The weevil with screws in its legs

June 30, 2011 • 1:49 pm

by Matthew Cobb

One of the things that natural selection hasn’t created is the wheel (and don’t give me all that stuff about the golden wheel spider rolling up and bowling down Namibian sand-dunes to escape from a predatory wasp! That doesn’t count and you know it. I mean a wheel and an axle). It also hasn’t created the screw. Or rather, it hadn’t. In a brief article that has just appeared in Science (subscription needed to get past abstract—grrr), Thomas van der Kamp, Patrik Vagovic, Tilo Baumback and Alexander Riedel from Karlsruhe and Berlin describe how the leg joints in the weevil Trigonopterus oblongus “work as a biological screw-and-nut system”. And it might just be related to why there are no wheels in nature, or at least in weevils.

Here’s the only figure in the article. It shows reconstructions of the ‘coxa’ (the first leg joint, where the leg joins the thorax) in green, and the trochanter, the first leg segment, in yellow. The coxa is the thread, the trochanter the screw. There’s also a nice scanning electron microscope image of the trochanter. The system works pretty much like your hip does – except the knobbly bit at the end of your hip has turned into a screw shape. (All the nerves and stuff run down the centre of the trochanter.) Most insect joints work like hinges.

The 3D reconstruction [(A) to (E)] of coxa (green) and trochanter (yellow) of left hind leg of T. oblongus. (A) Depressed position. (B) Elevated position. (C) Coxa cut horizontally along rotation axis; dorsal aspect of trochanter while leg depressed. (D). Isolated trochanter showing external spiral thread and tendon (t). (E) Dorsal portion of coxa corresponding to ventral portion of (C). (F) Scanning electron microscope photograph of the right metatrochanter showing posterior condyle (c) and external spiral thread (e).
So why doesn’t the leg unscrew itself? Most of the weevil’s leg motion will not involve a 360 degree rotation, or the hapless insect would get stuck after a couple of paces. Instead it will simply swing back and forth. Doesn’t the leg get blocked when it’s screwed in to the max? Presumably so, in which case careful observation should show weevils back-pedalling to unwind their legs. And conversely – why doesn’t the leg come unscrewed? The muscles appear to hold it in place. Phew!
The Science website has a little article about this and a nice image showing SEMs of both screw (r) and thread (l), and of the weevil itself:
Credit: Science/AAAS
Now why does the weevil have this odd arrangement? The obvious advantage is that you can rotate the leg right round. But in that case, why not evolve the axle/wheel combination? The authors speculate that the screw might be better:
We suggest that an advantage of this construction is that the leg comes to a stable resting position, preventing passive straining of leg muscles, which would not be accomplished by an axle construction.
Above all, they think that it might be the weevil’s unique feeding posture, where it shoves its rostrum (its ‘snout’) into its food, that holds the key. Substantial forces will be generated on the weevil’s legs as it tries to grip the substrate; having a screw would effectively block the rotating joint, stopping the weevil from ending up with its head smashed in the food.
If you don’t understand what I mean – try it; put your palms flat on the table, your arms braced, then put your face down onto the table; flex your arm muscles (the equivalent of the screw-lock) and you’ll be fine –  let them go (the equivalent of having an axle) and you’ll end up with your head on the table… Most undignified – even for a weevil.
[EDIT – various points clarified after a hasty first draft! h/t Jerry and Adam M. You can sort the rest of it out yourselves in the comments…]

22 thoughts on “The weevil with screws in its legs

  1. Curiosities:

    1. Can its legs be completely unscrewed?
    2. How does it get blood, oxygen, etc. into the leg? Or does it not need to?
    3. “flex your arm muscles then let them go and you’ll end up with your head on the table…” What?? I can’t figure out how that’s supposed to work.

    🙂

    1. I had just grabbed The Ancestor’s Tale – pages 558 ff in the paperback – The Rhizobium’s Tale…

      1. Haven’t read that one of Dawkins yet. I presume he discusses the flagella on rhizobia (some?), but his perspective on that phenomenon is what? Certainly seems to me to qualify as an example of a biological wheel and axle, even if the wheel – the ring – is internal rather than external.

