!  But let’s pretend for a moment that this beetle is one of the hydrophilids that don’t have a spine.  Then we need to look at a different part.

As you probably know, the abdomen and the thorax of insects are made up of several subsections.  The thorax is made up of three sections, with one pair of legs attached to each.  The abdomen is historically made up of 11 sections, but many insect groups have combined sections and now have less than 11.  The first section of the abdomen directly behind the thorax (also called A1) is an important section to look at when identifying beetles because it helps you determine which of the four suborders the beetle in question belongs to.  Dytiscids belong to the suborder Adephaga while the hydrophilids belong to the Polyphaga, so their A1 sections look different on the underside of their bodies.  Let’s look at this section on the hydrophilid first:

Hydrophilid A1

Hydrophilid A1

I know it’s hard to see in the photo, but the legs of this beetle sit on top of A1 and do not split it into two parts.  You can follow the line of the A1 section closest to the back end of the beetle across the entire beetle without interruption.  The dytiscids are different:

Dytiscid A1

Dytiscid A1

Their hind legs break the A1 section apart so that part of the section lies on either side of the legs.  In these beetles, you cannot follow the line of A1 closest to the back end of the bug all the way across the beetle without interruption because the legs get in the way.  If you look closely at the photo (which is admittedly not as clear as it could be), you can see the first section of the legs (those two little bumps the rest of the legs are attached to) extending beyond the A1 section and the two parts of A1 on either side.

It’s going to be impossible to figure out whether a beetle swimming in the water has a broken or unbroken A1.  Even if you scoop the beetle out of the water though, this structure can be difficult to see, especially on some of the smaller beetles.  My students have a very hard time with this characteristic and a lot of them never quite figure it out.  This is where the antennae come in handy!

The antennae of dytiscids and hydrophilids are very different, so it’s easy to tell the two apart.  Finding the antennae on these beetles is an entirely different matter though!  Insect mouthparts have a lot of little dangly bits called palps and it just so happens that a lot of aquatic beetles have their palps sticking out right where you’d expect to see antennae.  Let’s take a look, starting with the dytiscid.  This is not an antenna:

Dytiscid not antenna

Dytiscid, not antenna

Nor is this:

Dytiscid not antenna

Dytiscid, also not antenna

The hydrophilids aren’t any easier.  This is definitely not an antenna:

Hydrophilid, not antenna

Hydrophilid, not antenna

Nearly all of my students eventually try to run through the identification key using these parts and invariably end up in the wrong place, especially with the hydrophilids.  I don’t blame them!  The palps in both beetle groups are in the right place to be antennae and look a lot like what you’d expect an antenna to look like.  In my experience, aquatic beetles are sneaky buggers and like to hide their antennae, especially if you preserve them for a collection or for identification later.  You frequently find them folded down under their heads alongside the inner margin of their eyes.   This is definitely true in the dytiscid pictured above.  I pulled the antenna out so you can actually see it in this photo:

Dytiscid antenna

Dytiscid antenna

See why I say they’re sneaky?!  These antennae look a lot like the palps, only longer.  The antennae of dytiscids are filamentous, which means that each segment of the antenna is about the same length and width as the segment before and after.  Now let’s compare that to the hydrophilid antenna:

Hydrophilid antenna

Hydrophilid antenna

Hydrophilid antennae don’t look anything like the palps!  They have clubbed antennae, which means that the segments near the tip of the antenna are much wider than the ones near the base.  This means that the antennae of the hydrophilids look absolutely nothing like the antennae of the dytiscids!  If you’re looking at the right part, the actual antennae and not the palps, it is very easy to tell these two beetles apart.  Clubbed antennae = hydrophilid.  Filamentous antennae = dytiscid.  Simple!

You often need to look at a combination of these characters to be sure you’ve correctly identified one of these beetles, but it’s not too bad when you know what to look for.  I’ll end the post with a handy dandy chart summing up what I covered above.  Happy identifying!

Characteristic Dytiscidae
(predaceous diving beetles)
(water scavenger beetles)
Shape of cross-section of abdomen Approximately oval Approximately triangular or semi-circular
Spine along center of thorax Never has spine Spine often present
A1 segment Split into 2 parts by legs Continuous, not split by legs
Antennae Filamentous Clubbed


Unless otherwise stated, all text, images, and video are copyright © 2011

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:



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:



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 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!


Unless otherwise stated, all text, images, and video are copyright © 2011