Diplopoda

(Millipedes)

Phylum Arthropoda

Class Diplopoda

Number of families 148

Thumbnail description
Many-legged, often long-bodied, segmented animals with antennae; also possessing two pairs of legs on each body segment. The segments, which are actually two somites fused together, are called diplosegments.

Evolution and systematics

The class Diplopoda contains about 10,000 described species in 15 orders and 148 families. Scientists believe that as many as 70,000 additional species have yet to be identified. The millipedes were once classified as a subclass of the class Myriapoda, which also contained the centipedes (now assigned to class Chilopoda). Since then, all four major myriapod groups have been given class status. The other two classes are Pauropoda and Symphyla.

Many researchers think that the millipedes may have developed during the Carboniferous period (360–286 million years ago) from the genus Arthropleura, a possibly diplosegmented myriapod that grew to an impressive 5.9 ft (1.8 m) long and 1.5 ft (0.45 m) wide. The largest extant millipedes, Graphidostreptus gigas and Scaphistostreptus seychellarum reach 11 in (28 cm) in length. Although the evolutionary history of the diplopods is still a disputed subject, systematists have generally agreed that diplopods and pauropods are the most closely related of the myriapods, followed by the symphylids. Some biologists have even suggested that these three groups of myriapods may be more closely related to insects than to the fourth myriapod group—the centipedes—but this view is hotly contested.

Physical characteristics

Millipedes differ from all other myriapods in having two pairs of legs on each body segment. A few segments typically have no legs or have only one pair. The first three segments form the thorax of the animal; the first of these has no legs while the second and third have one pair each. The fourth segment, which begins the abdomen, also usually has just one pair. The legs, which are uniramous (unbranched), may number from about two dozen to several hundred depending on the species. The species Siphonophora millepeda and Illacme plenipes hold the record with about 750 legs. The adult size of millipedes ranges from 0.08 in (2 mm) long in the subclass Penicillata to 11.8 in (30 cm) in Triaenostreptus. In general, millipedes are described as either typical (subclass Chilognatha) or bristly (subclass Penicillata).

Typical millipedes have a calcified exoskeleton and are usually long and thin. Species within the subclass Chilognatha, however, vary widely in appearance. For instance, members of the order Glomerida not only look superficially like pillbug isopods, but they also share their ability to conglobate, or roll themselves up into a ball. One morphological difference between the two groups is that the balled diplopods have dorsal plates that are similar in size from front to rear, while the posterior dorsal plates in isopods are much smaller than the anterior plates. Many chilognaths are almost round in cross section, but some, like those in the order Glomeridesmida, are flattened.

Bristly millipedes at first glance look more like hairy caterpillars than typical millipedes. They commonly have numerous transverse rows of setae (bristles) across their dorsal surface. Unlike typical millipedes, they have an uncalcified exoskeleton, so their bodies are soft. Bristly millipedes are small, reaching only about 0.16 in (4 mm), and have at most about a dozen segments.

Overall, male and female millipedes are similar. The most outwardly noticeable difference between the two is leg length; males generally have longer legs than females. This characteristic likely assists the males in grasping females during mating.

Distribution

Millipedes do not travel far on their own, which helps to explain the vast number of species that may fill similar habitats just a few hundred miles apart. Humans, however, are efficient transporters of these animals, and have introduced species to new areas the world over. According to The Biology of Millipedes, "Just to give a few examples, 59 percent of the species recorded in Hordaland, Norway, by Meidell (1979) were considered to have been introduced there by man. Kime (1990b) recorded that half the species found in Britain have been introduced into North America."

Habitat

Millipedes are generally found in dark damp places, often under leaf litter, wood piles, and rocks, or in the top inch or two of soil. Most burrow by pushing their heads through the dirt; but some, like Polyzonium species, also use their bodies to widen their tunnels.

