Tetropters

Those who visited the land and sea pages on the site may have wondered why there is no page on the biomechanics of flight, seeing there are pages on 'Walking with...' as well as on 'Swimming with...'. So where is the 'Flying with...' page?

The simple answer is that I am not happy with it yet. There is material on ballonts and on tetrapterate (four-winged) large flyers, and some are in fact shown on the 'air' page. But these organisms are fairly like Terran birds. For a true oddity the tetropters should be considered; in their case, 'oddness' does not reside in them having four wings. After all, Furahan tetrapterates and Terran insects also have two pairs of wings, i.e., four wings, so that isn't really extraordinary. No, the ordinariness resides in the description: 'two pairs of wings', and that says it all: the wings are arranged in pairs. Insects and birds (and Furaha tetrapterayes) all have a body scheme with bilateral symmetry, so their limbs are arranged in pairs.

Not so the tetropters. To be honest, the very first sketches I did of them did show bilateral symmetry. The top animal in the following image shows that primordial tetropter. In fact, their wing movement patterns had already been worked out, and showed a pattern that was exactly the same for each wing. So the wings showed a radial pattern, like a four-pointed star, but the body hadn't kept up. Here is one of those early tetropter designs under attack from a larger tetrapterate.

With their radial wing pattern tetropters were excellent hoverers, and control over wing movements should have allowed omnidirectional movements. But the animals still had a front and aft side. If you have a flight system that allows such tremendous manoeuvrability, why limit it with a limited body design? That is where the concept of complete four-sided symmetry came from. The next sketch illustrates the next logical step in the evolution of the tetroper concept. The top animal has bilateral symmetry, but the bottom one represents a conceptual novelty: the body follows the wings! So the entire animal now shows complete quadriradiate symmetry. By the way, I was taught that you should not mix Latin and Greek roots, and that is explains the switches between 'tetra-' (Greek) and 'quadri-'(Latin). Can't be helped.


I know of no such designs on Earth, although there are animals with five-sided symmetry (starfish, sea urchins etc.). The rest of their body scheme hasn't been worked out in much detail. The design problems are like those of octapods, with eight-sided symmetry. Tetropters too have eyes above as well as below their 'equator', and the mouth parts are all on the ground side. But I haven't done a complete drawing or painting of one yet, so the details remain sketchy - for now.

I found that the flight patterns had much in common with the movement patterns of flying and walking organisms. After all, each leg or wing or flipper moves repetitively, and any gait is no more than a cycle in which the limbs move with a specific set of phase offsets. Although there are an infinite number of ways in which you can do so, only a limited number make much sense. And so tetropters have 'walks', 'trots' and 'paces'. The first pattern shown here is a rather silly one.



I can't show the animated gif here; to see it click here.

Imagine the sphere as the animal's body, seen from below and to the side. The four wings are rotated around their axis to provide lift while going clockwise as well as moving back anticlockwise. While such a pattern does provide lift, it would also result in a net rotation effect on the animal as a whole, which is impractical. To offset that, the rotation forces need to be cancelled by having two wings swing in the opposite direction from the other two. A rough animation of just such an effect follows:


I can't show the animated gif here; to see it click here.

You can see the wings going through one another; in reality they would of course not do so. That's why I need to do a better animation, but it is not as easy to do as it sounds. In the smallest tetropters the wings clap against one another and then to move in the other direction. Many Earth insects use this technique, by the way: the clap their wings together behind their backs, and this apparently generates lift when the wings move away from one another again. In insects, there is just one such 'clap' in each cycle. Tetropters have taken the idea one step further, and the clapping occurs twice in a wing cycle, not just once.

Larger tetropters usually avoid clapping the wings together, so they reverse direction without touching. Whether this is aerodynamicaly better or simply avoids damage to the wings remains to be seen. There is so much to be researched...