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CLASS DIPLOPODA

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Originally appearing in Volume V18, Page 472 of the 1911 Encyclopedia Britannica.
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CLASS DIPLOPODA. Structure.—The anterior extremity is provided with a distinct head which by its general form and the nature of its appendages is as sharply marked off from the body as is the case in the Hexapoda. It always bears at least three pairs of appendages, the eyes when present and, in the Oniscomorpha a peculiar sense organ. The inferior edge of the head plate overhangs the mouth and is termed the labrum. The exoskeleton of a typical somite consists of the following elements: a dorsal plate, a ventral plate, and a pleural plate on each side. To the external margin of the ventral plate or sternum is articulated a pair of legs and between the leg and the pleural plate is situated the spiracle of the tracheal system. But the segmentation of the Diplopoda presents two marked peculiarities. The first is the fact that, with the exception of a few of the anterior leg-bearing segments and perhaps one or two of those at the posterior end of the body, a single dorsal plate or tergum with its pleural plates overlies two sternal plates, two pairs of legs and two pairs of spiracles. Hence the segments appear to be double and to be furnished with twice as many legs as is normal in the Arthropoda —a peculiarity which has suggested the term " Diplopod " or " double-footed," for this group. It is generally believed that each tergal plate results from the coalescence of the terga of two originally distinct adjoining segments; but the same effect would be produced by the enlargement of one of a pair of terga and the complete excalation of the other. It is in favour of the latter view that there is only a single pair, and not two pairs, of stink-glands on each so-called double tergal plate. Unfortunately the history of the development of the segments does not clear up the difficulty since the terga of the double segments are single from the first, and no evidence either of fusion or excalation is supplied. The second of the two peculiarities above-mentioned is the great development of the tergal sclerite as compared with the sternal. Only very rarely (i.e. in Platydesmus) is there a broad sternal area. In the majority of cases the lateral edges of the tergum are bent downwards and inwards towards the mid ventral line; the sternum at the same time is so much reduced that the basal segments of the legs of opposite sides are almost in contact. The A h B .-, C After Silvestri, Ann. Mts. Genova, (2), xvi, figs. 17, 19, 2$. Chilongatha. A, of Spirostreptus. B, of Julus. C, of Glomeris. c, cardo. m, mentum. st, stipes. lg, linguae. pleural plate on each side usually disappears either by suppression or by fusion with the tergum. The sterna with their attached legs often remain free. But quite commonly the coalescence of the skeletal elements is carried to such an extreme that each segment is a solid ring with two pairs of movable appendages. The last segment is differently constructed from the others. It is always limbless, and usually consists of a complete tergal ring, a single sternal plate, and a pair of movable anal valves which are normally closed, but are capable of being opened for the passage of faeces. These anal valves are possibly the homologues of the plural scutes of a normal segment. The appendages are modified a. it9 After Pocock in Max Weber's Zoo!. Ergebnisse, &c., IV., Pt. XXI., fig. 8, 1894. c, head with eyes and antennae. tg I, tergal plate of first segment. a.tg, tergal plate of last or anal segment. a.st, sternal plate of ditto. a.v, anal valve. 19 pm, promentum. h, hypostoma. as a single pair of antennae, two or three pairs of jaws and a variable number of walking-legs, of which one or more pairs may be trans-formed into gonopods. The antennae are short and very similar to the legs. They are preoral in position, and usually consist of seven segments, the seventh or distal segment being small, as a rule, and furnished with a sense organ which is probably olfactory or tactile in function. The mandibles or jaws of the first pair are the most anterior of the postoral appendages. They are large, powerful, and usually consist of three or two segments, a basal or After Voges. It, Posterior margin of the body- t, Fine tracheae given off from it. ring (tergum). ms, Respiratory muscle attached r, Anterior border. to tracheal sac. st, Tubular chamber of tracheae. m, Ventral body muscle. cardo, which is sometimes absent, a second or stipes, and a third or mala, the latter being supplied with a strong tooth and pectinate lamellae. In all Diplopods, with the exception of the Pselaphognatha, there are only two pairs of jaws, those of the second pair forming a large plate, the gnathochilarium, which acts as a lower lip. It consists of several distinct sclerites, two external on each side, the proximal known as the cardo, the distal as the stipes, the latter being tipped with one or two lobes (malae) and far exceeding the cardo in size. Between the external plates there is a median proximal plate (mentum) generally of large size and often itself subdivided, and