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10 Slippery Facts About Slugs

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Slugs get bad press. Okay, they’re cold and slimy and sometimes eat your kale, but there’s so much more to them. They exude a liquid crystal, they have sex while dangling from a thread, and one time they beat a charismatic mammal in a dramatic popular vote.

1. SLUG OR SNAIL? IT’S A SPECTRUM.

People tend to call something a slug if it looks like a snail but has no shell. However, many distantly related critters among the gastropods—the group that contains snails and slugs—have independently evolved a sluggy, shell-free shape. So there’s no single, evolutionarily distinct slug lineage. To make things more complicated, a lot of slugs secretly do have shells—they’re just hidden within the slugs’ bodies.

And then there are the in-betweeners. So-called semi-slugs have tiny shells on the outside of their bodies that are way too small for them to retract into. (Honestly, they look pretty ridiculous.)

2. SLUGS HAVE TENTACLES, BLOWHOLES, AND THOUSANDS OF TEETH.

If you want to dream up an alien species for the next big sci-fi blockbuster, start with slug anatomy. First, check out the tentacles. Slugs have four, and they’re retractable. Two are for seeing and smelling, and they can be operated independently: a slug can gaze at you (or smell you) and a friend simultaneously. The other two are for touching and tasting.

Slugs also have thousands and thousands of teeth. These tiny chompers are part of a rasping structure called a radula that’s unique to mollusks. And in case that doesn't seem weird enough, slugs essentially breathe through a blowhole that opens up on one side of their bodies. This round pore is called a pneumostome.

But that’s just the anatomy of land-living slugs. Sea slugs have their own incredible features. For example, some breathe using delicate feather-like gills that surround their butt holes, and they smell with neon-colored, bizarrely shaped protrusions called rhinophores.

3. WHEN ATTACKED, SOME SLUGS LOSE THEIR TAILS.

Melibe leonina from Santa Cruz. Image credit: Robin Agarwal via Wikimedia // CC BY-SA 4.0

It’s an ingenious strategy for escaping a hungry predator: Break off a small tasty part of your body, and leave it behind as you make your escape. Some slugs do this. The aptly named taildropper slugs, such as the reticulated taildropper, can quickly amputate their own tails. And certain sea slugs have body parts that snap off safely and easily, leaving a would-be predator with a smaller, less desirable meal.

4. THEIR LOVE LIVES ARE ACROBATIC AND GORY.

Leopard slugs and their relatives will only mate when they’re dangling upside down from a thread of mucus. This position enables them to extend their gigantic, body-length penises and wrap them around each other. And yes, that’s penises plural: slugs have both male and female body parts.

And that’s just one example of surprising slug sex. The banana slug, a fixture of the Pacific Northwest, sometimes chews off its partner’s penis after mating. Then there’s the sea slug that removes its own penis post-sex and rapidly grows a new one. Another sea slug sets the mood by stabbing its partner in the head.

5. SLUG SLIME IS A LIQUID CRYSTAL.

Slugs are gooey and sticky, and they leave a trail of slime wherever they go. But that goo is pretty remarkable. It’s a liquid crystal, a substance that’s somewhere between a liquid and a solid. It flows a bit like a liquid, but at the molecular level, it’s more organized. It can be both adhesive and lubricating, and it actually slurps up water.

Why all the goo? Slug slime is multi-purpose. It helps these critters move and climb challenging surfaces. It also protects them from fungi and bacteria. Plus, slugs can learn about each other—and find potential mates—by examining slime trails. And, of course, mucus is a key part of a leopard slug’s daredevil upside-down sex life.

6. THEY CAN GET UP TO 30 POUNDS.

Some land-living slugs can get pretty large. Europe’s ashy-grey slug is 10 inches long. But that’s nothing compared to the sheer size of some sea slugs. Found in California, the black sea hare can reach nearly 40 inches and weigh 30 pounds.

