Crabs as spidrids, spidrids as crabs...

I recently realised that Earth crab locomotion resembles Furahan spidrid walking more than I expected. I may have spent too much time on spidrids or not enough on crab locomotion, as there was a lot to learn about radial walking, odd as that may sound. I found a very nice paper on walking patterns of decapod crustaceans (basically crabs and lobsters) beginning with the sentence 'Most decapod crustaceans can walk in any direction they please'. 

From: Vidal-Gadea et al. Arthropod Structure & Development 2008; 37: 95–108 (adapted)

The image above is from that paper and shows leg movements of a sideways-walking crab, a forwards walking crab, and a forwards-moving lobster. The ability to move in any direction without turning the body is one of the main features of a radial walking design, something I thought did not exist on earth. Apparently crabs, particularly forwards walking ones, are quite 'radial'. In fact, the paper uses the very word 'radial' to describe leg positions for the forwards walking crab. A peculiar convergence with spidrids is that its common name is 'spider crab' (Libinia emarginata).

 Source: http://www.youtube.com/watch?v=_wZZ6N8aM3Y

While most crabs preferentially walk sideways, they can combine directions and walk diagonally if they so wish. The video above shows a crab that starts walking backwards but gradually adds a horizontal element until it ends up walking sideways only. (Click the link to see the source at a better quality).

Source: http://www.youtube.com/watch?v=uYXJ5Ww361s

And here is an example of a forward walking crab. Again, the original has better quality. If you look carefully you will see that the legs do not all point sideways: the front ones are angled to the front, and the hind ones point almost backwards. In short, they are almost placed and held radially around the body. Are spidrids crustaceoid or are crustacea spidridoid?

Click to enlarge; copyright Gert van Dijk

Spidrid legs, although the mere result of a thought exercise, are rather like real crab legs. The image above shows the simplified leg anatomy, say of a sideways-walking crab (or of a spidrid leg). The bottom part shows that the leg can turn forwards and backwards around a vertical hinge close to the body, movements labelled 'promotion' and 'remotion' in technical papers. Let's call that the 'body-leg joint'. The other joints, the 'intraleg joints', in spidrids have horizontal axes allowing the leg to be straightened and flexed (see the top part). There would be muscles for every joint, but I only showed them for one.

If the animal moves in the direction shown here the leg does not need  action of the promotor and remotor muscles: the power for movement comes from the intraleg joints.  If you rotate the direction of movement 90 degrees, muscle force for this leg has to come from the body-leg joint, meaning the promotor and remotor muscles.

Click to enlarge; Copyright Gert van Dijk

So what does all this mean for spidrids? Well, regardless of the direction it walks in, a spidrid has some legs parallel to the direction of movement and some at a right angle to it. The image above shows how that relates to the direction of movement and to the necessary range of motion. The legs parallel to the movement function as the legs in sideways-walking crabs, and  depend on intraleg flexion and extension, pulling and pushing the beastie. The legs at largely right angles to the movement depend on promotion and remotion. The leg in between simply make use of both sets of muscles to varying degrees. (Mind you, the word 'promotion' in crabs always refers towards the front end of the animal; in adapting it for spidrid use it must mean 'in the direction of movement', there being no front end.)

The next evolutionary spidrid twist stemmed from the idea that one of these two types of force production might be superior to the other. How would spidrids make use of that edge, while staying radially symmetrical? Before tackling that I realised I had never shown the spidrid's ability to change direction without turning. Solving that posed some interesting Matlab programming problems, but never mind, it works. I added height for fun and slanted the body a bit when the beastie is on a slope to make it look more natural.

 Copyright Gert van Dijk

Here it is! Finally, a spidrid that negotiates terrain and make a sharp turn. As you can see, the sharp turn calls for some interesting leg movements. With a shallow turn you would not see the changes well. So this is how real radial animals walk. By the way, should a rich Hollywood director wish to buy the concept for a film, I am available! Anyway, it is now time to adapt this standard spidrid walk to more energy-efficient gaits.

Copyright Gert van Dijk

The one above is built on the assumption that flexion/extension is more efficient than promotion/remotion. So, this species uses its promotion/remotion muscles to swing the legs as far parallel to the direction of movement as they will go. There are probably anatomical limits to this, so some legs still stick out at a right angle to the direction of movement regardless. The turn becomes odd, as some legs have to swing a long way to end up in their new position.

Copyright Gert van Dijk

But the existence of forwards-moving crabs shows that under given circumstances using promotion and remotion as the power house is feasible. The animation above has a spidrid moving its legs with a preference for positions at a right angle to the movement. This movement also calls for large leg swings when the animal changes direction. The legs bump into one another, which can be solved with phase changes, but I left it as it is for now. The anatomy of the animal is the same in all three variants, which may be unwise; I can see the last type having shorter legs to improve leverage, at the cost of stride length.

