The sun broke as the clouds came, fracturing into a thousand rainbow-bright beams to sweep across the aetherglass desert. The lie-scent of damp dirt and rose perfume lured butterflies and bipeds into the magic maw, and in the teeth of the typhoon winds, Surzi finally ate well. --- ## ANALYSIS Petrichor is the distinctive scent that follows rain falling on dry earth. Some people claim humans and animals naturally love it, but if so, I've never actually smelled it. I've always hated the scent of asphalt after it's first hit by rain. The scent of wet leaves doesn't bother me, but the weird dustiness of the forest right after rain starts isn't pleasant to me. To top it off, the last time I smelled rain hitting the requisite dry earth for petrichor was in the Las Vegas desert. My husband and I were in town for a convention, and I decided that the one thing I really wanted to do while we were there was actually _see_ the desert. We got a (hilariously expensive) cab ride to a park at the edge of town and took some nice sunset photos of the little stream there, then decided to walk the little loop they had. The park closed at sunset, but after spending so much on the cab, we figured that "sunset" is a pretty fuzzy time and it wasn't like we were going to get lost — we could see the whole loop right from where we were standing. As we walked, enjoying all the little differences between the Las Vegas desert and the riparian landscape of home, we took note of storm clouds hovering over the mountains in the distance. My husband grew up in the Southwest and assured me that they were blowing away from us, and that the storm was pretty far away besides. We checked the radar, convinced ourselves we were fine, and kept walking, enjoying the desert. A little bit later we noticed the clouds getting closer. The wind started kicking up. Our phones honked with a dust storm warning. I covered up our camera and my face with my coat and we started heading back to the parking lot. Then it started to rain. As flash flood warnings start going off on my phone, the only thing I could think of was that we were trapped in the _only part_ of Las Vegas that actually has running water, no one knew where we were, and we were the only people in the park since it was twilight by this point. Studies show that [the scents we love are mostly informed by experience](newsletter.eleanorkonik.com/roses/). Humans don't come out of the womb loving the scent of petrichor, or roses, or the blend of diesel fuel and brackish water that smells like "home" to me as a child of the Chesapeake Bay. To me, the scent of petrichor has always meant "We screwed up the weather report and now we're trapped outside in the middle of the woods in a rainstorm, and if we're not careful, we could die." Eventually, we made it under cover — although even the outdoor-facing bathrooms of the nearby park museum were closed, so it was only partial cover. We called a Lyft and half an hour of torrential desert downpour later, made the mad dash to a parking lot they could get to. A cop car drove right past us as we walked (I was exhausted by this point), following a suspicious car — our Lyft! Terrified our ride would get sent on its way, my husband ran ahead — leaving me behind. The Lyft driver was half-afraid my husband was planning to rob him; the whole place was dark and deserted, and in 30 years of living in Vegas he'd never been to our location. As we headed back, we had to reroute because of all the downed trees, and several lights at major (think eight lanes) intersections on the strip were out. Maybe if I lived in the Swiss Alps or something I'd feel differently, but as an American suburbanite I'm not a fan. Still, the science behind petrichor makes for a fun inspiration for a story. Surzi is a magical beast who uses a lure scent to attract creatures to it. Pretty standard fare so far: lots of animals are attracted by scents. That's why sharks can detect blood at a ratio of one part per million in the ocean. Turns out, the petrichor after rain exists because of a similar evolutionary quirk — but it's directed at bugs instead of humans. Geosmin is a chemical compound released by bacteria from the _Streptomyces_ genus to attract tiny six-legged arthropods called springtails. It's a symbiotic relationship pretty similar to the way strawberries make fruit so birds will help spread their seeds. Unlike fruit trees, though, _Streptomyces_ bacteria tend to make spores when they're faced with _unfavorable_ conditions.  Generally speaking, the springtails eat the geosmin-laden spores, then excrete the spores in new places, helping the bacteria to spread. Sometime, though, it works a bit more like how bees and butterflies help pollinate flowers; the spores will stick to the bodies of the springtails and then slough off later. It turns out that _Streptomyces_ is one of the most important sources of antibiotics compounds. For a long time, scientists thought it propagated thanks to wind and water, but it's actually because of these tiny arthropods, which are immune to the many deadly compounds _Streptomyces_ usually uses to ward off insects and fungi. It's sort of like how clownfish are immune to anemone stings, except the relationship between _Streptomyces_ is probably about 400 million years old and so effective that basically ever single member of this genus makes geosmin. By contrast, only a tiny percentage make the antibiotic, antifungal and anticancer compounds humans find useful. Unlike the symbiotic relationships I've talked about so far, though, Surzi eats people when they try to cross into its territory. Kind of like how the American bolas spider females lure male moths by mimicking the sex pheromones emitted by female moths of that species. ## Further Reading - [How Animals Communicate Via Pheromones](https://www.americanscientist.org/article/how-animals-communicate-via-pheromones) by Tristram Wyatt over at American Scientist goes into a lot more detail about how pheromones work, from how they evolve to all the different ways animals use them. It also has some nifty infographics. - This 2020 [press release about geosmin](https://www.jic.ac.uk/press-release/research-unearths-the-science-behind-the-smell-of-spring/) was my main source for all this. It goes into a bit more detail about the nitty-gritty of stuff like the bacteria's genes and the precise biology of springtails, if you're interested.