7. SOME SLUGS ARE FIERCE PREDATORS.

Amgueddfa Cymru, National Museum Wales via Wikimedia // CC BY-SA 3.0 

Sure, many eat a salad-like diet of plants, mushrooms, and fruits, or chew up dead and rotting plants and sometimes even rotting animals. But others have a taste for living flesh. The ghost slug slurps up worms. The stunningly colored Spanish shawl (a type of sea slug) chows down on a particular marine creature called a hydroid, eating everything except the hydroid’s stinging cells, which the slug then uses for its own defense. The sea slug Pleurobranchaea californica eats other sea slugs, and it’s shown a remarkable ability to remember which species—like that stinging Spanish shawl—are no good to eat.

8. SOME SLUGS CAUSE TROUBLE.

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As people have spread across the globe, we’ve unintentionally introduced slug species into new locales. These slimy invaders lack natural predators in their new lands, so they flourish and even push out native slugs. The slugs that eat the veggies in your garden may very well be invaders, such as the Spanish slug or leopard slug—and that last species carries a parasite that can cause meningitis.

Slugs aren’t just spreading to new places on land, either. Ocean-going ships have also accidentally transported exotic sea slugs to new places.

And invasive slugs also cause trouble for other creatures. Some plants rely on ants to spread their seeds, and they reward the ants with a tasty treat called an elaiosome that’s attached to each seed. But invasive slugs can eat those treats before the ants find them.

9. SOME ARE IN TROUBLE.

Blue-grey Taildropper (Prophysaon coeruleum)

Only a few slug species are pests. Most are critical members of land and water ecosystems all around the world. And, like so many creatures, they’re suffering declines. One is the tiny and ridiculously colorful blue-grey taildropper of the Pacific Northwest. Another, the evocatively named snake skin hunter slug, is found in only a few spots in South Africa. Though these critters may not have the charisma of, say, a cheetah or a blue whale, they’re no less crucial to the health of ecosystems.

10. A SLUG IS THE SUBVERSIVE MASCOT OF THE UNIVERSITY OF CALIFORNIA AT SANTA CRUZ.

If you’re looking for a model of slug appreciation, take a look at UCSC. For a long time, students considered the banana slug, a gentle denizen of the area’s redwood forests, to be their unofficial mascot. Little did they know that their slug was in for a fight.

In 1980, UCSC entered the National Collegiate Athletic Association (NCAA). The university needed an official mascot—and some officials wanted a fiercer, sportier, and more traditionally charismatic species. A group of athletes proposed the sea lion, and it became the mascot. But other students refused to embrace the new choice. They continued to shout “Slime 'em!” and “Go slugs!” at basketball games. A fierce debate erupted, resulting in national media coverage. Finally, the matter was put to rest with a 1986 school-wide vote. Slug supporters slid into first place with an overwhelming five-to-one victory.

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Man Buys Two Metric Tons of LEGO Bricks; Sorts Them Via Machine Learning
May 21, 2017
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Jacques Mattheij made a small, but awesome, mistake. He went on eBay one evening and bid on a bunch of bulk LEGO brick auctions, then went to sleep. Upon waking, he discovered that he was the high bidder on many, and was now the proud owner of two tons of LEGO bricks. (This is about 4400 pounds.) He wrote, "[L]esson 1: if you win almost all bids you are bidding too high."

Mattheij had noticed that bulk, unsorted bricks sell for something like €10/kilogram, whereas sets are roughly €40/kg and rare parts go for up to €100/kg. Much of the value of the bricks is in their sorting. If he could reduce the entropy of these bins of unsorted bricks, he could make a tidy profit. While many people do this work by hand, the problem is enormous—just the kind of challenge for a computer. Mattheij writes:

There are 38000+ shapes and there are 100+ possible shades of color (you can roughly tell how old someone is by asking them what lego colors they remember from their youth).

In the following months, Mattheij built a proof-of-concept sorting system using, of course, LEGO. He broke the problem down into a series of sub-problems (including "feeding LEGO reliably from a hopper is surprisingly hard," one of those facts of nature that will stymie even the best system design). After tinkering with the prototype at length, he expanded the system to a surprisingly complex system of conveyer belts (powered by a home treadmill), various pieces of cabinetry, and "copious quantities of crazy glue."

