Diversity, fossils, habitat, and ecology
Defense secretions, warning coloration, and bioluminescence
What makes a millipede a millipede?
Millipede diversity in North America
The subclasses Penicillata, Chilognatha, and Pentazonia
The helminthomorph millipedes are the most diverse
Millipedes and humans
The importance of knowing organismal diversity
The most obvious feature of millipedes is the number of legs. The name, “millipede,” literally means “thousand feet,” and they are commonly referred to as “thousand-leggers.” However, this is a figurative term, as the “leggiest” millipede (also the leggiest animal), Illacme plenipes (order Siphonophorida, family Siphonorhinidae) in San Benito County, California, USA, has 750 legs (375 pairs) (Cook and Loomis 1928, Shelley 1996). It would have to grow by another one-third (33%) and add 63 segments, carrying 252 legs (126 pairs), to become a true “thousand-legger” and a “millipede” in the literal sense.
The Diplopoda encompasses a mind-boggling diversity of forms. Approximately 7,000 species have been described from a global fauna that is estimated, based on known degrees of endemism, to contain around 80,000 species, and we know very little about the fauna of China, which may really be immense. Millipedes are among the most ancient surviving terrestrial arthropod group. Retallack and Feakes (1987) describe millipede-like burrows in paleosols (ancient fossilized soils) from the Late Ordovician Juniata Formation (450 million years ago) in Pennsylvania. Some of the oldest known fossils of land animals are diplopods and modern forms had differentiated by the late Silurian period of the Paleozoic era, ca. 410 million years ago (Almond 1985, Shear 1992) . Wilson and Anderson (2004) documented a Silurian fossil with spiracles, indicating the existence of tracheae, proving that the earliest millipedes were fully terrestrial. More info about fossil millipedes. While occurring in all subarctic environments including deserts, they lack a waxy cuticle on the exoskeleton, which functions as a dessication barrier, and thus primarily require moist, deciduous habitats. With few exceptions, millipedes are exclusively “detritivores” (feed on decaying plant material or “detritus”) and are adapted for burrowing in the substrate, where they fill an important ecological niche by fragmenting accumulated detritus, thereby facilitating microbial decomposition and soil nutrient cycles. In tropical and subtropical forests, where earthworm populations are low, millipedes are the main debris-reducing, soil-forming organisms.
Millipedes are relatively inflexible, “progoneate” arthropods (reproductive tracts open on the anterior end of the body) with two body divisions, a head and trunk. In most orders, the species possess a row of “ozopores” laterally, the openings of the defensive glands from which noxious or toxic fluids that repel predators are secreted, this being millipedes’ primary method of defense. The arthropods exhibit a great array of body forms that are superimposed on a basic cylindrical pattern. Many species, particularly in the orders Polydesmida and Platydesmida, possess lateral expansions of the dorsum called “paranota” that increase the surface area and impart a flattened appearance to the organism, hence the term “flat-backed” millipedes. Some species have developed the ability to volvate into a perfect ball or sphere, and are convergent in this regard with the oniscoid Isopoda of the class Crustacea that have been introduced into North America and are common in urban environments. Some millipedes are dorsolaterally smooth while others are ornamented to varying degrees, sometimes elaborately so, with papillae, lobes, pustules, tubercles, ridges, crests, spines, and other projections; additionally, the paranota can be notched, indented, and modified to the point that they become spiniform in shape. Similarly, some species are brown or gray in color while others exhibit vivid “aposematic” coloration (warning colors that serve to announce the defensive secretions) with red, orange, yellow, blue, purple, and white spots and/or transverse or longitudinal stripes; still others are uniformly reddish or turquoise. In Tulare, Kern, and Los Angeles counties, California, USA, the species of Motyxia (Polydesmida: Xystodesmidae) bioluminesce. The whole animal lights up at night in a continuous, neon-white glow. These are the world’s only bioluminescent millipedes, and the phenomenon is thought of as “warning luminescence,” a nocturnal equivalent of aposematic coloration. The glowing millipedes are conspicuous at night in the southern Sierra Nevada and have been described as “resembling the starry sky on a dark night.”
