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In my story, there is a type of bee that releases a pheromone during mating season that is visible at night in brightly lit areas. This is similar to how you can see rain more easily at night if you look near a bright light source such as a streetlight.

After looking up pheromones, I understand that they are chemicals that are released as a way of triggering responses in members of the same species. They vary greatly in their uses: warning systems; mating calls; leaving trails; and so on.

Because these pheromones are often released into the air, I gathered that they must consist of particles of some kind.

To answer this question, I decided I needed the following information:

  • Just how big are pheromone particles, on average?

This is vastly important to the question. I haven't been able to find anything on this. I was hoping to find the minimum and maximum sizes of different kinds of insect pheromones and then using their average as my baseline. In addition, surely these chemicals are composed of different molecules of varying sizes, so I would need to take that into consideration as well.

I was able to find this article, but the size of it and the amount of technical jargon stressed me out and simply hurt my brain.

So, assuming we get past the first hurdle of figuring out the particle size variation, the following question comes up:

  • How dense would the pheromones need to be to see them?

After deciding on a size for the particles, I would need to know just how much of the pheromone these bees would need to release in order for the particles to be visible to a human under the nighttime lighting conditions described above.

For this question, let's define "visible" as being "very noticeable" or, for the sake of measurable criteria, 50% opaque or more.

So, fellow Worldbuilders, after trying to figure this out for several hours, I now turn my question to you:

Can an insect's pheromones be visible?

If the answer is yes, under what conditions can they become visible?

UPDATE: I thought of another way for them to be visible: The amount of pheromone you look through. If it stacks up, similar to fog in the distance, does this change its possibility? Perhaps for this version, even daylight viewing would be possible?

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  • $\begingroup$ it doesn't matter how big the particles are, you are never going to see one with the naked eye, only the density would matter $\endgroup$ – BKlassen Oct 11 '19 at 21:43
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    $\begingroup$ With the right instrument, of course they would be visible. We are able to image such molecules with atomic force microscopes, for example. With the naked eye, no, they wouldn't be visible. Think of perfumes -- they work similar to pheromones for humans; you can smell them but you cannot see them: they consist of a very small amount of substance, very finely dispersed. $\endgroup$ – AlexP Oct 11 '19 at 21:56
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    $\begingroup$ Judging from the paper the size is molecular. As in, made up of elements, so you'd need Avogadro's Number of these things to be visible. And the easiest way to make them visible is to cheat, and have another chemical (like say, condensed water vapor) released alongside the pheromones. $\endgroup$ – Halfthawed Oct 11 '19 at 22:03
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    $\begingroup$ @Halfthawed: If we are into cheating, one could imagine a genetically engineered strain of insects which release fluorescent pheromones, which could be made to glow and become visible in a very very dark room when lit by ultraviolet light. $\endgroup$ – AlexP Oct 11 '19 at 22:05
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    $\begingroup$ As to "concentration", insects have exquisitely fine tuned receptors which can be triggered by one single molecule of the right pheromone. For comparison, a cubic centimeter of air contains 26,500,000,000,000,000,000 molecules of oxygen, nitrogen, argon, etc. $\endgroup$ – AlexP Oct 11 '19 at 22:13
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Yes, but it's complicated

Yes, pheromones can be visible. How much is needed, and what we mean by "visible" is a little more complicated. This can be a bit difficult to understand, so strap in!

Pheromone Secretion

Pheromones are almost never secreted alone (I suppose it is possible, but I've never heard of such a thing). Pardon the crass analogies, but consider urea in urine or sperm in seminal fluid. Urine is approximately 95% water, but that other 5% is the whole point - your kidneys need to excrete salts, proteins, hormones, etc. But getting these solids out of your body is not fun without water. Ask anyone who has had a kidney stone. It's hell. The same goes for semen where only 2-5% is actually the sperm; the rest contains supportive materials: salts, sugars, proteins, vitamins (all to keep the sperm alive and moving). The wikipedia article is a good read on this.

It's somewhat similar with pheromones; they are complex substances, as one article states (regarding trail-following pheromones in ants):

These results on the trail pheromone of A. texana lead to the obvious conclusion that it is comprised of a mixture of compounds, all of which release trail following, but not all the compounds are equally active.

In other words, phermones are secreted with far more than the "active ingredient". To make things more complicated, these other "ingredients" in different forms can change how the pheromone is received or interpreted by others. This is, in part, why studying pheromones are so complicated. The pheromone may come with a few fatty acids, but how those acids are structured can change the signal communicated.

So What?

The density of the pheromones themselves are not relevant; the density of everything secreted is what you see (just like with urine or semen).

What this means is your specific type of bee can simply secret a pheromone along with other material which creates a type of observable mist or fog. Anything that's large enough to refract or reflect light is enough.

Quantity

One little bee may release a pheromone that's "visible", but bees are tiny; the issue here will be releasing enough pheromones. The advantage you have is the quantity of bees - the more bees, the more pheromones in the air, the more light gets refracted/reflected, and therefore the easier this will be to see to the human eye.

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