Why Does A Stinkbug Stink?

“Look, dad!” my son shouts, voice excited as he points. “A stinkbug!”

We’re walking up the front stairs to our condo. I glance in the direction he’s pointing, and see nothing.  “Cool,” I tell him.

“Why do stinkbugs stink?” he asks.

I unlock the front door. “I don’t know,” I answer. “I think maybe they spray stinky chemicals.”

What is a stinkbug?

Getting information on stinkbugs in general proved to be a little difficult, as Google just keeps giving me hits for the Brown Marmorated Stink Bug (Halyomorpha halys), an insect native to China and Japan that is an invasive species in North America.  Wikipedia, when you do a search for ‘stink bugs’, takes you to a page about family Pentatomidae, a family of insects belonging to order Hemiptera.  Shield bugs and stink bugs belong to Hemiptera, with stink bugs comprising Pentatomidae.

I couldn’t find a list of the species native to North America.  Heck, I couldn’t even find a list of the species of Pentatomidae, end of statement.  But, here’s a brown stink bug that is native to this continent:


That particular bug is Euschistus servus, and it is a common pest throughout southern Canada and the northern United States.  This particular bug is about half an inch long (10 to 15 mm), and lays a bright yellow egg mass (around 60 eggs) that turns pinkish as they get ready to hatch (between four and five weeks after being laid).  Larval stink bugs require about 29 days to reach maturity.  Adults are able to fly, and are primarily omnivores.

Brown stink bugs often feed on the vegetative parts, flowers, stems and foliage of the plant, as well as the seed, nut or fruit, and this makes them important pests of many crops.

Brown stink bugs are found on a variety of hosts, such as shrubs, vines, many broadleaf weeds, especially legumes, as well as cultivated crops such as corn, soybean, sorghum, okra, millet, snap beans, peas and cotton.

Do stinkbugs stink?

Yes, they do.  Here’s what Orkin has to say on the subject:

Their name comes from their smelly defense mechanism. Stink bugs have the ability to emit a strong deterring odor, from their body glands, whenever they feel threatened or injured — much like how a skunk defends itself. The smell varies depending on the species and the person’s olfactory senses, but it has often been compared to strong herbs and spices like cilantro and coriander.

Interesting enough, the composition of the odor is comprised of chemicals commonly used as food additives and is present in cilantro. This smell can linger for hours so, if possible, try to avoid stink bugs or carefully sweep or vacuum them up if they have entered your house, unless you want a face full of intense-smelling herbs and spices.

A more detailed description of the specific chemicals used by one particular species of stink bug may be found in Chemical Defense in the Stink Bug Cosmopepla bimaculata, an article published in the Journal of Chemical Ecology back in 1999 and freely available on the internet.  From the abstract,

Adult Cosmopepla bimaculata discharge a volatile secretion from paired ventral metathoracic glands (MIG) when disturbed. Collected volatiles were similar in both sexes and consisted of n-tridecane (67%), (E)-2-decenal (12%), (E)-2-decenyl acetate (12%), (E)-2-hexenal (3%), hexyl acetate (2%), n-dodecane (2%), a tridecene isomer (l%), and n-undecane, n-tetradecane, and n-pentadecane (all <1%). In addition. undisturbed males produced a novel insect compound. (E)-8-heneicosene. whose function is unknown.

No, I don’t know what any of those are.  But every resource I found agrees on a few things about stink bug spray:

  1. It smells vaguely of spices and cucumber
  2. Despite that, it does not smell good.
  3. No, really, it stinks like old garbage.

So, to answer the original question, why do stink bugs stink?  Because they don’t want to get eaten.  Again, as the abstract of Chemical Defense in the Stink Bug Cosmopepla bimaculata put it, “In feeding trials, killdeer (Charadrius vociferus), starlings (Sturnus vulgaris), robins (Turdus migratorius), and anole lizards (Anolis ccarolinensis) rejected or demonstrated aversion to feeding on the bugs. Furthermore. bugs that lacked the secretion were more susceptible to predation than bugs with secretion, suggesting that the secretion functions in defense against predators.”



What Is Glass?

My son and I were in my car when this happened. I was on my way to get an estimate on some body work for my car, and this got my son interested in things about the car. That combined with his interest in ice and cold, and he starts telling me about ice. “And then they melt the ice, and you can see through it!” he announces, because they’d been looking at ice. Then he asks, “how do they make the ice into windows?”

“They don’t,” I tell him. “Windows are made of glass.”

“Glass is made of ice!” he tells me. I… guess it makes sense? Glass and ice can look very similar, after all.