The class Diplopoda is defined by three autapomorphies (unique derived features): aflagellate spermatozoa, the presence of four or more apical sensory cones on each antenna, and the diplosegment condition. In the last, adjacent body somites, each carrying a pair of legs, tracheal (respiratory) openings, and a ventral nerve cord ganglion, become fused within the embryo to form diplosomites. The structures associated with the anterior of the fused somites have become relocated to the posterior somite such that in the resulting diplosegment (henceforth referred to as just “segment”), all four legs and tracheal openings and both ganglia are located in the metazonite (representing the posterior of the fused somites), while the prozonite (representing the anterior of the fused somites) lacks structures and thus inserts inside the preceding metazonite, which allows for a telescoping of segments and a more compact body form. The diplosegment condition is believed to have evolved in conjunction with millipedes’ burrowing habits, as the pushing force is more efficiently transmitted to the pushing surface when alternate segmental joints are made rigid and incompressible. The power for this pushing is generated by the legs, and at any point in time, most are in contact with the substrate in varying stages of the backstroke, pushing the millipede slowly and inexorably forward. The legs arise midventrally, which allows for the longest possible appendages and the greatest power with the least lateral extension, thereby minimizing the possibility that the appendages will extend appreciably beyond the sides of the body, where they are likely to be damaged or broken in the narrow spaces that millipedes inhabit.
Three burrowing mechanisms are known for millipedes. The first is bulldozing, in which the millipede lowers its head and rams straight ahead, with the “collum” or 1st segment constituting the pushing surface; this method is employed by the “juliform,” or rounded/cylindrical millipedes, and involves most representatives of the orders Julida, Spirobolida, and Spirostreptida. The second is wedging, in which the anterior end inserts into a crack or crevice, and the legs, by pushing upwards and straightening, cause the opening to widen, thus allowing further penetration by the anterior end. This method is employed by the flat-backed millipedes, primarily representatives of the order Polydesmida; the paranota constitute the pushing surface and tend to split matter in a horizontal plane, like matted layers of leaves. The third mechanism is boring, in which segments of progressively greater width are dragged forward thereby widening a crevice and allowing further penetration. This method is exhibited by forms in which the anterior end is narrow and the next several segments become progressively wider, as in the order Polyzoniida. In addition to these burrowing mechanisms, some millipedes are believed to have undergone “habit reversal,” and abandoned burrowing for a different lifestyle. We know they evolved as burrowers because they’re millipedes and possess the diplosegment condition that evolved in conjunction with burrowing, but they no longer burrow, or do so only infrequently or feebly, and now exhibit a different lifestyle. Some, like representatives of the order Callipodida, tend to be surface active and relatively quick (quick for millipedes), and not surprisingly, such species exhibit a higher instance of carnivory than most millipedes. Effective burrowing is also believed to be possible only within certain size limits. Thus, small, narrow-bodied (<1.0 mm wide) representatives of families like the Blaniulidae (order Julida) are too weak to burrow effectively and inhabit existing cracks and crevices instead. The largest millipedes, those upwards of 30 cm (1 ft.) in length, also tend to be surface active because too great an amount of force would be needed for so large and bulky an animal to burrow an opening. Consequently, videos of savannas and deserts in eastern and southern Africa sometimes show species of the genus Archispirostreptus (Spirostreptida: Spirostreptidae), the largest known millipedes, wandering across the grassy and sandy surfaces.
At present, the extant representatives of the class Diplopoda comprise 2 subclasses, 16 orders, and 145 families (text classification; tree diagram: families, orders). Forty-nine families and ca. 910 described species inhabit the US and Canada, but the Parajulidae (order Julida), the largest family on the continent, is essentially unstudied, and some 200 undiscovered species are anticipated in this taxon alone. The higher taxa (subfamilies and orders) are distinguished primarily by aspects of the exoskeleton, the number of legs and segments, the profile and general body form, the configuration of the head, and the presence or absence, and position when present, of the sperm transfer or copulatory appendages in males. Wilson and Anderson (2004) recognized the taxon Archipolypoda, for fossil paleozoic diplopods, as a third infraclass in the subclass Chilognatha and equivalent in rank to Helminthomorpha; it included three monotypic orders -- Euphoberiida, Archidesmida, and Cowiedesmida. A discussion and overview of fossil millipedes would require a separate website, so interested readers are referred to Wilson and Anderson's paper and the references therein.
The subclass Penicillata, with 160 known species (Nguyen Duy-Jacquemin and Geoffroy 2003), comprises forms in which the exoskeleton is soft, non-calcified, and covered with tufts of modified setae or bristles; additionally, males lack copulatory appendages, and reproduction occurs without contact between the sexes. Penicillates have a mechanical, rather than chemical, defense mechanism to thwart predation by ants. The caudal bristles have apical hooks and barbs along their lengths by which they interconnect; when penicillates are attacked, these bristles are detached, entangling and disengaging the ants, which become hopelessly entangled the more they struggle until they die (Eisner et al. 1996). All other millipede species belong to the subclass Chilognatha, which possesses a hard, calcified exoskeleton with at most only scattered setae. Chilognath males also possess reproductive appendages that are modified and specialized walking legs, and reproduction involves contact between the sexes. The infraclass Pentazonia, comprising three orders based primarily on the number of segments and whether or not the organisms volvate, contains relatively short, broad millipedes in which the five segmental sclerites (a dorsal tergite, ventral sternite, and two lateral pleurites) are separate and loosely connected by membrane. The last pair of appendages in males is modified into structures called “telopods” that either directly transfer the spermatophore to the female’s openings or function to clasp females during mating. Aspects of the configurations of the telopods constitute the primary taxonomic characters at the generic and specific levels.
