Sphaerotheriida (from the Latin sphaerium = ball) 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 sphaerotheriids reach a maximum size of a cherry  or golf ball, but some species from Madagascar can even reach the size of an orange (an example of island gigantism). 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 millipede taxa known to do this. This order of millipedes is also unique in that some African species are used for medicinal purposes.
Sphaerotheriida 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 (collum, thoracic shield, and ten normal tergites) and the anal shield. Ventrally, females possess twenty-one leg pairs (forty-two legs in total), 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 (stridulation). Furthermore unlike other large-bodied millipede orders, Sphaerotheriida do not have glands that excrete poisonous or ill-smelling substances. Therefore, they depend entirely on their rolling-up behavior for protection.
Sphaerotheriida somewhat resemble the North American and Eurasian pill millipedes of the order Glomerida, but are generally larger in size (20–80 millimetres or 0.8–3.1 inches body length): only the smallest Sphaerotheriida species are as small as the largest members of the order Glomerida. The orders differ in the number of tergites (10 or 11 in Glomerida, 12 in Sphaerotheriida) and legs (17 or 19 in Glomerida, 21 or 23 in Sphaerotheriida), and show great differences in their head morphology and genital openings, among other characters. Both orders have the ability to roll into a perfect ball, protecting the head, antennae, and the vulnerable underside. However, this rolled-up position (volvation) is achieved differently. In Glomerida, the enlarged second body ring (thoracic shield) 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 sphaerotheriids show the same gap as the Glomerida. Many giant pill millipede species have special ledges ('locking carinae') 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.
On Madagascar, some giant pill millipede species exhibit island gigantism, reaching more than 9.5 cm (3.7 in) in outstretched length and a size comparable to an orange when rolled up. Recently, a very small form was also described from Madagascar: full-grown individuals of the species Microsphaerotherium ivohibiensis being just the size of a pea.
In general, sphaerotheriids have a Gondwanan distribution (the exception is Zephroniidae from southeast Asia and adjacent regions). Gondwana was the large southern continent that formed after the breakup of the supercontinent Pangaea. It included the modern-day landmasses of South America (which however lacks sphaerotheriids), New Zealand, Australia, India, and Madagascar. It is believed that the Sphaerotheriida originated in Gondwana and then diverged into its various forms. As of 2010[update], 322 species in 23 genera are known, with numerous new species having been discovered recently. The sphaerotheriids are 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 (synonym Sphaeropoeidae) 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 (probably introduced). The Arthrosphaeridae are distributed in southern India and Madagascar.
A few giant pill millipede species have been dispersed by humans. Examples include the Sri Lankan Arthrosphaeridae species A. 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.
Little is known about the ecology, development and life history of sphaerotheriids, but all species are most likely detritivores, feeding on dead organic matter such as leaves and wood on the forest floor. Giant pill millipedes, like earthworms, play an important role in decomposition, releasing nutrients that are locked up in decaying organic matter back into the soil. This process is essential for plant nutrition. It is possible that giant pill millipedes use special bacteria in their gut, similar to the way termites do, to get energy out of non-easily digestible material such as lignin.
Like most millipedes, sphaerotheriids mainly inhabit the leaf litter of humid forests. Some species, however, show an arboreal (tree-living) lifestyle, and for these organisms the rolling-up reflex has been suppressed.
Defense against predation
Giant pill millipedes are preyed upon by a wide variety of organisms, and their rolling-up ability and tough skeletal armor offer protection against some predators. A few animals specialize on feeding on giant pill millipedes and have evolved special structures or behaviors to overcome their defensive mechanism. Examples include the South African snail family Chlamydephoridae which almost exclusively feeds on giant pill millipedes along with meerkats (Suricata suricata) which have been reported (at least in captivity) to throw rolled-up sphaerotheriids against rocks in order to break them. Besides their rolling-up behavior, it has been suggested that camouflage may be another defense mechanism that giant pill millipedes use specifically to protect them from animals that hunt using vision, such as birds. Sphaerotheriids also need to cope with internal parasites, with several species of nematodes living in them exclusively.
- Family: Arthrosphaeridae
- Family: Procyliosomatidae
- Procyliosoma Silvestri, 1917 – 11 species, Eastern Australia, Tasmania, New Zealand
- Family: Zephroniidae
- Bothrobelum Verhoeff, 1924 – monotypic, Borneo
- Cryxus Leach, 1814 – monotypic, Asia
- Indosphaera Attems, 1935 – 2 species N. India, Myanmar
- Kophosphaera Attems, 1935 – 5 species, N. India, Nepal
- Leptotelopus Silvestri, 1897 – monotypic, Myanmar
- Prionobelum Verhoeff, 1924 – 8 species, Vietnam, SW China
- Sphaerobelum Verhoeff, 1924 – 4 species, Vietnam
- Zephronia Gray, 1832 – 37 species N. India, Myanmar, Malayan Peninsula, Java, Sumatra, Borneo
- Sphaeropoeus Brandt, 1833 – 22 species, N. India, Myanmar, Malayan Peninsula, Java, Sumatra, Borneo
- Tigridosphaera Jeekel, 2000 – 4 species, Malayan Peninsula
- Castanotherium Pocock, 1895 – 50 species, Indonesian Islands, Philippines
- Castanotheroides Chamberlin, 1921 – 3 species, Philippines
- Sechelliosoma Mauriès, 1980 – monotypic, Seychelles
- Rajasphaera Attems, 1935 – monotypic, Borneo
- Family: Sphaerotheriidae
The first modern phylogenetic study of Sphaerotheriida (simplified below) 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.
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