Archive for December, 2010

The Macromite Before Christmas

December 24, 2010

Water-skating Homocaligus adorned with Roynortonella pustules

The winter solstice (adorned with a full lunar eclipse on an almost clear night here in Edmonton) is several days past and my brief Albertan ‘mid-winter’ holiday season has just commenced. In Australia the first month of summer is almost over – Australia begins its summer on the first day of December, presumably out of the usual nonconformity or some other reason that was never clearly explained to me – but their summer solstice is just past and it is also the holiday season (with snow in the mountains, but otherwise warmer than here). Celebrating the longest night of the year makes a certain sense. Although I still have 4-5 months before green returns to the landscape, I can optimistically assume that the sun will be shining longer and longer each day, even if it is on clouds that are dumping snow on me, and eventually the winter will end, at least officially. So, in the spirit of my holiday season, I wish my readers, wherever they are and whatever their holiday or not, a happy Christmas and productive, healthy, and intellectually stimulating New Year.

An undescribed, but checklist making, Annerossella from Queensland

Over the last few years I have gotten into the habit of tarting up one of my mites for a Christmas card. This year I picked an unidentified Albertan species of Homocaligus – one of the two genera of the raphignathoid family Homocaligidae. This mite is a festive bright red in life and skates over the shallow margins of lakes among emergent vegetation and aquatic mosses. Eggs are probably laid on vegetation as in Annerossella knorri Gonzalez, a homocaligid described from the leaves of water lettuce (Pistia stratiotes) near Bangkok, Thailand. I suspect it is a predator, perhaps of the springtails (Podura aquatica) that hop along in this habitat. I once kept an undescribed Australian species of Annerossella in a small aquarium, but other than watching it skate across the water, I was unable to add anything to the knowledge of its ecology (at about 0.5 mm in length, it is difficult to observe). However, I did make one of my early coloured SEMs of the mite and posted it on the Mite Image Gallery at the University of Queensland. Much to my surprise this was the first record of the family in Australia and my friend Bruce Halliday, putting aside his doubts about the validity of ephemeral web publications, cited the image in his Mites of Australia, a checklist and bibliography (1998, CSIRO Publications). Interestingly, the image at the top of a species of Homocaligus is probably the first record of the family from Alberta.

A pustule from the gymnodamaeid Joshuella agrosticula at 40,000x

Although festive enough for the holiday in itself, I thought the Homocaligus needed more adornment. The pine cone-like bulbs on the mite are cerotegumental pustules from another mysterious Albertan mite, Roynortonella gildersleeveae (Hammer, 1952). This mite used to reside in the genus Nortonella Paschoal, named after the great oribatologist Roy A. Norton. Unfortunately, in 1908 a certain Rohwer had already used Nortonella for a genus of tenthridinid sawflies; thus, the name was preoccupied. I suggested the new name as a replacement that was in keeping with the author’s original intentions. Like other members of its family (Gymnodamaeidae), the surface of the adult mite has scattered fields of strange and intriguing Bucky Ball-like pustules. The pustules arise as the cerotegument dries after the adult moult in what must be some interaction between microfibers and wax. Their elaborate form and species-to-species variants keep me, if not tied to a particular belief in the nature of the Universe, at least still amazed by how rewarding the study of even the smallest parts of Nature can be.

For more on Homocaligidae and Gymnodamaeidae see:

Fan Q-H. 1997. The Homocaligidae from China, with description of two new species (Acari: Raphignathoidea). Entomol. Sin. 4: 337-342.

Gonzalez RH. 1978. a new species of mite on water lettuce in Thailand (Acari: Homocaligidae). International Journal of Acarology 4:221-225.

Walter DE. 2009. Genera of Gymnodamaeidae (Acari: Oribatida: Plateremaeoidea) of Canada, with notes on some nomenclatorial problems. Zootaxa 2206: 23–44.

Wood TG. 1969. The Homocaligidae a new family of mites (Acari: Raphignathoidea), including a description of a new species from Malaya and the British Solomon Islands. Acarologia (Paris): 11: 711-729.