A few species are arboreal (found in trees), including some bristly millipedes. These diplopods forgo burrowing and instead live in tiny cracks in tree bark. A number of spirostreptids and spirobolids are also arboreal. By contrast, some bristly millipedes are known for their preference for dry habitats. A few species, like Archispirostreptus syriacus and Orthoporus ornatus, live in deserts.

Some millipedes, like Glomeris marginata, are commonly seen in the open and in broad daylight.

Behavior

Perhaps the most well-known millipede behavior is their defense mechanism of conglobation—rolling up into a ball if they are short or into a spiral if they are longer. Most diplopods conglobate except the bristly millipedes. By conglobating, millipedes protect their softer and more vulnerable undersides from predators, leaving only their hard dorsal surfaces exposed. Another behavior of millipedes that protects them from both predators and desiccation (drying out) is their general preference for such damp dark places as burrows or crevices within tree bark. A few diplopods break this pattern and spend considerable time in the open.

Millipedes also defend themselves against predation via poisonous—or at least noxious-smelling—secretions they emit through pores on their sides. A few of the larger tropical species can actually squirt their secretions. The secretions of polydesmids contain cyanide. Some, like the bristly millipedes, don't produce defensive secretions.

Millipedes exhibit some unusual behaviors. For example, when Diopsiulus regressus feels threatened, it heaves itself off the ground and jumps 0.8–0.12 in (2–3 cm). Upon landing, it runs forward, then leaps again. The males of a few members of the order Sphaerotheriida can stridulate, making sounds by brushing their legs against the sides of their bodies. A few nocturnal millipedes glow in the dark, including members of the genus Motyxia (= Luminodesmus). It is thought that their bioluminescence warns potential predators of their noxious qualities. In addition, some millipedes, like Calymmodesmus montanus, form mutually beneficial relationships with ants or termites. The insects protect the diplopods from predators, and the millipedes perform housekeeping duties by eating fungi and detritus in the insects' nests and bivouacs.

Feeding ecology and diet

Most diplopods are detritus feeders, chewing up and digesting decaying leaves or other vegetation. Many, including members of the family Siphonophoridae, will also eat tender shoots or roots; a few derive nourishment from organic matter in ingested soil. Some, including those of the order Callipodida and possibly a few other species, will eat animal detritus; a number of polydesmids eat fungi.

Although many species of millipedes eat their own feces, a habit called coprophagy, the practice is not universal. Some researchers suggest that millipedes don't obtain much nutritional benefit from the fecal matter itself, instead drawing food value from the fungi growing within it.

Predators of millipedes include amphibians, reptiles, birds, carnivorous invertebrates, and some insect-eating mammals.

Reproductive biology

The sexes are separate in diplopods; most millipedes reproduce sexually. Some species are parthenogenetic, especially those in the order Polydesmida; polydesmid females produce daughters without the contribution of male sperm. Some diplopod species perform courtship rituals. In Julus scandinavius, for example, the male presents a gift secretion to the female. Loboglomeris pyrenaica males stridulate in order to entice females.

The chilognaths and the bristly millipedes differ in their methods of insemination, with the former employing direct sperm transfer and the latter indirect transfer. A chilognath male generates a spermatophore (sperm jelled together to form a packet) and moves it from the genital opening to the gonopod, an intermittent organ. He uses this gonopod to place the spermatophore directly into the female's genital opening. A male bristly millipede instead spins a web and ejects his sperm into it. The female approaches the web and inserts the sperm into her genital opening herself. Females of all millipedes store sperm and fertilize the eggs as they deposit them.

Female diplopods lay eggs in nests in the soil, sometimes making capsules to help protect the eggs. Narceus species, for example, mold individual egg capsules from masticated leaves. In some species, the females guard the eggs; in a few, like some platydesmids, the males take over the sentry role. The young undergo two or three molts inside the nest, emerging in most cases with three pairs of legs. They gain segments, or "rings," and legs with each successive molt through a process called anamorphosis. Millipedes generally molt in such protected spaces as underground burrows, crevices, or even slight depressions in the soil. A few, like Narceus americanus and Orthoporus ornatus, seal themselves into chambers during this particularly vulnerable stage of their lives.