a pair of distal plates (linguae). Behind the base of the gnathochilarium there is a single large transverse plate, the hypostoma. In the Pselaphognatha, the jaws representing the gnathochilarium are differently constructed and an additional pair, the maxillulae, has been recently detected between the gnathochilarium and the mandibles. Behind the gnathochilarium, which from embryological data appears to result from the modification of a single pair of appendages, a legless so rite has been detected in some embryos. Possibly the plate referred to above as the hypostoma is its sternal element. The heart is a median dorsal vessel composed of a series of chambers each giving off a pair of arteries and furnished with a pair of orifices or ostia. According to Newport, the anterior chamber lying in the second segment is prolonged into an aortic trunk from which arise three pairs of lateral arteries dipping down on each side of the alimentary canal and uniting beneath it in a common ventral vessel. The heart is enveloped in a delicate pericardial membrane and is supported by lateral alary muscles. The alimentary canal is a simple tube extending usually straight through the body from mouth to anus. Only in the Oniscomorpha is it looped, thus suggesting the origin of this short-bodied group of millipedes from longer, more vermiform ancestors. A pair of so-called salivary glands opens into the fore-gut near its anterior extremity and one or two pairs of malpighian tubes communicate with the hind-gut at its junction with the broad mesenteric portion of the canal. Respiration is effected by means of tracheal tubes which communicate with the exterior by means of spiracles situated just above the bases of the walking limbs. Each spiracle leads into a longer or a shorter.pouch whence the tracheae, which are of two kinds, arise. In the majority of the orders the tracheae are tufted, that is to say, they form two bundles of short simple tubules springing from the innermost corners of each pouch. In the Oniscomorpha, however, each pouch gives rise to a number of long tubes which extend through the body and somewhat resemble those of the Chilopoda except that they neither branch nor are extensive. As in the Chilopoda and Hexapoda the tracheae are strengthened and kept expanded by a slender spiral filament. The ventral nerve cord consists of two strands so closely approximated as to be practically fused, with a small ganglionic enlargement for each pair of legs. Hence in the double segments there are two such ganglia, which in addition to the crural nerve give offon each side a large branching nerve to other organs in the segment. In the Opisthospermophora (lulus, Spirostreptus) and the Oniscomorpha (Glomeris, Sphaerotherium) the ganglia are spaced at equal distances on the cord, but in the Merochaeta (Polydesmus) they are grouped in pairs to correspond to the spacing of the legs. The apodous penultimate and anal segments are innervated from the last ganglion of the cord, as are also the gonopods of the males of cia . oc After G. C. Bourne, J. Linn. Soc. xix., Pl. 29, i8R6. rotherium obtusum, a South African species of Oniscomorpha. g12 and g122, Second and twenty- second ganglia of chain, the posterior nerve of each gan- glion, lg.n, supplies the leg, the anterior, tr.n, the tra- cheal sac and other organs. segments. n.gon, Nerve to gonopods. cb, Cerebral ganglia supplying tr, Tracheal tubes with spiral the eyes and antennae. filament. oes, Oesophagus, cut through. tr.s, Tracheal sac. sb.gl, Suboesophageal or first ganglion of ventral chain. the Oniscomorpha. The first (suboesophageal) ganglion of the cord supplies the mandibles and gnathochilarium and is connected by the oesophageal commissures with the bilobed cerebral nerve whence arises the nerves for the eyes, when present, and the antennae. Eyes are sometimes absent, as in all the genera of Merochaeta and in many genera of other groups, as in Siphonophora, one of the Colobognatha, and several of the Juloidea (Typhloblaniulus). In other cases they are represented by one or two ocelli on each side (Stemmiuloidea) ; or by a vertical series of ocelli as in the Glomeroidea and Polyzonium amongst the Colobognatha. But in the majority of the orders they are represented by triangular or subspherical. aggregations of ocelli recalling in a certain degree those of the Lithobiomorpha amongst the Chilopoda. They are simple in structure and consist externally of a cuticular corneal thickening or lens and internally of a retinular layer of enlarged epidermic cells, the c, Head. oc, Eye-cluster. ant, Antenna. md, Basal segment of mandible. tg2 and tgig, Part of the terga of the second and thirteenth internal or proximal ends of which are continuous with the fibres of the optic nerve. The ovary is unpaired and extends almost the entire length of the body beneath the alimentary canal. The oviducts are sometimes separate tubes (Lysiopetalum), some-times confluent and divided just before terminating in the two orifices behind the base of the legs of the second pair (Julus). The testes and seminal ducts occupy the same position and extent as the ovary and oviducts. The ducts are sometimes coiled, some-times divided, sometimes united. The two testes are sometimes united by transverse