Monday, 14 May 2018

How do tetropters walk? (Tetropters IX)

In a recent post I showed my latest animation of tetropter flight, using a brightly coloured farfalloid species as an example. As I wrote then, the reason to get down to the nuts and bolts of tetropter anatomy and movement was that I am painting a few tetropters paintings.

Click to enlarge; copyright Gert van Dijk
Here is a small fragment of the latest one. I had given most attention to tetropter wing movement, but naturalistic paintings also require details about the rest of their anatomy, such as eyes, mouth and legs. The radial nature of tetropters is very reminiscent of that of spidrids; tetropters obviously share a common ancestor with spidrids. On the whole, tetropters are much smaller than spidrids. Whereas spidrids are in the crab range, tetropters are more like insects in size. The Furahan atmosphere is denser than Earth's, which makes flying easier. The tetropter respiratory system does not wholly depend on passive diffusion, so it does not form a crucial limiting factor. Some tetropters, such as the Red Baron shown earlier, are quite a bit larger than current earth insects. There may well be tetropter species in remote areas that are as large as the giant dragonflies from Earth's Carboniferous era. These areas have not been explored in detail yet: they are far away and travel is expensive.

Tetropters have eight legs, just as spidrids do, and their gaits are in many cases exactly like those of spidrids. There are exceptions though. Tetropter legs differ in some aspects from spidrid legs. The most obvious difference is that the legs of a tetropter need not be all alike. In contrast,  all eight legs of any spidrid are virtually identical. Again, this is a bit like insects' legs, that usually differ markedly in size and shape between front, middle and hind legs. This probably makes sense because these legs have different mechanical roles, whereas tetropters do not even have a front or a back. The asymmetry of tetropter legs takes a shape that is peculiar to their radial nature, and quite fitting: there are four large legs and four smaller ones, and they alternate: big, small, big, small, etc. Over evolutionary time, the differences have become quite marked in some clades. In predators such as the 'Red Baron' the outer ring of legs has gained a grasping function. In most species both the outer and inner rings are used for walking. Some say that the differences came about in response to a need to stop the legs becoming entangled; that sounds good, but spidrids do not seem to suffer from tripping over their own legs! Others say that the small size of tetropters means they needed legs that are splayed very wide to stop them being blown over by the wind. But why should that hold for just four legs? Sometimes we just do not know... (meaning I will shelve the question for later, or perhaps I will leave it unsold. There are many things unclear in Earth biology, so perhaps I do not have to explain everything).

Anyway, here is a schematic tetropter using the 'double table' gait. Its wings are neatly held in their vertical resting position. At any time there are four legs of either the outer or the inner ring on the ground. For a brief moment there are eight. This system is just as stable as the 'double tripod' of insects. There is little or no chance of falling. Note that the body wobbles a bit. I did that just so you could see that there is a joint between the 'corpus' holding the legs and mouth on the one hand, and the cephalothorax holding eyes and wings on the other hand.

This specimen proves that the gait can be a bit more fanciful than the 'double table' without destroying overall stability. You may also note that the joints of the legs are arranged in a different way. In the previous species, and in all spidrids, the angles between the three big leg segments always bend in the same direction, so the leg gets curved more inwards and downwards as you progress from the proximal portions near the body to the distal parts at the tip. In this particular species, the first joint bends in the other way. In earth arthropods you can easily find these patterns too.

Finally, here is a walking tetropter in which the joints of the legs of the outer ring all curve inwards, while the inner legs have yet another pattern, starting with a downwards followed by an upwards bend. The gait is somewhat complex as well, which I like, as it gives the animal a more biological feel.

So there we are; now I can safely paint an explanatory diagram explaining how tetropters walk. After that, it's back to 'toe studies' again. I must say I am distracted because I watched season 4 of Game of Thrones again. There is a scene in a giant rides a mammoth. Hang on; as I calculated earlier, such a giant should weigh about 1440 kg! Mammoths are big and probably strong, but that is some weight! How much weight can a mammoth actually carry? That is obviously a very silly question, but also one quite worthy of this blog. I may need to find out...


L13B3 14641 said...

Very fun. I wasn't expecting that tetrapters would hold their wings straight up! It's quite visually interesting.

And I do hope that post on the possible strength of mammoths does does indeed come together, as that sounds like quite the interesting topic.

Sigmund Nastrazzurro said...

L13B3 14641: Thank you; glad you like it. Where to put the wings was something I had not considered previously. Tetropters cannot simply fold them over their backs out of the way, so they either just stick the wings out in all four directions or upwards. In a way butterflies and moths face the same problem: their wings are to large too fold away too, so there are only two options left: like an open book or like a closed book.

L13B3 14641 said...

I don't believe they're taxonomically distinct, but it is interesting that butterflies and moths both seem to have stumbled on different strategies in regards to put their wings,as you said. I believe dragonflies and damselflies respectively found quite similar solutions to a similar problem.

Perhaps there are tetrapters that use either strategy? Or tetrapters with narrow, downswept wings. Though, personally I find upswept wings to be the more visually appealing solution. Just musing.

It's interesting to me that tetrapters tend to be small. The portmanteau of "tetra" and "pter" evokes the word "raptor".

Sigmund Nastrazzurro said...

L13B3 14641: Many tetrapters are capable of holding their wings at rest in the horizontal cross position, as well s in a vertical 'out of the way' position. I can see some of them flicking their wings from one to the other postion, attracting mates, or warning competitors, etc.

I like the 'raptor' hidden in tetrapter. The Red baron iof a few posts ago certainly is a predator, but not very large. They are exoskeletal like spidrids. They can easily grow larget than their earth counterparts, so you can expect many spidrids as large as coconut crabs, and tetrapters as large as the extinct Meganeura. But not larger than that...