The remaining 12 orders, containing the vast majority of species, belong to the infraclass Helminthomorpha, which are elongate, worm-like millipedes with varying degrees of fusion among the segmental sclerites that culminates in the condition in the Polydesmida in which they coalesce into a complete ring with no evidence of suture lines. In males, either the anterior or both pairs of legs on segment 7 (subterclass Eugnatha), or the posterior legs on segment 7 and the anterior pair on segment 8 (subterclass Colobognatha) are modified into copulatory appendages called “gonopods.” As in the Pentazonia, aspects of the configurations of the gonopods are the primary taxonomic characters at the generic and specific levels, so males are usually necessary for determinations below the familial level. Representatives of the subterclass Colobognatha have triangular or pointed heads with relatively long mouthparts that culminate in the family Siphonophoridae (order Siphonophorida), in which they are prolonged into a pointed, tubular “beak” or “rostrum,” which they are believed to insert into plant roots; some platydesmids are thought to feed on fungi using sucking mouthparts. Representatives of the subterclass Eugnatha possess chewing mouthparts and strong mandibles with which they crush detritus into small pieces. Eugnathan families are grouped into superorders based on general body form, the degree of fusion among the segmental sclerites, and the positions of gonopods in males. However, this generally accepted arrangement, detailed by Hoffman (1980) and Shelley (2003), was recently questioned by Shear et al. (2003), who provided evidence to suggest that some posterior gonopods (replacing the 9th legs or the posterior pair on segment 7) aren’t true gonopods but are modified legs that are shortened so as not to interfere with the copulatory function of the 8th legs. They further suggested that an unnamed clade exists in the Eugnatha comprising taxa in which the gonopods arise solely from the 8th legs (the anterior legs on segment 7), which, if confirmed by further research, will alter the existing taxonomy and probably also the order Spirostreptida. Diplopod taxonomy is thus a fluid science, and changes are to be expected based on new discoveries and reinterpretations of existing knowledge.
We close this general section on millipedes by addressing their importance to, and effect on, humans. Non-scientists are understandably concerned about whether organisms can harm people and pets, and it is important to note that, unlike centipedes, millipedes are harmless if handled properly, because they lack structures to bite, pinch, or sting. Obviously, they should not be bitten or eaten, as the defensive secretions consist of noxious compounds that would cause problems if ingested. Some secretions can discolor skin, but this wears away after a few days without lasting effect; others, however, particularly from large “juliforms,” can be quite caustic and cause skin lesions. Some large, juliform, tropical species, particularly in the Americas, forcefully expel or “squirt” their defensive secretions a meter or so (2-3 feet) and can blind chickens and dogs. Persons in the Neotropics should therefore be cautious in handling such millipedes, because these fluids are particularly caustic and very painful if squirted into one’s eyes. The collector of the Haitian species, Haitobolus lethifer (Spirobolida: Rhinocricidae), was zapped in the face and left eye and experienced instantaneous, intense pain despite bathing the area repeatedly in water. The eyelid and cheek swelled rapidly, closing the eye. The next day the eyelid was still swollen shut, but the swelling was reduced by bathing the eye in ice water. On the third day, the skin of the cheek, forehead, and eyelid turned dark brown and blistered where the spray concentration was greatest; the blisters persisted for a week after which the discolored skin peeled off without leaving any scars (Loomis, 1936:70-71).
In any ecological or physiological study, it is essential to know the organism being investigated to ensure that the same species is employed throughout a study. Appreciation of the ecological importance of a group of organisms is directly proportional to the understanding of their taxonomy, which has advanced to the level at which broadly based biological research is feasible in only a few millipede families. Many species and genera are superficially similar such that only an experienced taxonomist can distinguish one from another, and many taxonomically complex and speciose families are uninvestigated and unavailable for other research. The Parajulidae is a classical example. It is the dominant North American family, ranging from southern Alaska and northern British Columbia- Québec to the Florida Keys and Guatemala, and exhibits a “trans-Beringian” connection with one species in Japan and China. The Parajulidae is also the dominant, and in many areas the only, representative in grassland habitats in the Central Plains, where they occur in association with decaying logs, under dung, and among whatever shelter is available. Consequently, one can reasonably conclude that parajulids are vital to the health of prairie ecosystems, but the nascent state of their taxonomy precludes their utilization in ecological research. Advancing parajulid taxonomy and opening up the family for other studies is a major collaborative research goal of Drs. Shelley and Bond.
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