All bugs are insects, but not all insects are bugs

I have always called insects bugs.  I know I’m using the wrong word most of the time since the word bug refers to a specific group of insects.  If I’m not talking about that one group, using the word bug instead of insect is technically incorrect.  If you’re around many entomologists , using the word bug to describe an insect that is not actually a bug is like scraping your fingernails down a chalkboard – highly annoying.  I don’t know. I’ve always found this attitude somewhat pretentious and counterproductive.  As an entomologist myself, I like to promote insect awareness and get people interested in what I do.   It’s hard to get people interested in what you do when they feel belittled because you corrected them for using a word they didn’t even know was wrong.  I use the word bug for all insects, even though I know I am wrong to use it most of the time (and yes, I have been corrected by other entomologists myself!), for several reasons:

1) It’s easier to say bug than insect.

2) I find people relate better to stories about bugs than stories about insects, especially when they know I’m an entomologist or if they are kids.

3) I happen to work with insects that are, in fact, bugs.  When you constantly use the word bug to describe your work, even in scientific papers, you get in the habit of using the word bug all the time.

Really, though, how many people who aren’t entomologists know the difference between an insect that is a bug and an insect that is not?  It’s a subtle distinction and most people don’t have any reason to learn the difference.  For those of you who don’t know what makes a bug a bug, allow me to enlighten you!

The insects are divided into about 25 smaller groups called orders.  All insects belonging to an order share certain traits, which is why they are grouped together in the first place.  A bug is an insect that belongs to the order Hemiptera.  Members of the order Hemiptera are also called true bugs, hence the word bug.  If you’re being technical, only insects that are true bugs should be called bugs and everything else should just be called an insect.  But how do you tell a true bug apart from other insects?  All true bugs share two main traits: hemielytra and piercing-sucking mouthparts.

The word hemielytra refers to the specialized top pair of wings (forewings) of the true bugs.  Most insects have 4 wings and true bugs are no exception.  Some insects, like beetles, have hardened forewings that protect the more fragile hindwings underneath.  These are called elytra.  Take a look at this palo verde beetle’s elytra:

palo verde beetle

A beetle. The arrow points to this beetle's elytra.

The true bugs have hemielytra, not elytra.  The forewings of bugs are only hard for part of their length instead of the entire length.  The upper part is thick and leathery and the lower part is membranous, about the same texture as the hind wings underneath.  Look at the forewings of this giant water bug and look for the differences in these wings compared to those of the beetle pictured above:


Giant water bug. The arrow points to the hemielytra.

See the dark section of the wing toward the back end of the bug (to the right of the tip of the arrow)?  That’s the membranous part.  The rest of the wing is the thickened, leathery part and a completely different texture.  True bugs often have a sort of V-shape to their wings.  See the V just to the left of the tip of the arrow?  For the most part, if you see this V-shape in the wings of an insect, you’re looking at a true bug.  (Note: I’ll talk about that big, long piece sticking off the back of the giant water bug in a future post.)

The other trait that all true bugs have is piercing-sucking mouthparts.  Different insects have different types of mouthparts, but most people are familiar with insect chewing mouthparts.  This is what caterpillars, beetles, and grasshoppers have.  They use these mouthparts to grind their food up before swallowing it as the first step in digestion.  Look at the chewing mouthparts of the palo verde beetle:

palo verde beetle head

Palo verde beetle head. The arrow points to the chewing mouthparts.

Palo verde beetles have really big mouthparts that are easy to see.  These things can actually bite quite hard, even drawing blood if they get you in the right place!

In contrast, true bugs have piercing-sucking mouthparts.  Instead of putting food into their mouths and grinding it up the way beetles do (and humans too!), they inject digestive chemicals into whatever they want to eat using their mouthparts (hence the piercing part of “piercing-sucking mouthparts”).  These chemicals break the food down into a soupy mess which the bugs then suck into their mouths through their mouthparts (that’s the sucking part of “piercing-sucking mouthparts”).  It’s a lot like eating your food with a straw!  Check out the mouthparts on the giant water bug:

true bug bealk

Giant water bug mouthparts. The arrow points to the piercing-sucking mouthparts.

The piercing-sucking mouthparts of true bugs are often called beaks because they are long and pointy like a bird’s beak.  Can you see the similarity?  Unlike birds, bugs can fold their beaks down under their heads, which is how the beak of the giant water bug above is positioned.

An interesting aside: Bugs can have a really nasty bite.  That little straw-like mouthpart might not look that impressive, but remember how bugs eat: they inject digestive chemicals into their food.  If you handle one improperly or startle one, those same digestive chemicals can end up in your fingers!  They can’t do any lasting damage, but it can hurt a lot as it digests some of your muscle.  It’s usually a good idea to handle true bugs with care.

So now you know!  A bug is an insect that has hemielytra and piercing-sucking mouthparts.  The next time you want to get technical and use the word bug for an actual bug, think about whether the insect in question has that V-shape to its wings and a long beak folded down under its head.  If so, it’s a bug and you can use the word bug without fear or hesitation!  If not, you can still call it a bug.  The choice is yours after all.  Just expect any entomologists nearby who happen to overhear you to correct you.