And the Answer is: Polyxenid Millipede

December 16, 2010

the presentable part of a polyxenid from Queensland

As Christopher Taylor deduced, the 3rd Electron Challenge is none other than a member of the millipede subclass Penicillata and its only order Polyxenida. The cephalic trichobothria and disaggregated eye cups are characteristic of this group. Müller et al. (2007) consider the eyes to be secondarily reduced, miniaturized ommatidia and used their study of eye ultrastructure to argue both for the homology of all mandibulate eyes and a possible synapomorphy of the Myriapoda (millipedes, centipedes, symphylans, and pauropods).

Polyxenid in the courtyard: tiny, but not defenceless

Christopher also hypothesizes that parthenogenesis may help them to colonize extreme habitats like the Spanish Moss (the lichen-like bromeliad Tillandsia usneoides) that dangles from trees, especially live oaks, in the south eastern USA. Some populations of species of Polyxenus, at least, are parthenogenetic, so I suppose, that is a possibility under the General Purpose Genotype Hypothesis about the persistence of parthenogens. But, both bisexual and unisexual polyxenids are unusual among Diplopoda in that many inhabit xeric environments such as rock surfaces, bark, and even Spanish Moss (Whitaker & Ruckdeschel 2010). Wright & Westh (2006) recently demonstrated that Polyxenus lagurus (L.) is capable of absorbing atmospheric water vapour down to relative humidities of 85% – so far the only known millipede to have this ability. So, this ability seems more useful for climbing trees than the ability to do without males.

Our somewhat deformed specimen is from Queensland. Three families of Polyxenida have been recorded in Australia (Lophoproctidae Silvestri, 1897; Polyxenidae Lucas, 1840; Synxenidae Silvestri, 1923), but I don’t know which one this Queensland specimen represents. Unlike all other millipedes, polyxenids (this ‘common name’ could be confusing since it can be applied both to the family and order – but I’ll use it for the order) are soft-bodied and preserving them for SEM is tricky (also the setae, especially in the posterior pencil-like tuft, fall out and get stuck to everything else in the dish). Only about 160 polyxenid species are known today, but the group is very ancient with fossils in amber known from the late Cretaceous – and all have the whorls of serrate setae and the dense pencil-like tuft of fine setae on the rear.

Eisner et al. (1996) have a fascinating (and currently freely available) paper in the unfortunately acronymned PNAS that demonstrates that a North American species of Polyxenus uses the pencil tufts of modified setae on their posterior to thwart predation by ants. In fact, they use the ant’s mechanoreceptor setae and grooming behaviour as a death trap. When an ant approaches, the polyxenid swings its butt around and brushes the tuft of setae against the ant. Grappling hook-like processes on their tips (see the excellent SEMs in the paper)  snare setae on the ants mouthparts and legs and are shed as the millipede moves away. When the befouled ant attempts to clean itself the jagged-edged bristles become entangled and an elaborate snare begins to envelop the ant’s legs and mouthparts, often resulting in the eventual death of the ant (at least in the lab).

The whorls of setae on the body lack the grappling hook ends, but easily fall off and may provide a similar, last ditch defence against being grabbed by a predator and allow the polyxenid a chance to bring its death brush to bear. Polyxenid fossils are only known from the late Cretaceous and Polyxenus from the Eocene (Nguyen Duy-Jacquemin & Azar 2004), so this behaviour may have evolved in response to ants, but millipedes seem to have originated by at least the mid Ordovician and the Polydesmida are either the sister group to all other millipedes or, at the latest, originated in the Carboniferous (Wilson 2006), so this defence may be more than just a myrmicide. Also, not all ants let polyxenids entangle them.