branches across the middle line, and are some-times branched posteriorly. They bear short caecal diverticula in which the semen is developed. There are no accessory glands associated with the generative organs; but in some forms, e.g. Polyxenus, there is a pair of receptacula seminis extending back-wards alongside the ovary and opening into the oviduct. After Pocock, J. Linn. Soc. xxi., Pl. 25. spermophora (Spiroboloidea). A, Anterior view, and B, lateral views of the apparatus. ant, anterior, and post, posterior portions of the coleopod ensheathing the phallopod, of which the proximal portion, ph, is shown. C, Phallopod removed from the coleopod. The secondary sexual characters of the males are of great taxonomic importance. The seminal ducts, like the oviducts, open behind the legs of the second pair. Associated with them in the Limacomorpha (Glomeridesmus), there is a pair of very long retractile penes. In the Spirostreptoidea and Juloidea the penes are much shorter and have coalesced. Sometimes they are undeveloped (Spiroboloidea). In other cases, the Merochaeta, Oniscomorpha, &c., the ducts traverse the coxae of the legs of the second pair. But in all these groups, with the exception of the Oniscomorpha, semen is transferred from the genital orifices, with or without the aid of the penes, either into the first or second pairs of appendages of the seventh segment which are modified in various ways, and are termed phallopods. When the posterior legs are so modified the anterior are as a rule even more profoundly altered to form a protective sheath, or coleopod, for the phallopod; and as a further precaution the entire apparatus is usually withdrawn within the seventh segment. In the Oniscomorpha the semen is transferred into a pair of receptacles developed upon the coxae of the legs of the last pair, which are chelate. The male appendages that are modified in the above described ways are comprehensively spoken of as gonopods. Other secondary sexual characters, like the stridulating organs of the males of some Oniscomorpha, the suctorial pads on the legs of Spirostreptoidea, the development of angular processes upon the mandible or first tergal plate, or of fine ridges in the gnathochilarium—all of which are concerned in enabling the male to maintain a secure hold upon the female—are of great taxonomic use in distinguishing the genera and species. The most important glands in the Diplopoda are the repugnatorial or stink-glands, which, except in the Oniscomorpha, Limacomorpha and Ascospermophora, open by pores upon the sides of more or fewer of the segments. They secrete a fluid with an unpleasant odour, breaking up in one case into cyanide of potassium, and are practically the only means of protection, apart from the hard exoskeleton, which Diplopods possess. In some millipedes silk glands also exist and open upon papillae upon the posterior border of the last tergal plate. They are found in the Ascospermophora, Stemmiuloidea and Proterospermophora, and are used for spinning nests for the eggs and protective cases for the young during exuviation. Classification.—The existing members of the class Diplopoda may be classified as follows: Subclass I. PSELAPHOGNATHA. Order: Penicillata (Polyxenus). 2. CHILOGNATHA. Order: Oniscomorpha (Glomeris, Zephronia). Limacomorpha (Glomeridesmus). Colobognatha (Polyzonium, Siphonophora). Ascospermophora (Chordeuma). Proterospermophora (Lysiopetalum). Merochaeta (Polydesmus). Opisthospermophora. Suborder: Stemmiuloidea (Stemmiulus). „ Spiroboloidea (Spirobotus). Spirostreptoidea (Spirostreptus). Juloidea (Julus, Nemasoma). Subclass PSELAPHOGNATHA. Diplopods with the soft integument strengthened by weakly chitinized sclerites and furnished above and on the head with trans-verse rows of short, stout, somewhat squamiform bristles; laterally, on each side of the principal segments, with a thick tuft of long bristles and with a large, silky, white tuft projecting backwards from the posterior extremity. Mandibles one-jointed. Behind them a pair of small, one-jointed maxillulae, attached to a median membranous " lingua.” Behind the " lingua " and maxillulae, a large, double, transverse plate with a long, external sclerite bearing distally in Polyxenus an inner short-lobate process and an outer long spiny palpiform branch. The latter, however, is absent in Lophoproctus. These sclerites probably represent the gnathochilarium of the Chilognatha, but the homology between the skeletal elements of the jaws in question is not clearly understood. It has been suggested that they represent two pairs of jaws, but embryological proof of this does not exist. A, after Carpenter,Q.J.M.S. 49, Pl. 28, fig. r. B, after Latzel, Die Myr. Osi. Ung. Mon. II., Pl. ii., 1884. A, Jaws of second and third pairs. mxl, maxillula; mx.p, palpiformbranch of maxilla; mx.lb, lobate process of maxilla; mx.ext, external plate of maxilla perhaps corresponding to the stipes of the gnathochilarium of the Chilognatha; mx.int, internal plate of maxilla, perhaps corresponding to the mentum and promentum of the gnathochilarium (by Carpenter mx.int is regarded as an appendage posterior to the maxilla) ; mb, membrane. B, Mandibles of Polyxenus lagurus. _ Order Penicillata ( = Ancyrotricha). Head large, usually with