Text and images copyright © 2009

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:


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.


Text and images copyright © 2009


Abdomen: the back section of the insects, behind the point of attachment for the wings and legs.

Ametabolous: in insects, a type of metamorphosis in which all stages but the egg look the same apart from size

Boundary layer: the layer of low flow that occurs when fluids (water, air, etc) flow over the surface of objects in the stream.  See the Water Penny entry for a more detailed description of how a boundary layer works.

Bug: an insect that belongs to the order Hemiptera.  See the post about bugs for more information.

Complete metamorphosis: see holometabolous

Decomposer: an organism that helps break down a dead organism

Elytra: the hardened forewings of beetles.  Elytra means “sheath” in Greek.  See the post about bugs for more information.

Entomologist: a scientist who studies insects

Exoskeleton: the hard, tough outer covering of insects

Exothermic: organisms that are largely unable to produce their own body heat and regulate their temperature mostly through behavioral means

Flier: in odonates, the behavioral group which includes insects that rarely land and observe their territories on the wing.  Contrast with perchers.

Forewings: the front pair of wings in insects with wings.

Hemielytra: the specialized forewings of the true bugs (Order: Hemiptera) that are leathery on the upper half and membranous on the lower half.  Hemielytra means “half sheath” in Greek.  See the post about bugs for more information.

Hemimetabolous: insects that undergo incomplete metamorphosis and have a life cycle that consists of egg, nymph, and adult

Hemiptera: the order name for the group containing the true bugs.  All members of this group have hemielytra and piercing-sucking mouthparts.

Hindwings: the back pair of wings in insects with wings

Holometabolous: insects that undergo complete metamorphosis and have a life cycle that consists of egg, larva, pupa, and adult

Incomplete metatmorphosis: see hemimetabolous

Instar: the stage between molts in immature insects

Immature: an organism that is not fully developed and not reproductively capable

Insect: an animal that has 3 body segments (head, thorax, abdomen), six legs, two pairs of wings (may be missing in some insects), and one pair of antennae

Larva: the immature stage of holometabolous insects

Metamorphosis: in insects, a process by which an insect changes from one stage and/or form to another.  See also holometabolous, hemimetabolous, ametabolous, and the entry on metamorphosis in insects.

Molt: in insects, breaking out of an exoskeleton that has grown too small and expanding the new, larger exoskeleton beneath; the stages between molts are called instars

Nymph: the immature stage of hemimetabolous insects

Odonata: the order name for the group containing the dragonflies and damselflies.

Odonate: an insect belonging to the order Odonata

Parasite: an organism that lives off another organism to the detriment of the host organism and the benefit of the parasite

Percher: in odonates, the behavioral group where the insects sit on perches and observe their territories while resting rather than on the wing.  Contrast with flier.

Piercing-sucking mouthparts: the specialized mouthparts of true bugs that inject digestive chemicals into their food, then suck the food up into their mouths after the chemicals have liquefied it

Predator: an animal that eats another animal

Prey: an animal that is eaten by another animal

Pruinose: in dragonflies, individuals with a waxy and/or dusty looking coating on their bodies

Pupa: in insects, the developmental stage between larva and adult where a complete rearrangement of tissues occurs

Sclerotization: in insects, a process by which a soft, flexible exoskeleton is hardened into a hard and/or tough covering

Spore: the reproductive body of molds that allows molds to spread to new areas

Territory: an area containing necessary resources (food, access to mates, habitat, etc) that an animal is able to protect sufficiently to exclude other individuals at will

Thorax: the middle segment of insects, where the wings and legs are attached


Text copyright © 2009

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.


Text and images copyright © 2009

Collecting in the White Mountains

I went on a camping/fishing trip with a good friend for part of last week.  The fishing ended up being really bad in the Black River (my friend got only one 6 inch trout and she did better than anyone else we talked to on the river) and the last day we were there it was very windy, so we spent a good part of our time in the mountains collecting aquatic insects.  Aside from one stream that was very cold and located in a very windy location, collecting was productive and pleasant – a great way to spend some time when we were unable to fish!

I’ve been to the White Mountains with my Aquatic Entomology students several times, but we have always gone in the spring.  I was excited to be able to collect during the summer during our trip last week.  Aquatic insect communities are known to vary greatly from season to season, so the insects that are available in the spring might be gone by the time summer comes around.  Alternatively, insects that are very common in summer might not be present during the spring.

We caught many of the same insects that my students and I caught a few months ago, but my friend and I did get some new things.  I was most excited about getting dixid fly larvae.  My students are expected to learn the dixids in class, but neither I nor my students have ever collected them during our field trips.  I found my dixids hanging onto the submerged portions of the grasses that were growing along the banks of the stream.  I couldn’t have been happier to find them!  It would have made the trip worth it for me even if the rest of the trip had been horrible.  My friend was most excited about the water pennies because she had heard about them from one of our professors, but hadn’t ever seen them.

Over the next few posts, I’ll write about some of the insects that we found on our trip.  The White Mountains have some really fascinating insects, so I hope you will enjoy learning more about them!


Text copyright © 2009