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Who doesn’t love to gaze at a beautiful butterfly fluttering by? Aesthetically speaking, they are simply wonderful to watch. Wonderful, that is, unless you are getting a rare glimpse of pheromone laced coremata.

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You can keep building your STEM  skills at with 20% off an annual premium subscription! [♪ INTRO] Butterflies are beautiful and never  sport absolutely horrifying tentacles. This is well known.

Or is it? Everyone likes butterflies, right? That’s not true, ​​lepidopterophobia  is a real thing and, like, why not.

Cockroaches are awful and everyone hates them, and though they certainly seem less magical than a monarch gliding gracefully  on a massive migration, cockroaches and butterflies are both just bugs. Still, most of us like butterflies  because they look nice, and that might actually end up being  good for them because it makes us humans, who have a pretty large amount  of control over this planet, more likely to protect them. But that is not why butterflies are pretty.

They are able to use the colors  of their wings for camouflage, to scare predators, to warn (or  bluff) that they’re poisonous. There’s also pretty good evidence  that, in at least one species, female butterflies prefer  more brightly colored males. That might be because it indicates  that they’re better mates and would have caterpillar babies  with a better shot at butterfly lives.

And so, in those cases, what a  female butterfly finds attractive also just happens to be what we find attractive. Unless we’re talking about coremata! Butterflies have some pretty  intricate systems for mate selection, and for them, pheromones play a major role.

Pheromones are chemicals  produced by one animal that affect the behavior of another animal,  and are often carried through the air. Butterflies use them for courtship. Often, a male will use pheromones  to attract females to them, and the airborne chemical signals are also thought to make females more  receptive to male advances.

This is not a thing that we  tend to notice about them, but it is a very big deal for the lepidopterans. That’s the umbrella term  for butterflies and moths, not the name for aliens from the planet Lepidop. And so some groups of butterflies  and moths have special organs for spreading their pheromones more effectively.

These include little patches of specialized  wing scales that you would never notice. And they also include coremata: inflatable hairy tentacles  that shoot out of their butts. The butterfly lobby has clearly  been trying to hide this from us.

When a male of one of these  species wants to attract a female, the insect inflates large organs  from the rear of their abdomen that increase the surface area from  which the pheromones can evaporate. The inflating happens either with air  or with hemolymph, aka insect blood. And this, basically, allows  the insects to be stinkier.

Sometimes coremata are small and not too terrible, but in some moths they can be absolutely massive. Like, up to 37 millimeters in one  species; as long as their whole wingspan. This particular species of tiger  moth has four of these structures that it can retract entirely back into its body.

But, when it needs to really  waft its scent out there in the hopes of attracting a  mate, it inflates these organs. Clearly there is some  significant evolutionary pressure to really get that smell going. This is likely because some species with  coremata use a reproductive strategy called lekking in which males all  gather together to display to females.

In general, a lek refers to a gathering of males, who might display with loud  sounds, bright tail feathers or, in the case of some Lepidoptera,  extravagantly wafted scents. Indeed, another species with impressive butts that form leks is more familiar: peacocks. Males who impress females  are then selected for mating.

That creates strong evolutionary pressure  for whatever trait is being selected for. In some moths from the Erebidae  family, including our tiger moths, it seems that trait is giant butt tentacles. In fact, this is a well-known problem in  evolutionary biology called the Lek Paradox.

If females are exclusively  selecting males for this trait, then it should cause a runaway lack  of diversity, with only males with the absolute best wafting organs  getting to spread their genes. That lack of diversity would then be bad  for the survival of the entire species. But, for some reason, this doesn’t  seem to happen with lekking species.

And there are a few hypotheses why. One idea is that even though the  selected traits do hold the species back, the males in question are so fit that  they still make worthwhile mates. But a more prevalent hypothesis is that somehow the selected trait  is itself a sign of fitness.

Like, maybe a moth doesn’t  grow big coremata because it has the right genes to grow big coremata. Rather, it has to be a successful individual  or else the coremata won’t get so big. That means that females aren’t selecting for specific genes that lead to big coremata.

They’re selecting for overall healthy individuals and genetic diversity doesn’t  take so much of a hit. For some Erebidae moths, this appears  to be exactly what is happening! The size and full development of their  coremata is directly influenced by how much of a specific chemical they’re able to  ingest while munching around as caterpillars.

Adult males who ate well as caterpillars  have the biggest coremata and the strongest pheromones,  and they gather in leks to signal to females that it’s time to mate. Unlucky males with smaller  coremata don’t even try to join in: they wait until the females that  haven’t mated yet start signalling later in the day and join in mating that way. We have no idea why these species  started doing this, though, so the lek paradox remains a mystery.

Anyway! It’s important to note… not all  butterflies and moths have coremata, and most are much more modest  affairs than these tiger moths. But this is a reminder that butterflies  and moths are not here to please us.

Their beauty is an accident,  as is their occasional horror. We are merely spectators. And one of the coolest things to observe  about butterflies is their ability to detect Earth’s magnetic field.

But they don’t have to be the only  ones exploring magnetic fields, because we have Brilliant. Brilliant is an online learning platform  that offers guided problem-solving based courses in math and  science, like their science course on electricity and magnetism. That’s a topic that is way more interesting  with interactive examples than it would be to read about in a textbook.

So Brilliant provides everything from  diagrams to quizzes to keep you engaged. Click the link in the description down  below or visit and you’ll get 20% off the  annual Premium subscription. Thanks to Brilliant for supporting this video! [♪ OUTRO]