lateral eyes. Antennae eight-jointed, attached near the middle of the front of the head. On the dorsal side of the body there are eleven segments, simple and compound. The first four of these bear one pair of legs each, the succeeding four two pairs of legs, the ninth segment one pair, making a total of thirteen pairs of legs. The tenth and eleventh or anal segment are legless. There is a narrow sternal area separating the bases of the legs of the two sides. There are no repugnatorial glands. In the male none of the legs are modified as gonopods, but the coxa of each of the legs of the second pair is furnished with a conical penis, which during copulation, it may be supposed, is inserted into the genital orifice of the female, which occupies a corresponding position in that sex. The young when first hatched has only three pairs of legs and five segments. The millipedes of this order are all of small size, measuring at most only a few millimetres in length. The best-known genera are Polyxenus and Lophoproctus, both of which occur in Europe. Other forms have been discovered in the West Indies, North and South America, and Ceylon; and it is probable that the group has an almost cosmopolitan range. They live under stones or the loosened bark of trees. The carboniferous fossil, Palaeocampa, is usually referred to this subclass. Subclass CHILOGNATHA. Diplopods with firmly chitinized exoskeleton, sometimes thickly, sometimes sparsely covered with short, simple hairs,, but never decorated with tufts or rows of peculiarly modified bristles. B C SI 19 91 After Bode. a, Position of generative openings. Mandibles, two- or three-jointed; maxillulae absent, the jaws of the second pair being represented by the gnathochiiarium 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 9A pen After Pocock, in Max Weber's Zool. Ergebnisse, &c., IV., Pl. xic. Oniscomorpha. A, Lateral view of the entire animal. c, head; ant, antenna; tg', tg2 and tg13, tergal plates of first, second and thirteenth segments; lg, extremities of some of the anterior legs. B, Gonopods of the male. gp' and gp2, anterior and posterior pairs of gonopods, both being chelate claspers; pen, processes arising from the basal segments of the gonopods of the second pair, which act as penes. C, Vulvae or genital plates attached to the basal segments of the legs of the second pair in the female. g.o, genital orifice. body without forming a chitinous ring round the anal valves and sternum. In the male the legs of the penultimate pair are some-times 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. Order Limacomorpha. 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 A Cap. After Pocock, J. Linn. Soc. xxiv., Pl. 37. A, Lateral view. c, head with antennae; tgl, tergal plate of first segment; an.tg, tergal plate of last or anal segment. B, Lower view of one of the segments. tg, inferior edge of the tergal plate; pl, pleural sclerite; lg', basal segment of leg. C, Posterior extremity of body. an.tg, tergal plate of anal segment ; cop.lg. gonopod or copulatory leg. D, Legs of the third pair with extruded penes, pen, in front of them. are divisible into three families: the Platydesmidae (Platydesmus, Pseudodesmus), Polyzonidae (Polyzonium, Siphonotus), Siphonophoridae (Siphonophora). Of these the Platydesmidae have de-parted 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. 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), Graspedosomidae (Graspedosoma), 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 o. B solidly welded to the rest of the exoskeleton of the segments, which are either nineteen or twenty in number, in the absence of eyes and of spinning papillae, and in having six- jointed legs. This order is cosmopolitan in distribution and consists of a very large number of genera which by some authors are referred to the single family Poly- desmidae; by others to numerous families. Many species are brightly coloured, and some individuals of the Oriental genus After Pocock, in Max Weber's Zool. Ergel,nisse, &c., ... t9 9 IV., Pl. xx. a.19 Platyrhachus may reach a length of g ins. The segments are usually provided with lateral laminate or tubercular expansions bearing the repugnatorial pores, which are only very rarely absent. Order Opisthospermophora. Resembling the Proterospermophora in possessing a large and variable number of segments, each of which, with the exception of the last and the anterior four or five, is furnished with a pair of repugnatorial pores, but differing essentially from them in that the posterior pair of appendages of the seventh segment are converted into phallopods, and the anterior into protective coleopods in the male, and that the seminal ducts in this sex do not perforate the coxae of the legs of the second pair but are usually associated with a distinct penis situated immediately behind them. The genera of this order present greater diversity of structure than is found in the other orders and are referred to four suborders, which by some zoologists are erected to ordinal rank, namely, the Stemmiuloidea (Monochaeta) ; the Spiroboloidea (Anochaeta) • the Spirostreptoidea (Diplochaeta); and the Juloidea (Zygochaeta). In the Stemmiuloidea the sterna are free and the pleurae partially so; the terminal segment of the