Sunday, 24 February 2013

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.

video


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?

video

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.

video

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'             

Saturday, 9 February 2013

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.