In a year or two, sometimes longer, the young diplopods molt into sexually mature individuals. Many male julids, especially Ommatoiulus and Tachypodoiulus species, are unusual in their ability to molt "backwards," so to speak, reverting from sexually mature adults to nonsexual stadia (known as intercalary or Schalt stadia). The purpose of this reverse molt, called periodomorphosis, is unknown.

The life span of millipedes varies among species and can range from one to 11 years, possibly longer.

Conservation status

No diplopods are listed by the IUCN.

Significance to humans

As detrivores, millipedes' major contribution lies in promoting overall plant decomposition. One study estimated that they add two tons of manure to each acre of the forest floor each year.

Occasionally, large numbers of millipedes are reported to damage gardens and crops. In Japan, outbreaks of the species Parafontaria laminata have been known to cause transportation problems. The diplopods are run over by trains and their flattened bodies stick to the rails. Large numbers of these crushed millipedes on the rail surface have actually caused railroad cars to lose traction.

In addition, humans who come in contact with some species of millipedes may have severe allergic reactions. Some species, like those in the genus Spirobolus, secrete a defensive chemical that irritates human skin.

Species accounts

List of Species

No common name

Underwoodia iuloides

order

Chordeumatida

family

Caseyidae (Underwoodiidae)

taxonomy

Underwoodia iuloides Harger, 1872, Simon's Harbor, Pukaskwa National Park, Ontario.

other common names

None known.

physical characteristics

This shiny, dark brown millipede is lighter on its ventral surface and has somewhat banded antennae. The adults average about 0.4 in (10 mm) in length. Like other members of this genus, these millipedes have three projections extending from the anterior gonopod.

distribution

Northeastern quarter of North America, stretching as far south as North Dakota, Michigan, and upstate New York, and west as far as Saskatchewan, Canada. A population has also been found in northeastern New Mexico.

habitat

Damp wooded areas with thick leaf litter or coastal barrens that experience frequent fog and rain.

behavior

Little is known of this diplopod's behavior, except that it prefers to remain under stones or logs. It apparently requires habitats with high moisture content.

feeding ecology and diet

U. iuloides is thought to feed on leaf detritus.

reproductive biology

Only a few males have ever been found, which indicates that the species may be parthenogenetic over much of its range. Little is known about its reproductive biology.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Pill millipede

Glomeris marginata

order

Glomerida

family

Glomeridae

taxonomy

Glomeris marginata Villers, France, 1789.

other common names

French: Gloméris marginé; German: Gemeine Saftkugler; Finnish: Mustapallotuhatjalkainen.

physical characteristics

Short dark brown or black millipedes with 12 dorsal diplosegments lined posteriorly in light brown or light gray. One segment is actually two diplosegments fused into one large plate. Pill millipedes are dome-shaped in cross section, with 17–19 pairs of legs. They reach 0.8 in (20 mm) long and 0.3 in (8 mm) wide.

distribution

British Isles; western and northwestern Europe.

habitat

Unlike most other millipedes, G. marginata is less prone to desiccation and is often seen in the open, even on sunny days. It is usually found under leaf litter in forests, fields, and gardens.

behavior

When threatened, G. marginata rolls into a ball that resembles an isopod. Another defense mechanism is a secretion that discourages most predators. Some males stridulate, and all are capable of producing a pheromone that attracts females for mating.

feeding ecology and diet

Pill millipedes eat decomposing leaves. One study in France showed that G. marginata was responsible for eating about one of every 10 leaves that fell to the forest floor every autumn. Its predators include hedgehogs.

reproductive biology

Reproduction is cyclical with eggs produced in spring and early summer. Brood size varies according to the size of the female, but six to seven dozen is common. Eggs are about 0.04 in (1 mm) in diameter and deposited inside a capsule. The young emerge as second stadia about two months after the eggs are laid. Temperature can affect the timing of development and delay hatching by several months. The young may take several years to mature. Females have an active reproductive life of several years; half a dozen broods in a lifetime is not unusual. G. marginata can live as long as 11 years.

conservation status

Not listed by the IUCN.

significance to humans

This species plays an important role in recycling dead leaves and similar vegetable matter.

