Millipede

 

Common Name: Millipede, iron worm, worm millipede – A direct use of untranslated Latin for one thousand (mille) feet (pedis) conveys the idea that it is a worm with a large number of legs, each of which ends in a foot.

Scientific Name: Narceus americanus, N. annularis, N. gordanus – The generic name may derive from the from Greek nark from narkoun “to benumb” in reference to their excretion of defensive chemicals. The trio of species names reflects a complex of divergent evolution in eastern North America. Class: Diplopoda, Order: Spirobolida, Family: Spirobolidae.

Potpourri: Most millipedes are concealed by the leaf litter of the temperate forest understory that they consume as herbivores. While they are prolific, exposing their habitat usually necessitates digging down a few inches to the hypogeal realm where they compete for nutrients with mites, springtails, and the various other worms that do not have legs at all.[1] Not so Narceus americanus, the millipede that trundles about with coordinated waves of footsteps with apparent purpose along the trail as if on expedition. They are so common in the Appalachian Mountains, that the hill people named them “iron worms” probably because of the annular red segments that divide the body longitudinally like so many iron bands. A consequence of their peripatetic nature is a diaspora of species-cum-subspecies that have migrated from their southern Appalachian Mountain homeland in every direction. The motivation to move somewhere else will remain speculative absent a series of improbable controlled experimentation. However, the characteristic behaviors and genetic history of the complex of species that comprise the Narceus genus provide some insight.

The glaciers of the Pleistocene Epoch extended southward ten times on a cyclic basis over the last million years―the Holocene Epoch began with the last retreat ten thousand years ago. The frozen breath of the Arctic advance forced mobile animals toward increasing warmth and killed everything else, including millipedes. The genetic history of the Narceus millipedes tells the story of resurgence northward and eastward as the ice receded. The current population consists of three species that are nearly identical in appearance, Narceus americanus, N. annularis, and N. gordanus and fourteen subspecies with an as yet unsettled taxonomy. The phylogenic history is based on a sample size of 296 individual millipedes from 96 different locations from throughout their current range from the Adirondack Mountains of New York west to Illinois and south to East Texas and the Florida panhandle. From their original refuge in the Blue Ridge Mountains confirmed by DNA testing of genetic diversity, they rebounded to reoccupy abandoned habitats and seek new frontiers The radiating star pattern of genetic diversity revealed six separate networks, a pattern not unlike the glacial recovery of other animal groups including salamanders, frogs, voles, and bears. [2] Geographically, the Narceus complex is probably the world’s most successful clan of millipedes.

The success of the iron worm, the millipede that conquered eastern North America, is a matter of fact―physiological and behavioral profiles must have given the iron worm an iron will to survive.  Its life starts as one of several hundred eggs laid by a gravid adult female. Each egg is protected with a specially prepared cup molded of chewed leaves polished and sealed rub rosa to guard against predation. Egg preparation is surprisingly meticulous, coating the egg at the mouth and then passing it down the long line of legs to place it in the rectum to remove moisture―the end result appears as a fecal pellet to deter all but the occasional coprophagous burrower.[3]  Maternal care and feeding of progeny by invertebrates, while not unprecedented, is unusual and certainly enhances the probability of survival. The legless pupiform blob that hatches has a ready meal to eat and progress through instar stages that start with a six-legged larva and end multiple molts later in a process is called anamorphosis. The end result is a head with a three segment thorax followed by a long chain of metameres, identical segments each with four legs (two on each side). The outer shell of each segment is comprised of chitin, the stereotypical body armor of arthropods like beetles, reinforced with calcium carbonate to increase rigidity. The ubiquitous detritus of leaf litter provides provender for growth, locomotion, and, eventually procreation. As an adult it is a very robust millipede with a blunt, bullet-shaped head, a streamlined cylindrical shell, and many-legged traction to burrow deeply for food, for protection, and to escape the freeze in winter, as long as it is temporary.  That many survive, grow to maturity, and procreate is an ideal recipe for population expansion and dispersion. [4]

Millipedes in general, and iron worms in particular, are bereft of any active defense, having no teeth, talons, or tentacles. The slow-moving plump tubes of protein are perfect for an avian entrée or a raccoon snack. The first line of millipede defense is to roll into a tight spiral to take advantage of the hardened dorsal carapace to protect the ventral legs and loins. For the persistent predator, the comeuppance, as is so often the case with plants and other lumbering animals, is chemical warfare. The random mutations that are the driving force for survival of the fittest have no plan, they occur by happenstance as DNA is remixed with every sexual generation. Many of the extant millipede species have endured by independently concocting repugnant chemical cocktails including cyanide, alkaloids, and phenols. Some tropical species can squirt a particularly virulent secretion out as far as a meter that can blind dogs and result in severe eyelid swelling and intense pain in humans. Some species are brightly colored in aposematic warning.  The iron worm emits para-quinone, a bleach-like irritant with the formula C₆H₄O_2, which can discolor human skin but is otherwise harmless unless ingested. [5] Chitinous armor is not a palladium even with a secret weapon, however.  Some predators have learned to rub the sides of millipedes to remove the repellant before eating them and at least one species of beetle larva paralyzes millipedes and eats them from the inside out, leaving the empty exoskeleton as evidence of a lobster-like feast. [6]

