Weekly Bits & Pieces: Oribatulid Leg-well Ornaments

A Dystopian Future Earth or on what a Zygoribatula rests its leg?

Although my postings have been infrequent, I think I’ve more or less reached my original goal of filling the mitey void left by the demise of the late UQ Mite Image Gallery with a set of false-coloured mite SEMs of equal or better quality and exceeded the original gallery in terms of scientific content (not to mention navel-gazing). Some day I will figure our how to use WordPress properly and have all of the images easily available for perusal without having to backtrack through all the omphaloskepsis, but until then I think I need to pay attention to Kaitlin’s point and try to post more frequently.

Since full-mite coloured SEMs take an extraordinary amount of time to compose, and I am flat-out fulfilling all of my other commitments, I’ve decided to start posting a few interesting bits & pieces of mites. I’m not sure that I even need to or should spend any time trying to tart these up with colour, because I find them extremely interesting just as they are. Well, I will let you all judge for yourselves:

Cerotegument in leg I well of Zygoribatula sp. 2

These images were grabbed at 18,000 magnifications and represent ‘ornamentation’ of what is called the cerotegument: a secretion alleged to be composed of waxes and proteins that coats the outer cuticle of oribatid mites. I thought that this ornamentation might be a useful taxonomic character in a messy genus, Zygoribatula, but other genera (Oribatula, Dometorina) in the family (Oribatulidae) have similar ornamentation.

Oh well, it is still interesting, but what is its function? The rest of the surface of these mites is covered with a more or less smooth and thin coating of cerotegument – presumably keeping water in the mite and other things out . But why these pedestals in the grooves where legs I are retracted when the mites are annoyed? (NB – much of the surface structure of oribatid mites can be explained as protection from predators grabbing hold of limbs.)

When I first saw this pattern, it reminded me of pictures in magazines of the pulp era showing cities of the future with interconnecting anti-gravity roadways: better yet, cities on some hive-planet of insectoid aliens (I prefer to think of our future in a more low density, back to nature, gardens in the sky kind of way). However, I am open to more realistic suggestions. I assume the flattened tops (maximum diameter about one half of a millionth of a millimeter) support the withdrawn leg and form an air chamber under the leg, but why?

9 Responses to “Weekly Bits & Pieces: Oribatulid Leg-well Ornaments”

  1. Walter S. Andriuzzi Says:

    “I assume the flattened tops […] support the withdrawn leg”. Perhaps I am being unimaginative, but I assume that’s why. Even if this pattern confers a very little advantage in comparison to more smooth coating, it could be enough to give an evolutionary advantage and be selected. Or perhaps it is neither advantageous nor disadvantageous and is mainly the result of genetic drift or genetic linkage with a favourable character. Or it’s just to make acarologists go crazy. I mean, crazier

    • macromite Says:

      Hi Walter,

      This type of cerotegumental structuring is limited to the leg wells, but why do these mites need a bunch of minute supports for the legs? Or perhaps what benefit does a layer of air under the legs confer? A pleiotropic effect is possible, all structure is not adaptive, but in this case (spread across several genera at least) I suspect a function. I need to look at unrelated mites that also retract their legs I into grooves and see what goes.

      Cheers,

      Dave

    • macromite Says:

      Hi Dave,

      I like your explanation better than Walter’s. The ‘cerotegument’ that forms these structures should repel water given its waxy nature. Using hydrostatic pressure to pop the legs out of the grooves may be difficult with a lot of polar water molecules hanging on.

      Perhaps checking related mites from dry habitats would be a test of this hypothesis, but I suppose even in dry areas these mites seek humid microhabitats and get flooded on occasion.

      Cheers,

      macromite

  2. Walter S. Andriuzzi Says:

    I am no expert on physics (well, not on mites either, though I know some stuff about them) so this could be a very clumsy suggestion: maybe the tiny structures form a sort of vacuum, whose resulting suction makes the legs’ adherence to the body quite stronger, thus enhancing the mite protection from predators. Uhm, maybe I have passed from unimaginative to overly imaginative, but I like the idea

  3. Walter S. Andriuzzi Says:

    (maybe we could throw in even Van der Waals interaction, like in geckos’ suckers)

  4. Dave General Says:

    I think the function is opposite of Walter’s explanation. The ornaments prevent the legs form being stuck in the leg-well. If the leg-well were smooth, then a thin film of water would present a very strong adhesive force between the leg and the leg-well, due to the surface tension of water. These ornaments prevent the formation of such a film, thus allowing the legs to move at will, even in the very moist conditions of leaf litter.

    • Walter S. Andriuzzi Says:

      I have to admit, this sounds more plausible to what I suggested. In any case I would say it is very difficult to test these hypotheses by observation alone, since, as the admin pointed out, even oribatids adapted to dry environment are likely to prefer moist microhabitats; and of course correlation would not necessarily imply causation. This seems to me a kind of matter to investigate with experiments – perhaps nanotechnology, since a large scale reproduction of those cerotegumental structures could be misleading. Alas, I don’t think there are many people out there ready and willing to fund nanotech in order to study… mite legs!

      • macromite Says:

        Funding for nanotechnology and mites – a natural combination! Bill Gates, are you reading? (he said from a Mac).

  5. Walter Andriuzzi Says:

    Looks Dave General could be actually right: similar structures in collembola do indeed repel water
    http://blogs.discovermagazine.com/notrocketscience/2011/10/02/incredible-skin-helps-springtails-to-keep-dry-underwater-and-always-stay-clean/

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