Sphaerotheriida is an order of millipedes[1] in the infraclass Pentazonia,[2] sometimes known as giant pill millipedes.[3] They inhabit Southern Africa, Madagascar, South and Southeast Asia, Australia and New Zealand.[4] 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 [5] or golf ball,[6] but some species from Madagascar can even reach the size of an orange[7] (an example of island gigantism; illustration - [1]). 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.[8] This order of millipedes is also unique in that some African species are used for medicinal purposes.[9]
Sphaerotheriidans are characterized by a relatively conservative body morphology; superficially all species and genera look the same.[4] Dorsally, the head of an adult is followed by twelve body tergites (collum, thoracic shield, and ten normal tergites) and the anal shield. Ventrally, adult females possess twenty-one leg pairs (forty-two legs in total), while adult 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.[4] 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).[4] 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.
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). 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.[10]Also 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.[7]
The orders differ in the number of tergites in adults (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 traits.[11] Millipedes in both orders develop by hemianamorphosis.[12]
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 sphaerotheriidans show the same gap as the Glomerida.[13] 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 do not need continuous muscle contractions to remain in the rolled-up position.