For years, a longtime friend and her erstwhile husband had their very own imaginary friend. Known simply as "Snail," he was rarely seen, but often heard from, in the form of cryptic notes, emails, and the occasional postcard sent from France (or London, or Italy, or wherever her itinerant artist spouse was traveling at the moment). Snail went on to found his own imaginary artzine, Gastropod Cineaste (with a heavy emphasis on French New Wave films). Yes, Snail had a touch of intellectual snobbery; Jen-Luc Piquant simply adored him, dubbing him "mon petit escargot," despite the whole mucus thing.

Mucus, you say? Mais oui! That's how a snail gets around, after all, even those that dwell in the realm of imagination. Snails store a crystalline form of their mucus and then mix it with liquid to produce the long sticky trails of slime they leave in their wake. It's all about equal and opposite reaction: the snails push backward to create a pressure gradient that propels them forward. Mucus might be a bit gross to homo sapiens, but for snails, it's a miracle substance, enabling the creatures to crawl over all kinds of uneven terrain: sand, mud, ground scattered with leaves or twigs. And if they encounter a wall or ceiling, no problem! Thanks to the sticky stuff, they can maneuver those non-horizontal surfaces just fine.

I bring it up, all these years later, because there is a nifty new video making its way around the science blogosphere featuring Toro II, a robot designed by researchers at Chuo University's Nakamura Lab to mimic the movements of snails. It's just the latest in a long history of biomimicry when it comes to robot design -- because after all, Mother Nature seems to have done a pretty admirable job when it comes to biomechanics.

But snails? Really? Yes! It turns out that snails have an unusual approach to locomotion, relying on two basic techniques: undulating (with a bit of mucus to help it along), and "galloping," in which the snail sticks the front of its suction-like foot to, say, a lab bench surface, then draws the rest of its body up behind it like an inchworm, pushing its body forward bit by bit. Toro-II doesn't use mucus -- more's the pity -- but it exploits the same basic principles to enable it to move in every direction while still remaining stable enough not to be thrown off track by an unexpected shove. Still in the works: snailbots that can literally climb the walls. The researchers envision using these snailbots in hospitals and factories, although personally I would be slightly unnerved at the sight of mini mechanical snails roaming about the place.

This is not the first instance of scientists developing a snailbot. Anette "Peko" Hosoi and a few of her students at MIT introduced their own "Robosnail" back in 2003: a six-inch long machine, just one inch wide, that could "glide" over a thin film of silicon oil -- a synthetic version of mucus, called Carbopol, which is simply a gel-like, water-based polymer solution. Then, in 2006, came Robosnail 2.0: it can climb walls and move upside down, thanks in large part to its light weight (less than 31 grams) and a slick layer of another mucus-like substance called Laponite.

There's much to learn from building snailbots. Among other things, working with Robosnail I and II could provide insights one day into how blood flows through a vein. Like blood flow, snail movement is essentially a fluid flow contained by a flexible boundary. Another promising application is the use of Robosnails to aid in oil exploration by maneuvering through harsh, hard-to-reach environments. Surgeons might also find them useful to crawl to hard-to-reach areas of the body. These are pretty far afield in terms of development but Robosnails I and II are the first exploratory tools to that end.

There are some drawbacks to the snail's otherwise ingenious approach to locomotion, namely, it's not exactly energy-efficient (or fast). Slugs, for example, also move along trails of excreted slime. Marine scientist Mark Denny of Stanford University, who studies slugs, says they expend 70% of their energy making those mucus trails. That's about 10 times more the energy expenditure of animals that move by running, swimming, or flying. Mile for mile, it might not be the best choice for locomotion, unless there's no better alternative means of movement.

Then again, slugs make handy batteries for swarms of devices known as slugbots. Ian Kelly of the University of the West of England, has been working on this since, oh, about 2000. His little battery-powered robot is designed to patrol fields looking for slugs and scoop them up, using their decaying bodies to recharge its batteries -- a concept that seems like he ripped it straight out of The Matrix. British farmers purportedly spend some 20 million pounds a year trying to prevent invading armies of slugs from devouring their tender crops. Kelly estimates that as many as 200 slugs could be found for every square meter of a wheat field, providing a bounteous source of energy for his slugbot.

So gastropods are fertile fodder (sometimes literally fodder) for biomechanically robust robotics. What about fish? Northwestern's Malcolm MacIver has been studying the movements of the Amazonian black ghost knifefish for years, doing countless computer simulations based on his observations, culminating in his prototype "GhostBot" that debuted earlier this year. It's just what you'd think: a robotic fish, capable not just of swimming forward and backward, but also switching almost instantly to swimming vertically, thanks to the inclusion of a ribbon-like fin modeled on the black ghost knifefish. It comes in very handy for maneuvering around tree roots and other obstacles in the waters of the Amazon basin as it hunts.

How does this fluid locomotion work? The computer simulations showed that the fin creates only one traveling wave when swimming horizontally (forward or backward), but generates two such waves when swimming vertically. One moves from head to tail, the other from tail to head, and they collide in the center of the fin. This cancels out the forward thrust motion, funneling it into a downward-moving jet. Model those fluid dynamics computationally, and you get a sort of mushroom cloud effect in the water. GhostFish's ribbon-like lycra "fin" has 32 little motors each controlling artificial rays, giving it a wider range of motion.

The fish itself is pretty darned interesting in other ways, too, according to MacIver. Not only does it have that cool ribbon-like fin for locomotion, it also hunts for prey at night by surrounding its entire body with a weak electric field. MacIver says that unusual feature could teach scientists something about how the nervous system sends messages through the body to trigger motion. In the meantime, he's used it as a model for the GhostFish's electosensory system, enabling it to detect an object in its surrounding and then position itself near that location. Ideally, such robots would be useful for things like underwater recovery operations, like capping the BP oil spill.

Jellyfish are also a popular organism with robotocists. For instance, scientists at Tokyo University have been working to mimic the swimming motion of a jellyfish by building a micro-robot out of an unspecified "soft material." Sure, mostly jellyfish seem to just float about, waiting for the perfect opportunity to sting someone, but they can produce thrust, and thereby some momentum, by creating a "vortex ring." They can expand and contract their mushy little bodies and expel air into the water behind them, creating a little whirlpool that gives them a shot of propulsive power.

Caltech scientists are interested in the unusual fluid dynamics and locomotion of the humble jellyfish -- like John Dabiri, who is an expert on jellyfish propulsion and is designing "soft robotics" and tiny pumps for medical applications based on the creature's movements. "At the end of the day, when you look at fluid flows, whether it's air or water or blood, they can all be described by the same equations," he told the Los Angeles Times last fall. "So you can start to understand what makes a jellyfish efficient, and then use that information to design submarines that are more efficient, or diagnose when the heart is no longer performing efficiently." Or you can use the knowledge gleaned about fluid dynamics to build a more efficient wind farm, modeled on the dynamics of large schools of fish.

See? Mother Nature turned out to be quite the ingenious mechanical engineer.