Mandibles, two- or three-jointed; maxillulae absent, the jaws of the
second pair being represented by the gnathochilarium described
above.
Order Oniscomorpha.
Body short and broad, hemispherical in transverse section; convex above, flat below, and capable of being spherically coiled. The exoskeleton of a typical compound segment consists of a vaulted tergum, a pair of free pleural sclerites, two pairs of small tracheal sclerites and two pairs of legs, the latter attached to the ventral membrane, which has no sternal plates. The tergal plates are twelve or thirteen in number, whereof the first is very small, the second enormously expanded laterally, and the last, also enlarged and probably representing at least three segments, extends laterally and posteriorly like a hood over the posterior end of the body without forming a chitinous ring round the anal valves and sternum. In the male the legs of the penultimate pair are sometimes modified as claspers; those of the last pair are always enlarged and prehensile, and bear on their coalesced basal segments a pair of sperm-carrying processes analogous to the phallopods of other groups. Apart from these organs the male has no penis, the seminal ducts perforating the coxae of the legs of the second pair. This order contains two well-marked suborders, the Glomeroidea and the Zephronioidea. The Glomeroidea, comprising the families Glomeridae, Gervaisiidae, Onomeridae, have the antennae approximated on the head, the eyes uniserial and twelve (rarely eleven) tergal plates. To this group belong the common pill-millipedes of Europe and North Africa. In North America the Onomeridae alone are found. The Zephronioidea, with the single family Zephroniidae, have the antennae at the sides of the head, the eyes composed of a spherical cluster of ocelli, and always thirteen tergal plates. This group is common in the tropical and southern continents of the Old World, having representative genera in South Africa, Madagascar, India, Malaysia, Australia and New Zealand. They are much larger forms than the Glomeroidea, large specimens reaching two or three inches in length. In addition to the characters mentioned above the Oniscomorpha differ from all other Diplopods in having long tubular tracheae and the alimentary canal bent upon itself.
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| After Pocock, in Max Weber's Zool. Ergebnisse, &c., IV., Pl. xx. |
Fig. 8.—Sphaeropoeus hercules, a Sumatran species of the Oniscomorpha. A, Lateral view of the entire animal. c, head; ant, antenna; tgl1, tg2 and tg13, tergal plates of first, second and thirteenth segments; lg, extremities of some of the anterior legs. |
Order Limacamorpha
Resembling the Oniscomorpha in the shape and structure of a typical segment, except that the tracheal plates are unrepresented; in the facts that the last tergal plate does not form a complete ring round the anal area, and that the last pair of legs in the male are modified; but differing from them in that the body consists of nineteen or twenty segments, is elongate, and tapers anteriorly and posteriorly, the second and last tergal plates being small; in the presence in the male of a pair of long hairy protrusible penes between the legs of the second and third pairs, and in the structure of the gonopods, which, instead of being chelate, terminate in a slender, tapering tarsal segment. This order contains two families: Zephroniodesmidae (Zephroniodesmus) and Glomeridesmidae (Glomerisdesmus), the former from tropical Asia, the latter from tropical America. The largest of these millipedes reach a length of only about 7 mm. Nothing special is known of their habits.
Order Colobognatha.
Body elongate, capable of being spirally coiled, consisting of a large and indefinite number of segments, each being furnished with a distinct often large sternal area, and with the pleural sclerite or membrane distinct from the tergum. The last tergal plate forms a complete ring round the anal valves. Legs with coxal pouches; those of the seventh segment transformed into gonopods of a very simple type in the male, which is also furnished with a double penis completely or partially confluent with the coxae of the legs of the second pair. Head always small, frequently triangular or piriform, in the latter case the gnathites reduced in size and complexity. Repugnatorial pores present and lateral. The genera of this order are divisible into three families: the Platydesmidae (Platydesmus, Pseudodesmus), Polyzonidae (Polyzonium, Siphonotus), Siphonophoridae (Siphonophora). Of these the Platydesmidae have departed least and the Siphonophoridae most from the typical Diplopod in the structure of the mouth parts. The group is for the most part tropical, one genus only, Polyzonium, extending as far north as Central Europe.
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| After Pocock, J. Linn. Soc. xxiv., Pl. 37. |
Fig. 9.—Glomeridesmus marmoreus, one of the Limacomorpha. A, Lateral view. c, head with antennae; tg1, tergal plate of first segment; an.tg, tergal plate of last or anal segment. |
Order Ascospermophora.
Body elongate, consisting of from twenty-six to thirty-two segments, but not varying within specific limits; the pleurae coalesced with the terga, the sterna free. More or fewer of the anterior ten pairs of legs may be modified in the males, but no true phallopods are differentiated, the function of seminal receptacles being performed (according to C. Verhoeff) by the exsertile coxal pouches of the two pairs of legs of the eighth segment. The seminal ducts in the male perforate the coxae of the legs of the second pair. There are no repugnatorial pores, and the terga are furnished with three pairs of symmetrically placed hairs or bristles. On the posterior border of the last tergal plate there is a pair of spinning papillae. The millipedes of this order, also called Coelochoeta, are referable to several families: Chordeumidae (Chordeuma), Craspedosomidae (Craspedosoma), Heterochordeumidae (Heterochordeuma), &c. The Heterochordeumidae belong to the Oriental region, extending from India to New Zealand. The others are particularly abundant in genera and species in North and Central America and Europe; but are unknown in Africa, south of the Sahara.
Order Proterospermophora.
Differing from the Ascospermophora in that the number of segments is large and variable; they are furnished with repugnatorial pores, and not with the three pairs of setae. In the males the anterior appendages of the seventh segment are modified as phallopods, and the seminal ducts perforate the coxae of the legs of the second pair.
This order, containing the family Lysiopetalidae (Lysiopetalum), is widely distributed in Europe and North America. Large examples of some of the species, e.g. L. xanthinum, reach a length of 4 or 5 ins.
Order Merochaeta.
Resembling the Proterospermophora in having only the anterior appendages of the seventh segment converted into phallopods and the seminal ducts perforating the coxae of the second legs in the males; but differing essentially in that the sterna are