“No,” I tell him. “Glass is made of sand.”

He laughs at me. “No it’s not!”

“Yes, it is.”

“Then,” he asks, with the air of a prosecutor delivering the final damning bit of evidence, “how do you see through it?”

It’s a good question, really. The idea that sand – something he’s seen a lot of and [i]knows[/i] is opaque – can be seen through is really absurd sounding. I don’t have a good answer for that, really.

What is glass?


Dictionary.com defines glass as:


  1. a hard, brittle, noncrystalline, more or less transparent substance produced by fusion, usually consisting of mutually dissolved silica and silicates that also contain soda and lime, as in the ordinary variety used for windows and bottles.
  2. any artificial or natural substance having similar properties and composition, as fused borax, obsidian, or the like.

The Corning Museum of Glass offers this definition instead:

Glass is a rigid material formed by heating a mixture of dry materials to a viscous state, then cooling the ingredients fast enough to prevent a regular crystalline structure. As the glass cools, the atoms become locked in a disordered state like a liquid before they can form into the perfect crystal arrangement of a solid. Being neither a liquid nor a solid, but sharing the qualities of both, glass is its own state of matter.

Intuitively, I think most of us know what glass is. We interact with it all the time, in the form of windows if nothing else. Our everyday experience of glass tells us that it is hard (but obviously not indestructible, particularly when thin) and transparent.

How is glass made?

British Glass has a page titled All About Glass, which provides a general description of how glass is made:

Glass is made by melting together several minerals at very high temperatures. Silica in the form of sand is the main ingredient and this is combined with soda ash and limestone and melted in a furnace at temperatures of 1700°C. Other materials can be added to produce different colours or properties. Glass can also be coated, heat-treated, engraved or decorated. Whilst still molten, glass can be manipulated to form packaging, car windscreens, glazing or numerous other products. Depending on the end use, the composition of the glass and the rate at which it is allowed to cool will vary, as these two factors are crucial in obtaining the properties the glassmaker is seeking to achieve.

So, in other words, you melt sand with other stuff. Going back to the Corning Museum of Glass, they explain that “typical glass contains formers, fluxes, and stabilizers.” A former is the thing the glass is made of (silicon dioxide, aka silica, in standard windows and bottles). A flux is something that lowers the melting temperature of the former – soda ash (sodium carbonate) is a flux, as is potash (potassium carbonate). A stabilizer is something that makes the glass stronger (and often water resistant) – limestone (a form of calcium carbonate) is a stabilizer.

Interestingly, the page also states that without the stabilizer water will dissolve glass. Put this firmly into the category of Things I REALLY Didn’t Know.

Window glass is generally 73.6% silica, 16% soda ash, 5.2% limestone, 0.6% potash, and 4.6% other materials. By contrast, your glass baking dish is 80% silica, 4% soda ash, 0.4% potash, 2% alumina, and 13% boric oxide. Your fine lead crystal is 35% silica, 7.2% potash, and 58% lead oxide.

Why can we see through it?

Because it’s transparent.

No, really.

No, seriously. Transparent Glass doesn’t absorb photons of light in the visible spectrum. A glass like volcanic obsidian, on the other hand, does. In fact, it absorbs nearly all the light in the visible spectrum, which is why it looks black.

Could you be more specific?

I’ll try. Let me warn you in advance that I’m leaning heavily on Transparency and translucency from Wikipedia for this.

Any given wavelength of electromagnetic energy will be either reflected, absorbed, or transmitted by a given material. The human eye interprets reflected (visible) wavelengths as color, radar dishes and detect reflected radio waves to calculate distance and direction, and so on. Absorbed wavelengths increase the temperature of the material, because they’re pumping energy into the material. Transmitted wavelengths pass through unhindered to a greater or lesser degree. A material that transmits no portion of the visible electromagnetic spectrum is optically opaque, and a material that transmits all of the visible spectrum is optically transparent. Here’s a good quote from the article:

The atoms that bind together to make the molecules of any particular substance contain a number of electrons (given by the atomic number Z in the periodic chart). Recall that all light waves are electromagnetic in origin. Thus they are affected strongly when coming into contact with negatively charged electrons in matter. When photons (individual packets of light energy) come in contact with the valence electrons of atom, one of several things can and will occur:

  • A molecule absorbs the photon, some of the energy may be lost via luminescence, fluorescence and phosphorescence.
  • A molecule absorbs the photon which results in reflection or scattering.
  • A molecule cannot absorb the energy of the photon and the photon continues on its path. This results in transmission (provided no other absorption mechanisms are active).

