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.
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