So there we are! Rampaging spidrids! What else is left for spidrid movement? An obvious additional adaptation would be to include slanting, but I will not provide an animation of that; what you see here was quite complex. Then again, I now have a program resulting in 3D coordinates for any part of a spidrid negotiating a 3D terrain. Perhaps I should go for a photorealistic animation? How about the 'Crown of Thorns' (Coruna spinea) making its way over rocks? Or the 'Blue Jester' (Fossor azureus) walking on the forest floor? The 'Lesser Strandsprab' (Nepa aranea) would do well on a beach, but the 'Hairstar' (Coma confusa) would be difficult to depict, with its hair cover. By the way, all of these appear in paintings I am working on...                    

The challenge: Tarquin's Jackjaw

So the challenge was a difficult one... Both rodlox and Petr were right: the animal indeed came from the Dr. Grordbort collection of nicely mounted Venusian insectoids. I had hoped for a proper species identification though. I guess the walls of the Virtual Main Hall will have to remain in their pristine state for the time being. Oh well.

Click to enlarge; copyright Weta Limited.

Click to enlarge; copyright Weta Limited

The animal in question is a Tarquin's Jackjaw. I found it along with four other unreleased 'Framed Venusian Wildlife Specimens' in the book "Weta, the Collector's Guide". The top image above one is a scan from the book, and the one below that was taken directly from the Weta site. The sculptor's name is not completely visible, but credit should go to where it is due: Jamie Beswarick. You can find more about him on the Weta side. 

I would rather like to own one of these display specimens, and am convincing myself that I in fact need one. Mind you, the book also contains rather large trophies of Venusian animals' heads, to hang on your wall. perhaps I will show these later, as I am rather fond of the Dr Grordbort universe.

But not next week; I will be returning to Furahan spidrids..

A challenge for the speculative exobiology experts out there

I decided to NOT present another chapter on spidrids this time, seeing as how I presented two posts in a row on that subject already. There is definitely more to say on spidrids, and I will do so, but the next spidrid post will be better if I take more time. Another argument to step away from all those little and not-so-little spidrids scurrying around my feet is that I found myself thinking "Earth crab locomotion really resembles Furahan spidrid walks more than I expected". When you start taking fictional animals as the yardstick against which Earth animals are compared, it is perhaps time to tone the level of 'biogeekery' down a bit. Well, not that much, really, as instead of a post on spidrid gaits I will present you with a question challenging your level of expertise in this particular field.

Click to enlarge

What's the animal you see here? Mind you, I altered the image a bit to make it slightly more difficult. I will reveal the source in a week. The prize is a virtual plaque with your achievement on it on the wall of the Virtual Main Hall of the Institute of Furahan Biology. That's something, isn't it?   

Skitters ('Falling Skies'), spidrids and footless crabs

The previous post was devoted to spidrids, Furahan radially symmetrical walking animals. In thinking about their movement my attention was drawn more and more to Earth crabs, because the anatomy of their legs proved to resemble the anatomy I had chosen for spidrid legs. I admit that spidrid leg anatomy mostly came about because of a wish to see them walk. There are programming problems that I cannot solve myself (inverse kinematics) and I do not have any of the high-end graphics programs that can do it for me. So that is why I came up with the simple scheme I discussed before: at the hip joint there is a vertical axis of movement allowing clockwise rotation, but all other leg segments have horizontal axes enabling shortening and extension of the legs. Later, the exact same scheme proved popular with robot builders.

Last time I discussed a feature of crab (or spidrid) legs that I still do not understand well: slanting. I tried to contact arthropod experts but without luck so far. But there is another feature of crabs I do not understand: why do they not have feet? Crab legs more or less just end in rounded points. This is odd, because crabs, often living in tidal water, need to be able to get to good hold on the ground they are walking on, or else they may be swept away by the current. You would expect feet with curved claws for that. I have not found a discussion of why crabs do not have feet, just one mention of the fact that they do not. As I was considering adding feet to spidrids, I wondered whether there could be an anatomical reason for their absence. That will be discussed first, and only then will the 'skitters' of  'Falling Skies' be discussed: they have feet.

Click to enlarge; copyright Gert van Dijk

In the past I discussed the influence of leg posture on the anatomy of leg joints, and in particular how many axes are needed. The image above shows a new version of an old design (Mechanicus disneius). In the left image the leg is simply rotated forward around the vertical axis of the hip/shoulder joint: the foot ends up rotated with respect to the ground. While walking, the foot should move along a path shown as a stripe on the ground; in reality the animal moves forwards and the foot stays on the ground; it's the same relative movement. The foot should point forwards during the entire step, and that requires rotation around an axis parallel to the most distal leg segment, shown by an axis sticking out of it. The image on the right shows where the foot should be: many joint positions need to be adapted, but the one important here is the rotation of the feet around that longitudinal distal leg axis. By the way, rotate your hands palm up and palm down: what you are doing is pronation (palm down) and supination (palm up), which is just such a movement, due to the long bones in the forearm crossing and uncrossing.