Most of the time, it is a combination of the above that happens to the light that hits an object. The states in different materials vary in the range of energy that they can absorb. Most glasses, for example, block ultraviolet (UV) light. What happens is the electrons in the glass absorb the energy of the photons in the UV range while ignoring the weaker energy of photons in the visible light spectrum. But there are also existing special glass types, like special types of borosilicate glass or quartz that are UV-permeable and thus allow a high transmission of ultra violet light.

Interestingly enough, when I first wrote this out I wrote the following statement: “a material that transmits some portion is optically translucent to a greater or lesser degree”. This is actually incorrect. Optically translucent objects are considered optically transparent, but there is a quality to the material that prevents image formation. That is, visible light will pass through but it is scattered in such a way that you can’t make out what the source image on the other side is.

So. Glass is transparent because the molecules that make up the glass literally cannot absorb most of the electromagnetic energy in the visible spectrum (although it can reflect it, which is why you can sometimes see yourself in glass).

Is it a solid or a liquid?

You hear quite often that glass is a really slow-moving liquid. It isn’t. Glass is an amorphous solid, and to understand what that means we’ll need to explain what a crystalline solid is first.

Please do.

A crystalline solid is also called a crystal, and it is a solid where the atoms or molecules that make up the substance are arranged in a highly organized and periodic structure. Think of the atoms or molecules as the bricks in a brick wall or Lego structure. Individual crystalline solids can get really, really big.

A real picture, from the Naica Crystal Cave

Most solid objects are polycrystalline, meaning they are made up of multiple crystals. Each crystal can be arranged in a haphazard fashion relative to the other crystals – imagine taking that Lego structure above, breaking it into chunks, and then covering those chunks with glue and tossing them in a box. (Clearly, this analogy breaks down quickly. Just bear in mind that we’re imagining structures here, not chemical properties.)

Amorphous solids have no organization to the component atoms or molecules. Using the Lego analogy, they’re a sack full of individual Lego bricks (that were all coated with glue). They’re still solids – the individual atoms still have strong connectivity – but they have some liquid-like properties. They can flow, slightly, as the disorganized components try to arrange themselves into crystalline structures. As the Scientific American article I linked to notes, though, this is not why some antique glass looks thicker at the bottom. “[A]ncient Egyptian vessels have none of this sagging, says Robert Brill, an antique glass researcher at the Corning Museum of Glass in Corning, N.Y. Furthermore, cathedral glass should not flow because it is hundreds of degrees below its glass-transition temperature, Ediger adds. A mathematical model shows it would take longer than the universe has existed for room temperature cathedral glass to rearrange itself to appear melted.”

When Ice Is on Fire, Does The Ice Melt?


“Yes, son?”

“When ice is on fire, does the ice melt?”





It was one of those questions, the random sort of question a five-year-old (he just turned six last week, but he was five when he asked it) will ask. I don’t have any idea what prompted the question, or where it came from. But, the more I thought about it, the more interesting it sounded.

Can ice burn?

Well, it depends on what you mean by ‘ice’ and by ‘burn’. Here’s how Merriam-Webster defines it:

Full Definition of ice

  1. frozen water; a sheet or stretch of ice
  2. a substance resembling ice; especially : the solid state of a substance usually found as a gas or liquid <ammonia ice in the rings of Saturn
  3.  a state of coldness (as from formality or reserve)
  4. a frozen dessert containing a flavoring (as fruit juice); especially : one containing no milk or cream; British : a serving of ice cream
  5. slang : diamonds; broadly : jewelry
  6. an undercover premium paid to a theater employee for choice theater tickets
  7. methamphetamine in the form of crystals of its hydrochloride salt C10H15N‧HCI when used illicitly for smoking —called also crystal, crystal meth

For these purposes, we’ll stick with the first two definitions. ‘Frozen water’ and ‘the solid state of a substance usually found as gas or liquid’.

Burning, more properly called a combustion reaction, is a little more complicated. There’s an entire subfield of chemistry called thermochemistry that deals with burning (or, more properly, the energy release from a combustion reaction). In general, though, you need a compound to combust and an oxidant to react with the combusting compound, and some energy to get it started. The oxidant and the combusting compound then combine in a chemical reaction to produce one or more new compounds, and since the reaction is exothermic the process of making the new compound(s) generates more energy than it gives off.

Yes, that does mean that once you get a combustion reaction started it will continue as long as it has combustable compounds and oxidants. That’s why fire spreads.

So, can ice burn?