Snake millipede

Julus scandinavius

order

Julida

family

Julidae

taxonomy

Julus scandinavius Latzel, 1884, from several localities in Austria including Kirchdorf and Wien (lectotypification is required to establish a type locality).

other common names

German: Schnurfüsser.

physical characteristics

These black to brownish black millipedes are long and cylindrical in shape with a heavily armored dorsal surface. Adult males are identified by mesial (located toward the middle) ventrally directed processes on the first joint (coxa) of the second pair of legs. Adult females range from 0.6–1.5 in (16–38 mm) in length and 0.06–0.11 in (1.5–2.7 mm) in diameter, and males from 0.5–1.1 in (13–29 mm) long and 0.06–0.07 in (1.4–1.9 mm) in diameter. Immature stadia are light brown.

distribution

British Isles and western Europe, with an introduced population in Massachusetts, northeastern United States.

habitat

Prefer sandy soils in forested areas; often found under leaf litter.

behavior

Snake millipedes, like other julids, are sometimes referred to as "bulldozers" for the way they push with their legs and force their heads through the soil. Mating among julids occurs when the male locates a female and climbs onto her back. He twists himself around her body until the gonopores are adjacent. He then produces a glandular secretion and presents it to the female before copulation.

feeding ecology and diet

Eats decaying leaf matter, preferring ash to oak leaves.

reproductive biology

After the female lays her eggs in April, the young emerge in about four to six weeks as stadium-III individuals, having undergone the first two molts in the egg. The young molt three or four more times by the end of their first winter, and twice again during each of the next two winters. The females, now almost three years old, are finally ready to mate. They die shortly after laying their eggs.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Flat-backed millipede

Polydesmus angustus

order

Polydesmida

family

Polydesmidae

taxonomy

Polydesmus angustus Latzel, 1884, Normandy, France.

other common names

French: Polydesme; German: Bandfüsser; Dutch: Grote Platrug; Norwegian: Flattusenbeinet.

physical characteristics

Flat-backed millipedes resemble typical centipedes in form; they are flattened dorsoventrally and have long legs. They also possess long segmented antennae and sculptured dorsal segments. The adults are dark brown, have about 20 segments, and range from 0.6–1.0 in (14–25 mm) in length and about 0.16 in (4 mm) wide.

distribution

Northwestern Europe, now also introduced to the southeastern United States.

habitat

Prefers compost piles, hiding places under tree bark, crevices in decomposing trees, and loose soil packed with organic matter.

behavior

Like other polydesmids, this species has paranota or keels on its dorsal surface. The paranota help them to burrow into the soil by providing wedges that they can lift and lower to open up the soil in front of them and move forward.

feeding ecology and diet

Flat-backed millipedes feed on roots, dead leaves, and other vegetative detritus, as well as on strawberries and other fruits.

reproductive biology

Breeding season generally runs from late spring through the summer or from late summer through mid-fall. The females store sperm and may produce several broods, although the males typically breed only once. The young become sexually mature in 1–2 years, with those hatched in the earlier breeding season maturing in one year, and those hatched in the second season in two years. According to research published in 2003, the second group is sensitive to light, which affects both development and reproduction. This study was the first to show the influence of photoperiods among diplopods.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Bristly millipede

Polyxenus lagurus

order

Polyxenida

family

Polyxenidae

taxonomy

Polyxenus lagurus Linnaeus, 1758, Sweden.

other common names

German: Pinselfüsser; Dutch: Penseelpoot; Swedish: Penselfoting.