The need for pronation and supination must be present for any animal with sprawling legs. That does not only include lizards and turtles, but arthropods -and spidrids and radial robots!-. I have not found any mention of how arthropods solve this problem. The robot builders just ignore it. Insects have a series of short segments at the need of their legs, the tarsus, linked through ball and socket joints. Are they how insects cope? Does the tarsus more or less flip around during a leg cycle? As said, crabs have no feet, so the end segment of their legs must turn in place during a step, rotating against the ground. Do crabs have no feet because there is no mechanism for pronation and supination? I cannot believe that. All this rotation with friction without a proper way to grab the ground seems an odd way to design a leg, and yet it seems to be there. If anyone knows an arthropod limb expert, please ask them...

Click to enlarge

What does all this have to do with the skitters of the television series 'Falling Skies'? Well, they have radial legs with feet, in an interesting example of 'convergent speculation'. A skitter is shown above, along with my simplified version to illustrate its anatomy (Disneius horrificus). Skitters have invaded an conquered Earth, but as the story unfolds it seems they did so under duress, so to speak. Their top end shows bilateral symmetry but their nether end has radial symmetry, which is odd. There are more aspects that suggest we should be well prepared to suspend disbelief (I like the series). For one, the legs are overly thick given that there are six of them. In itself that might indicate evolution in a very high gravity, but the sprawled position of the legs argues strongly against that. They can walk along vertical walls, a feature so unlikely there is little need to discuss it. They communicate through radio, an old favourite of speculative biology that seems difficult to get underway in an evolutionary sense. (All right, here is why I think so: the evolution of biological radio might well start with a primitive capability to receive radio waves, but can you see/hear with that sense? Where is the benefit?).

                            
The video fragments above show skitters in action. I have repeated the very short fragments to make it easier to see what is going on. It is clear that their nether ends are indeed fully radial, and that the feet accordingly stick out in all directions.

Above you see another fragment, one I rather like. I have repeated this one a few times as well. The skitter turns as it negotiates the path between the furniture. It must turn, as its top end has clear front and back sides: if it has to face the humans, it has to turn. With that clear preference you would expect its bottom end to have an equally clear fore to aft distinction, but that part is radially symmetrical. The one distinguishing feature of radial symmetry is that it allows movement in all directions. With such feet, a skitter might be expected to walk in any direction with equal ease, even directions in which it cannot see...  

The feet remain planted on the floor during the turn, so the legs in fact rotate around a longitudinal axis of the distal leg segment. That is nice; a pity that the anatomical mechanism is not visible. I very much like the way the animators solved the problem of how an organism with such a wide leg base negotiates the limited space between the furniture, designed for the much narrower forms of humans. The skitter behaves like an all-terrain vehicle and simply puts its legs on the furniture where needed, evening out the differences in height between its feet as it goes. That is very well done, I think.

But still... Leaving spidrids with leg points that pirouette against the ground at every step is very unsatisfying. The feeling is a bit like when you are unable to solve a puzzle, the answer of which must be staring you in the face. I think I will equip spidrids with feet, if only to end the irritation. That still leaves crabs, presenting the same puzzle...    
          

Sprirally slanted spidrids

Arthropod leg design seem straightforward, but there are several aspects about them that i do not understand, so I have been trying to find out more about them. Answers prove hard to find though, and so for this post I will limit myself to just one thing: why do Earth arthropods such as crabs and scorpions often walk with their legs slanted with respect to the vertical?

Click to enlarge; copyright Gert van Dijk

This image shows my prototypical spidrid. Most joints are fairly simple with just one axis of rotation. The legs turn clockwise or anticlockwise at the joint with the vertical axis near the body; let's call that the coxa, to keep the arthropod analogue going (the word means 'hip'). The other joints simply bend and straighten the leg. The result of this simple design is that each leg operates in a vertical plane. I felt that this makes sense from a construction point of view; no slanting here.

Click to enlarge; from Wikipedia

But this wonderful image of a crab shows a different pattern: the plane in which each legs lies is heavily slanted with respect to the vertical. From a point of countering gravity this design does not seem wise, but crabs are not very large, so gravity is less a constraint than it is for animals with a larger mass. Slanting must be good for something. It makes the crab flatter while the reach of the leg is not compromised. Perhaps flattening is good for animals living in crevasses. Another possible explanation might have to do with propulsion. In your typical spidrid the propulsive force of the legs at the side comes from muscles that rotate the leg clockwise or anticlockwise with regard to the body. This joint lies far from the point where the leg exerts force on the ground, which may weaken the design. The muscles that extend and flex the leg do not help much in propulsion. But slanting the legs would mean that those same muscles can now add their force to pushing against the ground. Perhaps that is it; if so, I will have to look harder for evidence.

