Photo: A millipede (Sigmoria aberrans) displaying warning colors. (Photo by Gilbert S. Grant/Photo Researchers, Inc. Reproduced by permission.)
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 myria-pod 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 diploseg-mented 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.
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 Chilog-natha) 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 ex-oskeleton, 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.
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."
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 spirostrep-tids 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 or-natus, live in deserts.
Some millipedes, like Glomeris marginata, are commonly seen in the open and in broad daylight.
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.
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 Cal-lipodida 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.
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 amer-icanus 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.
No diplopods are listed by the IUCN.
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.
1. Pill millipede (Glomeris marginata); 2. Flat-backed millipede (Polydesmus angustus); 3. Snake millipede (Julus scandinavius); 4. Bristly millipede (Polyxenus lagurus). (Illustration by Amanda Humphrey)
No common name
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