What do Roman soldiers, trilobites and the Furahan Droodle have in common?

Click to enlarge; copyright Gert van Dijk

Before answering that, you are probably wondering what a Furahan 'droodle' is. Well, one description found in the 'Annals of IFB Field Expeditions' reads as follows: 'The droodle is a slow an silly creature, behaving in its snug little world like the Lord of Creation, notwithstanding its utter insignificance'. Apparently the Annals did not require any semblance of scientific impartiality before accepting contributions, but that is not the point. Further on the author continues: "It can behave this way because it is well protected because of its foul taste and because of the overlapping armoured plates covering nearly its entire body."

An early version of the droodle is shown above. And there we are: the droodle has an armour consisting of overlapping plates, and so did trilobites, and so did Roman soldiers. I came across the subject when I was preparing to paint the droodle anew for The Book. Most of my new paintings have very little to do with the old ones, but I like some old designs enough to go over them again, taking the opportunity to improve them in as many ways as I can. I started wondering how animals manage to move while covered with what seem like very stiff plates. How are these plates attached to one another? Obviously, in arthropod legs the exoskeleton of adjacent parts of the leg form joints that often have just one axis of movement, much like our own knees: we can bend it stretch a knee but it does not move sideways nor can we rotate the leg and foot backwards. I started thinking about whether that also applies to the plates covering the droodle.

Click to enlarge; copyright Gert van Dijk

Above you see the result of simple experiment: I wanted to form successive hoops curving around the animal's back while widening at the sides. I imagined a hinge between two hoops with the axis of rotation about halfway up the animal. Of course, such an axis of rotation would make sideways movement impossible, but so be it. I assumed that the plate in front would slide over the plate in back. I could imagine that in my mind's eye for half-circular hoops, but felt I needed some visual help with hoops that widened at the side: could they in fact slide over one another over their entire length, or would they intersect, making the movement impossible? So I made a rough shape like that in Vue, of which the top surface represents the plate. As you can see, at the centre one hoop can easily slide under the next one while it wants to move over it at the sides. This does depend on the site of the hinge and some other aspects, but it does show that you cannot assume any angle or shape to work. I could of course still paint it the intended way and no-one would be the wiser. But science has preference over art in such matters. So what was wrong?
 

Click to enlarge; Manton, The Arthropods 1977

I then thought of trilobites: their name indicates that their bodies had three lobes lengthwise, with a thick part in the middle and much narrower side flanges, much like the droodle. And trilobites could roll up their bodies, so they solved the problem how to slide one hoop under the other better than I had (weel, they had more time...). I obviously needed expert guidance, perhaps a book called 'Biomechanics of trilobite intertergite movement' (the hoops are called 'tergites', just so you know). I found something close: 'The Arthropoda' by S.M. Manton, 1977. I am not new to reading scientific papers, but this book is as intricate as it is condensed: the reader is assumed to be rather well-versed in arthropod classification and anatomy. The book contains sentences like 'There were no coxal endites or gnathobases.', in its own way as wonderful as 'It was a dark and stormy night'.

Click to enlarge; Manton, The Arthropods 1977

So here is Figures 1.5 from that book: have a good look at the top right in particular: the tergites are connected by a fold of skin, doubling back on itself. There is no hinge to be seen anywhere. If the tergites are indeed connected only by such a fold over their entire length, they would have much greater freedom of movement than if there were just one axis of rotation: this is a good idea.

I then wondered if his is how all armour segments are connected in arthopods, and browsed through the book. As you would expect, there are a myriad adaptations of tergite movement. In species that burrow, successive tergites are kept from sliding over one another and have a built in 'door stop', allowing the animal to push the soil out of its way. In other species there are additional small tergites normally hidden between larger ones. When the body is flexed, the gap that would otherwise appear between the large ones is filled by the small ones. In many cases tergites only cover the back of the beast. There may be other bits of hoops at the belly (sternites) or the sides (pleurites), all of which are connected to one another by the folds of skin.

Click to enlarge; from The Arthopods, SM Manton 1977

Here is an example of the intricacy of the internal anatomy of a millipede. The large top image shows the poor millipede cut lengthwise with its body flexed (the back is at the top). The lower left image is a horizontal section of the body rotated sideways. Complex, aren't they? The feature I would like to call your attention to is the folding of the skin between tergites: I found that in all such plates.

So I learned from all this that arthropod tergites can in fact be connected by 'hard points', but in many cases the skin folds allow flexibility and freedom of movement. So that was one problem solved, but all this did not answer the question how to ensure that the tergites do not 'intersect' while rotating, as they did in my simple model? Or how do you avoid having large gaps form when the animal moves? There may be several answers to these questions. Perhaps the tergites should be flexible.

( video does not seem to work; I will check later...)
Have a look at the YouTube video above, of a millipede flexing its body in all directions: the tergites do not show any gaps at all, and yet they slide over one another in at least two directions of rotation: they must be flexible. But would that work for a big animal, in which you would expect the tergites to be stiff? (but never brittle: the armour must be capable of withstanding blows, and allowing it to deform it a bit should absorb the energy of a blow). 

Another solution would be to forgo tergites that run from one side of the animal to the other; split them up in separate parts instead. These smaller plates could each be tough, and be connected with skin folds. At the top of this post you'll find a simple model I made to see what a droodle designed in this manner might look like. Mind you, this is not what the painting look like: the droodle has already evolved some more and no longer looks like this, but it does still have multiple overlapping tergites.

Click to enlarge; source here

So what about the Romans? Everyone who has ever seen a film with Roman soldiers in it, or who has read an Asterix book, knows that their armour consisted of metal hoops circling the soldier's body. This seemed very similar to the tergites of trilobites or other artropods, so I wondered how the legionnaire's hoops were connected, That was easy to find out: there are books describing actual archeological finds. The image above is from tha reconstruction based on such finds. The Latin word for this particular type of armour is 'lorica segmentata' ( I also leaned that all these films might be wrong: this plate armour type may not have been the standard type of armour; chain mail may well have been more common.) And here is how the loops are connected: not by hinges, but by leather straps on the inside of the hoops.

( video does not seem to work; I will check later...) 
The video above shows the Roman lorica in action (the inside is well visible two minutes into the video). The hoops could slide and rotate a bit with this arrangement, in exactly the same way that the tergites of trilobite could slide over one another thanks to being connected by folds of skin. Only the trilobites had their armour some 520 millions of years before the Romans invented their lorica.

So this is how the droodle came by its armour and by its scientific name of 'Lorica segmentata'. There is a long list of items the Romans did for us, to which I would humbly like to add that they can make you think how exoskeletons work. Not a bad thing at all.