Water does not burn particularly well, because it’s already a product of a combustion reaction. Burning just about anything with hydrogen produces water. Burning hydrocarbon (sugar, wood, meat, alcohol, whatever) produces carbon dioxide and water and heat, burning hydrogen generates water and heat, burning acetylene gas forms carbon dioxide and water, and so on. Because it is the product of a combustion reaction, water is already at a low energy. You’d have to add energy (breaking the chemical bonds between the hydrogen and oxygen) to burn it, and then you’re not actually burning the water. You’re burning the hydrogen gas you released from the water.

Other ices can burn, however. Ethanol (a hydrocarbon) burns, and it freezes at -114 degrees Celsius (-173.2 degrees Fahrenheit). Gasoline (another hydrocarbon) certainly burns, and will freeze between -40 degrees and -60 degrees Celsius (-40 to -76 degrees Fahrenheit), depending on the exact properties of the substances in the gasoline. Acetone (nail polish remover) is another hydrocarbon, which freezes at -95 degrees Celsius (-139 degrees Fahrenheit) and which burns. Other liquids that burn also exist, obviously. And most of them require dangerously cold temperatures to freeze, and then will have fire. Use caution, lots of caution, if you actually plan to try this at home. And then, having exercised caution, you probably shouldn’t try this at home.

But, I really want burning water ice. For reasons.

Well, there’s a couple of different things you can do. The first is to put a layer of (water) ice cubes on top of a layer of calcium carbide. As the ice melts, the water reacts with the calcium carbide to produce hydrogen and acetylene gas, both of which will burn – old fashioned mining lamps actually used this reaction (something I learned when I went spelunking as a Boy Scout). The results look like this:

Another option is to pour alcohol on top of your ice, and light it up. The ice won’t burn, but there will be flames on the ice.

Regardless of which one of these you do, if you do one of them, please exercise caution. Lots of caution, because you’re playing with fire.

But… does the ice melt?

Yes. Because there is something hot near the ice, which will cause it to melt.

Why Does Oil Make A Rainbow?

It’s summer, and it’s just finished raining, and we’re walking across a parking lot on our way back to the car from running an errand. My son is, as five-year-olds are wont to do, taking the opportunity to jump in puddles and laugh as they splash. Suddenly, he stops. “Look!” he cries, pointing at the ground. “There’s a rainbow!”


I go and look. Sure enough, there’s a small puddle with a thin film of oil slicking the top. I nod at him, and looks at it again. “Why is there a rainbow on the ground?” he asks.

“There’s oil on the puddle.”

He looks at it for a moment, then looks up at me. “How does oil make a rainbow?”

Yeah. You’ve just stumped me son.

So, what’s up?


Specifically, PHYSICS!

Could you… elaborate? Just a little?

It all starts with the nature of light, which as we all know is simultaneously a particle and a wave. Here’s how Professor Emeritis Dinesh O. Shah explains it in Scientific American:

Light reflects upward both from the top of the oil film and from the underlying interface between the oil and the water; the path length (the distance from the reflection to your eye) is slightly different depending on whether the returned light comes from the top or from the bottom of the oil film. If the difference in path length is an integral multiple of the wavelength of the light, rays reflected from the two locations will reinforce each other, a process called constructive interference. If, however, the rays reach your eye out of step, they will cancel each other out due to destructive interference.


Sunlight contains all the colors of the rainbow–the famous ROYGBIV (red, orange, yellow, green, blue, indigo, violet). Each color of light has a different wavelength. Hence, a given disparity in the path length will cause constructive interference of certain colors, whereas other colors will not be observed because of destructive interference. Because the oil film gradually thins from its center to its periphery, different bands of the oil slick produce different colors.

Constructive and destructive interference?

Think of the classic sine curve you had to draw (we all had to draw them) when you took algebra or pre-calculus in high school. The wavy line that looks kind of like a snake. That thing. It’s the most common way to represent a wave of any sort, with the height of the “hills” of the wave representing amplitude (how intense the wave is – brightness for light, loudness for sound, and so on) and how close together the “hills” are representing frequency of the wave (how energetic it is).

Now, think hard about that math class. Do you remember what happens if you add two sine waves together? It changes the nature of the wave. Two identical waves will end up dobuling the amplitude (making it brighter or louder), while two utterly opposite waves will flatten the wave into a line. Check out the image below, if that isn’t clear.


The amplitude of the light is how bright it is, so the areas of the puddle where the reflected light interferes constructively you can see it clearly while you can barely see it when the light interferes destructively. The colors shift because the frequency of the reflected light (which determines the color) vary with the thickness of the oil on the water and the angle at which the light hits your eye.