Millipedes are in the class Diplopoda (two feet) of the subphylum Myriapoda (many feet) along with the centipedes and two minor classes: the soft, small, and pale pauropods and the symphylan garden centipedes. Millipedes are the fourth largest group of arthropods with eighty thousand species worldwide of which seventeen hundred are indigenous to North America. The confusion between centipedes and millipedes is a matter of perspective as all are vermicular with many legs. The common names suggest the main distinction is having either a hundred feet or a thousand feet. While the key difference is pedal variance, it is in the leg count on a per segment basis and not the total number. Centipedes have two legs per segment and millipedes, except for a few front and rear segments, have four legs per segment. No millipede yet discovered has a thousand legs, the record is 750, but a fair number of centipedes have more than a hundred. A second significant distinction is that millipede legs project directly downward as digging appendages whereas centipede legs extend outward away from the body as running appendages. This is because most centipedes are hunting carnivores, chasing down their prey with the celerity of a wolf spider. They are also usually endowed with powerful pincers and poisons as coup de grace for their victims. The class name Chilopoda means “lip feet” to account for the dexterity and lethality of the pinchers called forcipules that project from just behind the head.[7]

The iron worm’s occasional sidekick walking along forest trials is a member of the largest order of millipedes, Polydesmida, which literally translates from Greek as “many bonds” but which is usually referred to as “flat-backed.” The flatness is similar to that of centipedes, a case of mistaken identity …  the four feet per segment establishes the correct taxonomy. The distinctive feature of the species that make up the order is that they all produce cyanide, a potentially deadly toxin to any nucleated eukaryote because it disrupts the production of ATP (adenosine triphosphate) in the mitochondria. ATP is the source of energy for cells so stopping it can have dire consequences, particularly if the cell is in the cardiovascular system.  These millipedes can be safely picked up without hazard but a subsequent hand washing is advisable. [8] The most common of the flat-backed millipedes is Apheloria virginiensis, which has no common name, but “Virginia red scute” is one idea.  Millipedes walk like all other arthropods with the posterior leg starting first followed by the next in line going forward. With multiple legs, sequencing is required and metachronal waves (produced by sequential action) pass from the back to the front with a frequency that matches the duration of each step with a wavelength proportional to the time lag between adjacent leg movement. [9]

Myriapods represent one of the earliest terrestrial life forms, emerging from the vestigial aquatic ooze during the Silurian Period about 435 million years ago (mya), long before the first tetrapod amphibians. It is generally accepted that their pelagic progenitors first appeared in the oceans in the late Cambrian Period with extensive cladogenesis to form new species in the Ordovician. [10] From a strictly physiological standpoint, it does make sense that a worm-like creature would be well suited to littoral habitats forming multiple segment copies in bulking up. A mutation of ambulatory knobs would at some point facilitate an amphibious landing. A fossil of a flat, segmented invertebrate was extracted from rock strata deposited 560 mya that had a trail behind it with side grooves that were probably left by pedal appendages of some sort. [11] There is also fossil evidence that the millipedes were the first land animals adapted for air breathing. The Silurian millipede Pneumodesmus newmanii was unearthed in the United Kingdom early this century with spiracles or holes through its sheathing cuticle. The only postulated reason for gaps in an otherwise protective carapace would be to allow air access to the trachea, branching tubes supplying air directly to circulating blood for oxidation energy. [12]

Living ashore requires a necessary and sufficient suite of adaptations to sustain life including breathing, water retention, locomotion, and sensory perception. This was a difficult challenge―only 9 of the 58 animal phyla ever identified have managed to sustain land colonization. The buoyant forces of aqueous habitats that provide support equal to the weight of water displaced allow for boneless gelatinous bodies like jellyfish. The crushing burden of gravity necessitates either an internal scaffold or an external shell to keep the internal organs from being mashed flat. The exoskeleton of arthropods was adapted in the millipede as telescoping segments that are its hallmark with feet added for improved mobility. Living in water is mostly about keeping water out … living out of water is the opposite as retaining moisture becomes the challenge. The hard casing provides a nearly impermeable barrier to the water vapor that would otherwise emanate as evaporation. While subterranean millipedes may only need the senses of touch and maybe smell to survive, the epigeal species like the iron worm needed ocelli with some light sensitivity and tactile antennae to negotiate spatially. Evolution’s “intelligent design” millipede was a success; by the Permian Period 300 mya, they were among the apex species. Evidence of the largest land dwelling arthropod of all time was found in Nova Scotian sandstone in the form of an ichnofossil, a remnant of biological activity. The tracks of what must have been an extinct millipede named Arthropleura were six feet long and two feet wide. [13] In the forests of the current geologic era, there are up to one thousand millipedes in a square meter of soil consuming about ten percent of annual leaf litter.

