Saturday, 1 October 2011

"Maybe if you stick on another leg at the end of the tail?"

Designing a novel way of walking for extraterrestrial animals is complicated. I tried my hand at designing gaits for large hexapodal creatures (see the main Furaha site), radial walking patterns and also explored walking with an odd number of limbs. In all such efforts the trick is to achieve something that looks interesting as well as believable. In the context of speculative biology 'believable' is balanced somewhere between 'Earth normal' and weirdness. One thing is clear though: you cannot get a believable result by assembling an animal of leftover bits and pieces, such as just sticking an extra leg on the end of a long tail.

Or can you? As usual, evolution on Earth manages to come up with designs that, if invented by a mere human, would fall in the category of unacceptable weirdness. The following video shows an insect that looks odd, but oddness by itself is fairly normal for insects. Look how it moves: most of the time insects walk with a double tripod gait: the front and hind legs on one side move in unison with the middle leg on the other side. When these three legs touch the ground they form a stable tripod. The other three legs meanwhile are lifted and swung forwards, and when they touch the ground, they will form a tripod as well. The two tripods are exactly out of phase, so when one hind leg is on the ground the other should be in the air. Now have a look at the hind legs of this interesting beastie, a trilobite beetle from Borneo. The original is here.

Its pairs of legs are in phase, a bit unexpected, but slow-moving insects can do that. But that is not all: it uses the tip of its abdomen as an additional unpaired leg. It curves its abdomen forwards, plants its 'leg' on the ground, and pushes backwards with it. Anatomically this may not be a proper leg, but functionally this animal certainly uses seven legs: it's a heptapod!

Here's another video. The beginning shows that this species can also walk with the front legs out of phase, but you do not get to see all legs that well. It is clear though that it uses the end of its abdomen as a seventh functional leg.

Why do these animals walk in this weird fashion? The gait does not look quick or agile. In fact, the animals appear to be rather slow and clumsy. A bit of research points to an answer. These 'trilobite beetles' are said to belong to the genus Duliticola, and using Google with that name results in a paper starting with the brilliantly surrealistic sentence 'There are two trilobite larva species in Singapore.' Apparently, the male and female of these species differ greatly in shape: the males look like typical beetles while the females are neotenous. Now neoteny is a condition in which sexual maturity occurs while the body is still in a larval stage. The axolotl is a famous example, and humans are sometimes thought to display neoteny as well.

But what does that mean for the strange gait of this apparently female insect? Well, it looks a bit like a regular adult insect, with a hard exoskeleton and all, but its general body shape is in fact that of a caterpillar. Caterpillars display complex gaits, not too surprising if you think about their body plan: six regular legs that will become the legs of the adult insect, a number of 'prolegs' (the knobby stumps further along a caterpillar's body), as well as final 'anal prolegs'. All of these are attached to a boneless body, providing endless opportunities of combining walking with stretching of the body. So that explains the trilobite beetle's walk: its' a caterpillar in disguise. Never underestimate insects' capability of oddness.

There is of course more to be told about caterpillar movement. In fact, at least in some species their gut moves inside their body before the outside follows up. The following video show that very nicely as well as the combination of body stretching with using legs. Perhaps there is a risk that you will learn more about caterpillar movement that you bargained for, but personally, I love details.


Evan Black said...

It's important for speculators to note that coming up with a new gait may certainly be interesting and have real-world precedent (such as is presented here) but that doesn't necessarily make their idea plausible. Sure, biomechanics and real examples verify the feasibility of the locomotive pattern, but speculators often get into trouble when they stop there and don't figure out why their imagined creature shares the gait. i.e. What works for a caterpillar doesn't necessarily work for a large predator.

The moving caterpillar guts is a fascinating video, and I wonder how it can be adapted to serve the organism in other capacities. Perhaps it can be used to pull in vital organs so that whatever end is attacked by a predator is empty for the most part and comes out unharmed? Or maybe an extreme case could be used to stretch out the body as a whole, creating a tension that can be used to launch the organism into the air; it's kind of like stretching out a rubber band and letting fly. What do others think?

Spugpow (aka Luke) said...

Good point Evan.

This is out there, but I can imagine an animal in the (near) vacuum of space propelling itself by slinging its guts backward, then slowly recoiling them. It might inhabit the rings of a jovian, "hopping" from ice-chunk to ice-chunk. To observers, it would appear to be spookily gliding forward in pulses with no discernible means of propulsion.

mithril said...

Spugpow, your idea a space travel system probably wouldn't work unless your also flinging away disposable mass. when the guts reach full extension, they'd slow to a stop due to the connection, which would negate your momentum vector, bringing you to a stop. differential rate of movement doesn't matter (otherwise the Dean Drive would have worked. )

you'd only get a distance travelled roughly equal to the length of your 'guts', and the act of expelling and retracting your counermass would make it highly inefficent in terms of energy use.

however, on a planet a system which can push against the ground to make the animal jump would make for a useful means of movement. especially if it gives longer distances or higher arcs than using it's legs.

speaking of which, the 'tail-leg' was one of the things i though Fahy's Fragment used well..the idea of a tail that evolved to be a 'jumping leg' is a nice alien touch.

Evan Black said...

mithril, that jumping technique is also used by springtails, foldiong their tails underneath and straightening it to catapult themselves out of danger (hopefully).

Luke/Spugpow, the idea is interesting, but I don't think it can work for continual propulsion. Perhaps if it used this mechanism to push against ice chunks and launch itself, but such a motion in the air would cancel itself out and the spooky gliders would be dead in the water... um, air... no, vacuum... ;)

Luke said...

Yeah, I thought there was something fishy about that idea, but I decided to post it anyway :) .

This is getting away from the topic of the post, but another idea I like is animals that inhabit the ice chunks in a Jovian ring, grazing on photo/chemosynthetic microbes (or plants?). They fire eggs explosively through the void to disperse their young, aiming toward chunks that appear promising.

I might actually invent an entire ring biota...