But instead of doing so I wondered whether slanting could work for spidrids, so I played a bit with Matlab and produced some animations. My first idea was to put an angle to the vertical axis connecting the leg to the body. Doing so would slant the leg when it rotates with respect to the body, and more so the more the leg is rotated. When the leg is just sticking out from the body it would not be slanted. What you see above is the result. As you can see, the leading and trailing legs are not rotated in a clockwise or anticlockwise direction, but 'just stick out'. They do not exhibit slanting at all. The other legs are slanted when at maximal angles, and the extension muscles in them could help push the animal forward. It looks intriguing, doesn't it?

Of course, I could not resist having the legs slant the other way, but that was probably a mistake: I do not see them providing additional propulsion this way. The spidrid is not flattened, because the leading and trailing legs still lie in a vertical plane.

If flattening is needed, all legs will have to be slanted, and that idea resulted in the spidrid above. Aas far as propulsion is concerned there may be a problem. Crabs are bilaterally symmetrical, so the left and right legs can both push against the ground in the slanted position. But spidrids have radial symmetry, and that means that legs on one side can provide an additional push while the ones on the other side do not. But I am not certain that that is what slanting is for, so perhaps it is no problem. Meanwhile, I rather like the somewhat sinister aspect of this 'spirally slanted spidrid'             

Another grouillard: Eructator admonitionis

After an involuntary six month hiatus I have finally been able to pick up painting again, and have since finished one small painting, one big one, have started a new large one, and have generally been working toward producing a pdf file that I can send to publishers as a sample of the book that should persuade them to publish it. I need to do just one additional illustration for the marblebill page, one that will probably show a dawn scene with screeching marblebills outlined against the sky.

I decided to show you one such additional illustration, showing a species of grouillard related to the one on the Furaha site.

Click to enlarge; copyright Gert van Dijk

 And here it is. As you can see, it has a banded colour pattern that just screams 'Here I am!' to any animal with a decent vision. Remember that the grouillard you are familiar with (Oructator olidus) was black and brightly red, so it wasn't exactly given to camouflage either. These warning colours, because that is what they are, tell other animals not to fool around with a grouillard: except for its extremely nauseating and somewhat toxic spit, the animals taste extraordinarily awful. Young predators may bite into a grouillard once, and that grouillard will not live to tell about the experience. But the predator will, and will not bother another grouillard for a long time, if ever.

So there you are. It was fun painting the mossy fungoid thingy on the branch. I still have no good notion of what all the various brushes in Corel Painter 12 can do, my favourite painting software. When I find a better fungoid-producing bbrush, I may erase this one and do it again. So far I set out to copy my usual method of working in oils as closely as possible. That seems to work nicely, so it is time to see what else I can do with this box of tricks.

Furaha on the silver screen: "How to describe a cloud"

In previous posts I had mentioned that Furaha would appear in a film, hadn't I? Yes, I had; the proof is here, in a post of almost one year ago.

Well, the wait is almost over. Next Saturday the film will see its world première at the International Film Festival in Rotterdam. The director is David Verbeek, and the film is produced by Conijn Films. The film is in Chinese, but it is a Dutch film nevertheless. I will write more about it later. Meanwhile, here is the information from the website of the festival (right here):

"A young woman in Taipei is confronted with the blindness of her mother who is convinced she has a sixth sense. Shot with little facilities, but it is Verbeek’s most mature film - about uprooting, spirituality and modernity. Nominated for The Big Screen Award.

David Verbeek made this film on a tiny budget in the lull before his larger Dead & Beautiful had been fully financed. With its reflections on the role of spirituality in modern society, this film may be a precursor to his next, a socially critical vampire film.
 
But How to Describe a Cloud can stand on its own two feet. The story of Liling seems to unfold intuitively. When her mother goes blind, the young musician is suddenly forced to leave her big-city cocoon and return to the small island where she grew up. There, her scientific approach to blindness, in which she presents the world to her mother through words on the advice of the doctor, clashes with her old mother's spiritual approach. She argues that she can’t see the world around her any more, but can still sense it. Could the science fiction drawings of the former scientist with whom she flirts in the city provide mediation? Also see Immortelle in Tiger Awards Competition for Short Films."


Just guess who made the 'science fiction drawings of the former scientist'... To prove that, here is the film's trailer (there is a much better quality version on YouTube here).

 Exciting, isn't it?