Sphaerotheriida


Sphaerotheriida is an order of millipedes in the infraclass Pentazonia, sometimes known as giant pill millipedes. They inhabit Southern Africa, Madagascar, South and Southeast Asia, Australia and New Zealand. Like the Northern Hemisphere pill millipedes of the order Glomerida, these millipedes can roll into a ball when disturbed. When they are rolled-up, most sphaerotheriidans reach a maximum size of a cherry or golf ball, but some species from Madagascar can even reach the size of an orange. When rolled-up, predators are unable to unravel giant pill millipedes since the margins of their second and last dorsal plates fit perfectly into one another, creating a sealed ball. A few giant pill millipede species are able to produce sound, the only millipedes known to do this. This order of millipedes is also unique in that some African species are used for medicinal purposes.

Morphology

Sphaerotheriidans are characterized by a relatively conservative body morphology; superficially all species and genera look the same. Dorsally, their head is followed by twelve body tergites and the anal shield. Ventrally, females possess twenty-one leg pairs, while males carry two additional modified leg pairs, the anterior and posterior telopods under their anal shield. The telopods resemble chelae and/or clamp-like structures, which are probably used in holding the female during mating. In the position of the male telopods, the females instead have a sclerotized subanal plate, which in some species such as those belonging to the family Arthrosphaeridae, is enlarged and is used to produce vibrations. Furthermore, unlike other large-bodied millipede orders, Sphaerotheriida do not have glands that excrete poisonous or ill-smelling substances. Instead they depend entirely on their rolling-up behavior for protection.
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Sphaerotheriida somewhat resemble the North American and Eurasian pill millipedes of the order Glomerida, but are generally larger in size. Until the end of the 20th century only the largest known species in the order Glomerida rivalled the size of even the smallest known Sphaerotheriida, but in the early 21st century a much smaller Sphaerotheriid was described from Madagascar: full-grown specimens of Microsphaerotherium ivohibiensis are just the size of a pea.
Also on Madagascar, some giant pill millipede species exhibit island gigantism, reaching more than in outstretched length and a size comparable to an orange when rolled up.
The orders differ in the number of tergites and legs, and show great differences in their head morphology and genital openings, among other traits.
Both orders have the ability to roll into a perfect ball, protecting the head, antennae, and the vulnerable underside. However, this rolled-up position is achieved differently. In Glomerida, the enlarged second body ring has a more or less visible gap within which fit the tips of tergites 3–11, whereas in Sphaerotheriida the tips of tergites 3–12 fit perfectly into a groove on the thoracic shield. Juvenile sphaerotheriidans show the same gap as the Glomerida. Many giant pill millipede species have special ledges on the underside of the tergite tips and the anal shield which can be moved above a brim on the thoracic shield. These millipedes remain passively locked-up since they need not continued muscle contraction to remain in the rolled-up position.

Distribution

In general, Sphaerotheriida have a Gondwanan distribution. Gondwana was the large southern continent that formed after the breakup of the supercontinent Pangaea. It included the modern-day landmasses of South America, New Zealand, Australia, India, and Madagascar. It is believed that the Sphaerotheriida originated in Gondwana and then diverged into its various forms.
Sphaerotheriida is divided into four families whose distributions do not overlap: Procyliosomatidae, Zephroniidae, Sphaerotheriidae and Arthrosphaeridae. The most basal family, Procyliosomatidae, lives in Australia and New Zealand. The Zephroniidae occurs in southeast Asia from the Himalayas and China south and east to Sulawesi and inhabits some Philippines islands. The family Sphaerotheriidae only occurs in South Africa with isolated populations in Zimbabwe and Malawi. The Arthrosphaeridae are distributed in southern India and Madagascar.
A few giant pill millipede species have been dispersed by humans, probably inadvertently. Examples include the Sri Lankan Arthrosphaeridae species Arthrosphaera brandtii which has established a population in the Usambara Mountains, Tanzania, as well as some South African Sphaerotherium species which have isolated populations in Malawi. Another likely candidate is Sechelliosoma forcipatum, a small species of the southeast Asian family Zephroniidae, currently only known from a single island in the Seychelles.

Ecology

Little is known about the ecology, development and life history of Sphaerotheriida, but apparently all species are detritivores, feeding on dead organic matter such as leaves and wood on the forest floor. Like earthworms, they play an important role in decomposition; by breaking down decaying organic matter they release locked up nutrients back into the soil. Such recycling is essential for plant nutrition and accordingly for the entire ecology. It is possible that giant pill millipedes rely on special bacteria in their gut, much as termites do, to exploit the nutritional value of otherwise indigestible material such as lignin.
Like most millipedes, Sphaerotheriida inhabit mainly the leaf litter of humid forests. Some species, however, show an arboreal lifestyle, and in these the rolling-up reflex has been suppressed.

Defense against predation

The rolling-up ability and tough skeletal armor of the Sphaerotheriida offer protection against some predators, but a wide variety of predators feed on them, or even specialise in them as a source of food. Species that specialize in feeding on giant pill millipedes necessarily have evolved special structures or behaviors to overcome their defences. Examples include the South African snail family Chlamydephoridae which almost exclusively feeds on giant pill millipedes Another example is the meerkat which has been reported to throw rolled-up sphaerotheriids against rocks in order to break them. This behaviour however, is their way of breaking open many refractory food items, such as snails and hard-shelled eggs.
Apart from the rolling-up behavior of giant pill millipedes, camouflage may be an important defense mechanism against predators that hunt by sight, such as birds.
Sphaerotheriida also are subject to internal parasitism, and several species of nematodes are obligate parasites of particular species of giant pill millipedes.

Taxonomy

By 2014, over 326 species in around 34 genera in approximately 20 genera had been described.
The first modern phylogenetic study of Sphaerotheriida was conducted by Wesener and VandenSpiegel in 2009, using morphological data from 36 species in 10 genera. The South African family Sphaerotheriidae was found to be sister to the Madagascar family Arthrosphaeridae. The Australian genus Procyliosoma was found to be distinct from all other genera and placed in its own family, Procyliosomatidae.