Neotropical ants of the genus Thaumatomyrmex (they feign death when disturbed) hunt the polyxenids abundant in leaf litter (Brandão et al. 1991). A polyxenid is seized by the ant’s antennae, snapped by the wicked-looking mandibles, and then stung and carried back to the nest. In the nest the paralyzed polyxenid is turned belly up and stripped of its setae using the fore tarsi which have “small but stout setae” (perhaps too stout to be engaged by the grappling hooks) and the mandibles. This can take 20 minutes, interrupted by bouts of grooming, so it seems the polyxenid setae may still be fighting back. Brandão et al. thought the setae must have a noxious chemical – this being the normal millipede defence – but Eisner & Deyrup have shown that the morphology of the setae themselves can be fatal and no chemical defence need be invoked. The hunter then eats most of the polyxenid and feeds the remains to a larva.

Polyxenus Latreille, 1803, seems to have given its name to this strange and ancient group of millipedes, but I’m not sure where ‘Polyxenus’ (‘very or many strange’ or ‘very hospitable’ are two possible translations) comes from. Polyxena, the daughter of King Priam of Troy, who came to such a gruesome end on the pyre of Achilles, would seem to be one possible answer, but ‘Polyxenus’ is not feminine and the animal is not hospitable and anything but a willing victim. Polyxenidas was a renegade Rhodian admiral known mainly for treachery and losing naval battles to the Romans, but there is nothing marine or ship-like about these dry-adapted animals (although they may be found on beaches). I think it must be the many strange setae that inspired Latreille and that seems very fitting.

Short Bibliography

Brandão, C. R. F., Diniz, J. L. M. & Tomotake, E. M. (1991) Thaumatomyrmex strips millipedes for prey: a novel predatory behaviour in ants, and the first case of sympatry in the genus (Hymenoptera: Formicidae). Insectes Sociaux 38: 335-344.

Eisner, T., M. Eisner and M. Deyrup. 1996. Millipede defense: use of detachable bristles to entangle ants. Proceedings of the National Academy of Sciences 93: 10848–10851.

Müller CHG, Sombke A & Rosenberg 2007. The fine structure of the eyes of some bristly millipedes (Penicillata, Diplopoda): Additional support for the homology of mandibulate ommatidia. J. Arthropod Structure & Development 36: 463-476

Nguyen Duy-Jacquemin M. & Azar D. 2004. — The oldest records of Polyxenida (Myriapoda, Diplopoda): new discoveries from the Cretaceous ambers of Lebanon and France. Geodiversitas 26 (4) : 631-641.

Nguyen Duy-Jacquemin, M., and J.-J. Geoffroy. 2003. A revised comprehensive checklist, relational database, and taxonomic system of reference for the bristly millipedes of the world (Diplopoda, Polyxenida). African Invertebrates. 44(1):89-101.

Whitaker JO Jr & Ruckdeschel C. 2010. Spanish Moss, the Unfinished Chigger Story. Southeastern Naturalist 9:85-94.

Wilson HM. 2006. Juliformian millipedes from the Lower Devonian of Euramerica: implications for the timing of millipede cladogenesis in the Paleozoic. J. Paleont. 80: 638–649

Wright JC & Westh P. 2006. Water vapour absorption in the penicillate millipede Polyxenus lagurus (Diplopoda: Penicillata: Polyxenida): microcalorimetric analysis of uptake kinetics. The Journal of Experimental Biology 209: 2486-2494.

A long and more normal millipede:

A more traditional Australian millipede

3rd Electron Raster Challenge – Hint

December 12, 2010

Here's a hint - diagnostic character is visible

Hmm, I’m surprised no one has guessed this one. I wonder if everyone is busy shoveling snow? I would be myself, except a neighbour did my driveway for me, so we can do our food shopping this morning with no hassle. Since my neighbour was so kind, I’ll pass it along as a hint: a diagnostic character for the class this animal belongs to is visible in the new image (and the orientation is more realistic).


Macromite’s 3rd Electron Raster Challenge

December 7, 2010

Now that I’m back in the groove, more or less, I suppose I should offer up a new Electron Raster Challenge. This is an easy one, these things are everywhere, so for full credit, how about naming three of the structures visible too. In case anyone needs a hint or two, well, they are unusual  among their close relatives for two reasons that I can think of:  (1) they have a physiological ability that allows them to live in Spanish Moss and (2) they use their morphology to confound ants. Extra credit for explaining these two feats.