        But fascinating mechanism. Seems like it would be interesting or illuminating to question why it hasn’t manifested itself in larger structures – issues of providing nutrients, of growth and of scaling might head the list.

        Interesting also that it figures centrally in Behe’s Darwin’s Black Box which he, of course, uses to buttress his untenable argument of intelligent design – at least insofar as the author, the designer, shows some passing resemblance to Jehovah. Interesting, though somewhat sad, example of the “sacrifice of the intellect” to religious dogma …

  2. Sorry Jerry, but Sexual Selection evolved another screw; the penis of a pig. As an old farm boy I have seen this phenomenon many times when we were breeding sows to the boar. GOTO the internet and Google, ‘penis, pig’ and SELECT, “WATCH PIG PENIS VIDEO” for a closeup of the boar’s screw! You will also find other article on the subject and it is a much longer screw with several threads compared to one in your post.

    1. Does it actually twist as it goes in? (Surely in the weevil it is significant as it is all in one creature?)

      1. It’s not the screw shape of an organ or body part that’s the problem; it’s the problem of having two parts of a single body that fit together like a screw and nut. It’s the same problem as a wheel and axle: how do you get two completely disjunct parts of a body that nonetheless each get oxygen, etc. if they’re separate? (See Adam M.’s comment above.) This is not a problem with the pig penis as what it fits into is part of ANOTHER pig!

        1. No, on pigs. I seem to remember a bad rhyme about the tragedy of a man who was similarly um, endowed, and when he finally met a woman with the same configuration, she had the wrong chirality for him.

          I do remember that “left-hand thread” rhymed with “he dropped down dead”.

  3. It [evolution] also hasn’t created the screw.

    Well then how in the Sam Hill did we all get here? It seems to me screwing must have evolved quite some time ago.

    (Yeah, couldn’t resist. Sorry.)

  4. I think filaree seeds (Erodium sp.) with their corkscrew attachments would qualify as a screw. These clever devices respond to soil and atmospheric moisture to auger themselves into a seed bed.

    For a wheel, what about a rolling tumbleweed? Ok, so maybe that’s more like a ball.

  5. Fascinating! And in the nonbiological realm, the invention of the common screw & driver dates only to the late 1400’s with mass production naturally coming much later. There’s a neat little book on it: One Good Turn: A Natural History of the Screwdriver and the Screw (Witold Rybczynski)

  6. That doesn’t count and you know it. I mean a wheel and an axle

    Thanks, I’ve never heard of the spiders before.

    Btw, better examples may be self-powered rollers:

    “… I’m talking about real animals that will roll away from danger, under their own power.

    The first one was discovered in 1979 by Roy Caldwell, an animal behaviourist at the University of California at Berkeley. It was a tiny ocean crustacean called Nannosquilla decemspinosa. It’s related to the prawn that you chuck on your BBQ. This creature is around 25 mm long, and very skinny. In fact, they’re so long and skinny, that they can’t walk or crawl on land. So when the ocean waves throw them up on a beach, they curl themselves up, and do up to 40 backwards somersault rolls in a row, to get back into the water. They literally roll like a little wheel.

    Now the only known land creature that will deliberately itself roll away from danger is the caterpillar of the Mother-Of-Pearl Moth, Pleurotya ruralis. This research was done by John Brackenbury at University of Cambridge in the United Kingdom. Most caterpillars have some sort of defence against attackers – such as a warning set of colours, irritant hairs, or chemicals that make them taste really bad. The mother-of-pearl caterpillar doesn’t have any of these, so it relies on rolling away at enormous speed.

    … But if you give it a really good poke, it will start off in its reverse gallop, and then, with its tail on the ground, push off from its front legs, curl itself into a ball, and roll backwards. Depending on how flat and level the ground is, a decent push will set it off into five complete revolutions, travelling at a speed of about 40 centimetres per second – about 40 times its normal speed.”

    Still no axle, though.

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