Here's a video showing the current system running at low speed:

The key part of the system was running the bricks past a camera paired with a computer running a neural net-based image classifier. That allows the computer (when sufficiently trained on brick images) to recognize bricks and thus categorize them by color, shape, or other parameters. Remember that as bricks pass by, they can be in any orientation, can be dirty, can even be stuck to other pieces. So having a flexible software system is key to recognizing—in a fraction of a second—what a given brick is, in order to sort it out. When a match is found, a jet of compressed air pops the piece off the conveyer belt and into a waiting bin.

After much experimentation, Mattheij rewrote the software (several times in fact) to accomplish a variety of basic tasks. At its core, the system takes images from a webcam and feeds them to a neural network to do the classification. Of course, the neural net needs to be "trained" by showing it lots of images, and telling it what those images represent. Mattheij's breakthrough was allowing the machine to effectively train itself, with guidance: Running pieces through allows the system to take its own photos, make a guess, and build on that guess. As long as Mattheij corrects the incorrect guesses, he ends up with a decent (and self-reinforcing) corpus of training data. As the machine continues running, it can rack up more training, allowing it to recognize a broad variety of pieces on the fly.

Here's another video, focusing on how the pieces move on conveyer belts (running at slow speed so puny humans can follow). You can also see the air jets in action:

In an email interview, Mattheij told Mental Floss that the system currently sorts LEGO bricks into more than 50 categories. It can also be run in a color-sorting mode to bin the parts across 12 color groups. (Thus at present you'd likely do a two-pass sort on the bricks: once for shape, then a separate pass for color.) He continues to refine the system, with a focus on making its recognition abilities faster. At some point down the line, he plans to make the software portion open source. You're on your own as far as building conveyer belts, bins, and so forth.

Check out Mattheij's writeup in two parts for more information. It starts with an overview of the story, followed up with a deep dive on the software. He's also tweeting about the project (among other things). And if you look around a bit, you'll find bulk LEGO brick auctions online—it's definitely a thing!

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Creative Bar Owners in India Build Maze to Skirt New Liquor Laws
June 20, 2017
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Facing a complicated legal maze, a bar in the southern Indian state of Kerala decided to construct a real one to stay in business, according to The Times of India. Aiswarya Bar, a watering hole that sits around 500 feet from a national highway, was threatened in 2016 after India's Supreme Court banned alcohol sales within 1640 feet of state and country-wide expressways to curb drunk driving. Instead of moving or ceasing operation, Aiswarya Bar's proprietors got creative: They used prefabricated concrete to construct a convoluted pathway outside the entrance, which more than tripled the distance from car to bar.

Aiswarya Bar's unorthodox solution technically adhered to the law, so members of the State Excise Administration—which regulates commodities including alcohol—initially seemed to accept the plan.

"We do [not] measure the aerial distance but only the walking distance," a representative told The Times of India. "However, they will be fined for altering the entrance."

Follow-up reports, though, indicate that the bar isn't in the clear quite yet. Other officials reportedly want to measure the distance between the bar and the highway, and not the length of the road to the bar itself.

Amid all the bureaucratic drama, Aiswarya Bar has gained global fame for both metaphorically and literally circumnavigating the law. But as a whole, liquor-serving establishments in India are facing tough times: As Quartz reports, the alcohol ban—which ordered bars, hotels, and pubs along highways to cancel their liquor licenses by April 1, 2017—has resulted in heavy financial losses, and the estimated loss of over 1 million jobs. Aiswarya Bar's owner, who until recently operated as many as nine local bars, is just one of many afflicted entrepreneurs.

Some state governments, which receive a large portion of their total revenue from liquor sales, are now attempting to downgrade the status of their state and national highways. To continue selling liquor in roadside establishments, they're rechristening thoroughfares as "urban roads," "district roads," and "local authority roads." So far, the jury's still out on whether Kerala—the notoriously heavy-drinking state in which Aiswarya Bar is located—will become one of them.

[h/t The Times of India]

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