Sex on land is also a challenge. For many aquatic animals, male sperm deposition is a simple matter of ejaculation into the water followed by sequestration as desired by the female. Land sex must be more direct and specialized organs for sperm insertion evolved as copulation succeeded aquatic social distancing. Appendages called gonopods, arthropod penises, extend from a number of modified body segments in male millipedes near the anterior end to serve as spematopositors to ensure injection of semen into the female vulva during mating. Gonopods come in many shapes with complex and intricate structures. For this reason, they are used as a basis for speciation of a number of arthropods, notably millipedes and beetles. Most animal species engage in some form of mating ritual that involves visual and olfactory recognition clues to identify potential partners, a matter of mate choice. In the arthropods, at least to some extent, it is more constrained. The gonopod of one species will only fit in the vulva of a female of that same species, and, like the notorious O. J. Simpson glove test, if it doesn’t fit it won’t work. [14] There is at least one case of a millipede with gonopods that fluoresce under ultraviolet light, which is surprising because the species has no eyes. [15] There are many, many aspects of  nature that are yet to be understood.

 

References:

1. Nardi, J. Life in the Soil, University of Chicago Press, 2007, pp. 67-116.
2. Walker, M. et al “Pleistocene glacial refugia across the Appalachian Mountains and coastal plain in the millipede genus Narceus: Evidence from population genetic, phylogeographic, and paleoclimatic data” BMC Evolutionary Biology 30 January 2009. Volume 9, Article number: 25. https://bmcevolbiol.biomedcentral.com/articles/10.1186/1471-2148-9-25
3. Hoffman, R. “Myriapod” Encyclopedia Britannica Macropedia Volume 12, University of Chicago, Chicago, 1972 pp. 768-772.
4. https://cronodon.com/BioTech/diplopoda.html – Cronodon is a web based portal in which the arts, sciences, the quest for truth, and learning are prioritized with a twist of pointless sarcasm.
5. Shelley, R. “Millipedes”. University of Tennessee: Entomology and Plant Pathology. https://ag.tennessee.edu/EPP/Pages/Nadiplochilo/Millipedes.aspx
6. Eisner, T. et al (1998). “Rendering the inedible edible: circumvention of a millipede’s chemical defense by a predaceous beetle larva”. Proceedings of the National Academy of Sciences of the United States of America. 3 February 1998. Volume 95 (3) pp. 1108–13.
7. Shelley, R. M. “Centipedes and millipedes with emphasis on North American fauna”. The Kansas School Naturalist. March 1999. Volume 45 (3) pp. 1–16. https://web.archive.org/web/20161112025334/http://www.emporia.edu/ksn/v45n3-march1999/
8. https://bugguide.net/node/view/97404 An excellent overview of arthropod taxonomy.
9. “Locomotion” Encyclopedia Britannica Macropedia Volume 11, University of Chicago, Chicago, 1972 p. 20.
10. Shear, W. et al. “The geological record and phylogeny of the Myriapoda”. Arthropod Structure and Development. 6 December 2009. Volume 39 (2–3) pp. 174–190. https://pubmed.ncbi.nlm.nih.gov/19944188/
11. Pennisi, E. “Ancient wormlike animal caught in its tracks sheds light on early locomotion” Science, 4 September 2019.
12. Garwood, R and Edgecombe, G. “Early terrestrial animals, evolution and uncertainty”. Evolution: Education and Outreach. 24 August 2011 Volume 4 (3) pp. 489–501. https://evolution-outreach.biomedcentral.com/articles/10.1007/s12052-011-0357-y
13. Sues, Hans-Dieter “Largest Land-Dwelling “Bug” of All Time”. National Geographic 15 January 2011. https://blog.nationalgeographic.org/2011/01/15/largest-land-dwelling-bug-of-all-time/
14. Golovatch, S. and Kime, R. “Millipedes (Diplopoda) Distributions, A Review.” Soil Organisms, Volume 81 (3) 2009 pp. 565-597. https://web.archive.org/web/20160303230452/http://www.senckenberg.de/files/content/forschung/publikationen/soilorganisms/volume_81_3/24_golovatch.pdf
15. Daley, J. “Millipede Genitalia Glow in Ultraviolet Light” Scientific American, 27 May 2019.