physical characteristics

These light brown millipedes have an uncalcified exoskeleton, which gives them a soft body. They also possess characteristic bristles that look like transverse rows of fringe separating their diplosegments. Splotches of dark brown are visible on the dorsal surface. Bristly millipedes reach only 0.08–0.12 in (2–3 mm) in length, and have 13–17 pairs of legs (12 pairs on average) on their 11–13 segments.

distribution

Found in Europe, northern Africa, and western Asia, as well as northeastern North America and the area around Vancouver on the western coast of Canada.

habitat

Polyxenus lagurus does not burrow; instead it seeks refuge beneath or between pieces of loose bark.

behavior

Unlike the chilognath millipedes, P. lagurus employs indirect sperm transfer. In this method, the male spins a small web onto which he deposits his sperm. He finishes the web with a string of so-called "signal threads" that attract the female and lead her to the sperm. The female then approaches the web and gathers the sperm into her genital opening.

feeding ecology and diet

This species finds its meals on tree bark, where it feeds on the algae growing there.

reproductive biology

Although P. lagurus does reproduce sexually, some populations are parthenogenetic. Females in these groups produce offspring from unfertilized eggs. In sexual reproduction, however, males and females mate to generate fertilized eggs. The female lays her sticky eggs in a loose mass and then uses her tail brush to fashion a protective sheath around them.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Resources

Books

Hopkin, S. P., and H. J. Read. The Biology of Millipedes. Oxford, U.K.: Oxford University Press, 1992.

Minelli, A., ed. Proceedings of the Seventh International Congress of Myriapodology. Leiden, The Netherlands: E. J. Brill, 1990.

Periodicals

David, J. F., J. J. Geoffroy, and M. L. Célérier. "First Evidence for Photoperiodic Regulations of the Life Cycle in a Millipede Species, Polydesmus angustus (Diplopoda: Polydesmidae)." Journal of Zoology (London) 260 (2003): 111–116.

Dove, H., and A. Stollewerk. "Comparative Analysis of Neurogenesis in the Myriapod Glomeris marginata (Diplopoda) Suggests More Similarities to Chelicerates Than to Insects." Development 130 (2003): 2161–2171.

Niijima, K., and K. Shinohara. "Outbreaks of the Parafontaria laminata Group (Diplopoda: Xystodesmidae)." Japanese Journal of Ecology 38 (1988): 257–268.

Shear, W. A. "Millipedes." American Scientist 87 (1999): 232–239.

Shelley, R. M. "The Millipede Genus Underwoodia (Chordeumatida: Caseyidae)." Canadian Journal of Zoology 71 (1993): 168–176.

Organizations

British Myriapod and Isopod Group. Web site: <http://www.bmig.org.uk/>

Other

"Diplopoda." Studies in Arthropod Morphology and Evolution. [4 Aug. 2003]. <http://www.life.umd.edu/entm/shultzlab/vtab/diplopoda.htm>.

"The Diplopoda (Millipedes)." Earth-Life Web. 20 June 2003 [4 Aug. 2003]. <http://www.earthlife.net/insects/diplopoda.html>.

"Glomeris marginata (The Pill Millipede)." Casual Intruders. 2000 [4 Aug. 2003]. <http://www.the-piedpiper.co.uk/th11b(2).htm>.

"Myriapods." Nature of the Northcoast: Millipedes. 27 Nov. 2001 (4 Aug. 2003). <http://www.humboldt.edu/natmus/NorthcoastNature/Miriapoda/myriapoda.html>.

"Orders of Millipedes." Herper.com. [4 Aug. 2003]. <http://www.herper.com/myriapods/orders.html>.

"Species Spotlight: Millipedes." Illinois Natural History Survey. 1 Aug. 2003 [4 Aug. 2003]. <http://www.inhs.uiuc.edu/chf/pub/surveyreports/may-jun95/milli.html>.

"Systematics: Myriapods." 11 Feb. 2003 [4 Aug. 2003]. <http://www.biols.susx.ac.uk/ugteach/cws/syst/myriapods.htm>.

Leslie Ann Mertz, PhD