Click to enlarge; copyright Gert van Dijk
Rusps feature on the 'walking with rusps' page on the main Furaha site (on the 'land' page). The image above shows the first ever sketches of 'rusps'. (Their name is inspired by how Dutch children may have difficulty with the word for caterpillar, and end up saying 'rusp' instead of 'rups'.) As you can see, rusps have multiple legs and are protected by a strong carapax. My notes alongside the sketch stated whether such presumably slow animals could survive in an environment of agile large animals. But I also noted that sea urchins are bright nor fast, but do quite well. Such considerations made me add stings, evolved from stings alongside the body through poisonous hairy plumes into mean-looking fore and aft tentacles equipped with poisonous barbs. Their bodies are segmented, and there are multiple eyes on the front and back; in fact the lower rusp also has secondary eyes along the body. The reason there is a tentacle at both ends had to do with my wish to protect the animal. I have always been surprised by the inability of earth's animals to protect their rear end well, a weakness exploited with great success by predators such as lions and hyenas. Of course, this 'active rear' design, with eyes, a tentacle and the corresponding neural circuitry, represents a departure from Earth patterns, where no large animal has a 'split brain' to this degree. It reminds me of the claims for a 'secondary brain', such as was once implied for stegosaurs? Such a design goes against the theme of 'cephalisation', and I am not at all certain that it could actually work. Then again, I see no definite reason why it could not.
Click to enlarge; copyright Gert van Dijk
I almost immediately wished to make them bigger, at least elephant sized. Here is another early sketch, showing how a large rusp crosses a stream sunk into the ground. Animals such as elephants have great difficulty in doing so, as they cannot jump at all and dislike steep inclines. I thought that rusps would have the advantage of being able to use their long bodies: they can hold the front part of their body in the air at some energetic cost until they are able to find a foothold on the other side.
Rusps probably evolved from millipede-like ancestors. In contrast to Earths arthropods these ancestors were obviously equipped with respiratory and circulatory systems allowing adaptations for large bodiess. They were also endoskeletal, for similar reasons. The carapax is not a part of the skeleton, but simply an outer protective covering; it is leathery rather than hard.
Click to enlarge; copyright Gert van Dijk
The top image shows one from a low viewpoint, a trick that by itself suggests large size, as viewers often assume a viewing point at eye level. What you can also see is how the legs move in a sort of ripple, with adjacent legs moving a bit out of phase with the ones in front and behind. That is the usual gait of Earth's millipedes and centipedes, by the way (this is also animated on the main Furaha site). The sketch in the middle show transverse sections, illustrating my thoughts on their possible body shapes. I did not want their legs knocking into one another, so I thought that successive pairs could be offset to the left or the right, giving each room to move. To evoke an atmosphere of large size, I played with the idea of making them high and narrow rather than wide and squat. If you take a good look at elephants and many dinosaurs you will see that they are often relatively narrow. I also played with the site of attachment of the legs: underneath the animal or along its sides? The sketch at the bottom shows a resulting rusp of the narrow type. I rather like their tentacles, but realised that they woud pose biomechanical problems: holding them horizontally with muscle power alone would be as comfortable and efficient as humans holding their arms horizontally in the air all day: try to do it for 10 minutes...
Click to enlarge; copyright Gert van Dijk
Rusps only made it to a painting once, a detail of which is shown above. But even on that painting they were in the middle distance. One of the reasons they never received their own painting was that I had problems in finding a suitable design. With their long horizontal bodies they present a somewhat passive shape. One solution would be to show them from the front with a considerable degree of foreshortening, such as in the images above, or I could show only a part of their body, such as in the 'fording a stream' design above. But there were other problems as well, such as the relative positions of the legs, the 'active rear' with its corollary 'split brain', and of coarse the tentacles, that cannot just be held in the air horizontally like that. For a structure held up with muscle power only, such a position is virtually impossible; just try to hold your arms with extended elbows horizontally in the air for 10 minutes, and you will find out why...
Click to enlarge; copyright Gert van Dijk
I keep coming back to rusps, and above is a recent study done with Vue Infinite. Here, too, the successive legs are offset resulting in a double track on each side. It has as a disadvantage that the hip joint is not directly above the foot. As a painting it results in a rather sedate view, but the well-lit afternoon sunshine could make up for that. Rusps ought to look grandiose, and perhaps a design like this brings that out.
The one thing I am not worried about at all is the leg pattern. The two animations are newer 3D versions of rusp locomotion. The essence of centipede or millipede locomotion is that there are successive phase differences between successive legs. Depending on whether a leg is either just ahead or behind in its cycle compared to the leg behind it, the legs tend to clump together either on the ground or in the air. The effect is that the movement appears to ripple either forwards or backwards along the animal. For centipedes and millipedes the choice seems to depend on how far sideways the legs are held. In rusps, the legs are always held vertically, so leg tangles have to be avoided in another way. Possible solutions are of course to make the legs shorter, place them further apart, or, as I wrote above, to have their feet offset. On the other hand, the animation shows that manipulating the phase differences can avoid leg entanglement quite well, so the simplest solution is probably the best one. In short, there seems to be little need to offset the legs.
Click to enlarge; From: Steven Vogel. Comparative biomechanics. Princeton Oxford 2003
Click to enlarge; from:Klein et al; biology of the sauropod dinosaurs. Indiana University Press 2011
As for the tentacles, some further thought suggested a solution. Sauropod necks and tails are very long structures held out horizontally in positions that at first glance seem impossible. This is possible because they are not tentacles but trusses: there are bones resisting compression at the bottom and tendons to withstand pulling forces at the top. With a design like that, rusps whips could function and wreak havoc on any hexapod predator foolish enough to enter the rusps' range of motion, at its front or back. Rusps whips will not curve in three dimensions as gracefully as they do in the sketches above, but will be a bit stiffer. Expect the cross section of future rusp whips to be narrow and high.
The next diversion into the archives will explain the relation between rusps and major Gruber, a creation of the late Moebius, a French grand master of bandes dessinées (comics).
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