legs is bisegmented; there are two pairs of spinning papillae on the last tergite; the penis is a single long tube, and the eyes are represented by one or two large lenses on each side of the head. The genus Stemmiulus, constituting the Stemmiulidae, is represented by a few species recorded from the Oriental, Ethiopian and Neotropical regions. In the possession of silk-glands this suborder resembles the Ascospermophora and Proterospermophora, and should perhaps rank as an order apart from the Opisthospermophora. The Spiroboloidea, containing one family, the Spirobolidae (Spirobolus, Rhinocricus, &c.), have the sterna and pleurae coalesced, the tarsi undivided; no spinning papillae, no penis, the eyes represented by an aggregation of ocelli; and the first five segments each with a single pair of legs, the sixth carrying two pairs. This group attains its maximum of development in the tropics, where species and genera are numerous and specimens of large size, i.e. 6 ins. or over, are met with. The Spirostreptoidea resemble the Spiroboloidea in many particulars, but the fourth segment is footless, and the fifth has two pairs of limbs; the male has a distinct and double penis, and in both sexes the stipites of the gnathochilarium extend to the proximal end of the mentum, which is relatively small. The distribution of this order, which contains several families: Spirostreptidae (Spirostreptus, Rhynchoproctus), Cambalidae (Cambala, Julomorpha), &c., is practically the same as that of the Spiroboloidea. Specimens over 6 ins. in length are met with in the tropics of Africa and Asia. The Juloidea differ from the Spirostreptoidea in having the third segment limbless, the first, second and fourth with a single pair of appendages, and the stipites of the gnathochilarium much expanded and meeting for a considerable distance in the middle line behind the very small promentum. The best marked family of this group is the Julidae, which is widely distributed in the northern hemisphere. Its species and genera (Julus, Pachyiulus) are abundant in Europe. Another European family, the Nemasomidae, is founded for the genus Nemasoma, which is remarkable for having the sterna free. Habits, &c.—Millipedes are principally cryptozoic, living understones or logs of wood in damp, secluded localities. They feed almost wholly upon decaying vegetable matter, and drink a considerable quantity of water. Some of the tropical species emerge in numbers from their hiding-places after heavy rains, and crawl over the ground and bushes in search of moisture in broad day-light. Their method of progression over level ground is quite peculiar. The body is held in a straight line and is propelled by a succession of wave-like movements of the legs, which are moved in groups, the groups on the right and left side exactly corresponding. Some forms, e.g. Stemmiulus, have been described as attempting to evade capture by a hopping action caused by vigorous jerking and wriggling of the body. Many of the species are very conspicuously coloured and the association of brilliant colouring with the existence of the nauseous secretion of the repugnatorial glands suggests that the coloration is aposematic or of warning significance. Copulation between the sexes takes place before oviposition. In the Opisihospermophora the males and females coil together with the ventral surface of the anterior ends of their bodies opposed, the male holding the female securely by the head while the extended phallopods carrying the semen are brought into contact with her genital orifice. In the Polydesmidae pairing is effected in the same way except that the male and female instead of intercoiling remain extended, the male clasping the female with his legs. In the Oniscomorpha the sexes also pair front to front, not head to head, however, but head to tail, so that the gonopods in the anal segment of the male can be applied to the second pair of postoral appendages in the female. Some males of this group, e.g. Sphaerotherium, have a stridulating organ on their posterior gonopods and stridulate when finding the females. The method of disposing of the young, which usually have only three pairs of legs at hatching, differs in various groups. In Julus and Polydesmus the female burrows below the surface and makes a subspherical nest of small blocks of earth which are moistened with the salivary secretion and moulded to the proper shape between her jaws and anterior legs. When the receptacle is nearly finished she deposits her eggs in it, and, closing the aperture, leaves the whole to its fate. On the other hand, a female specimen of the South African species, Archispirostreptus erythrocephalus, that lived in the London Zoological Gardens, buried herself, coiled round the eggs, and remained with her young for some time after they were hatched. Again, millipedes, like the Stemmiuloidea and Ascospermophora, which possess silk-glands, spin silken, cases for the protection of their eggs. From Balfour, after Metsch- Immature specimens of these groups mkov. spin similar silken cases at the time of FIG. II.—Larva of exuviation; and cases, resembling the Strongylosom¢ Guerinii, nests, are likewise made for purposes of one of the Polydesmidae, moulting by immature forms of some just hatched. exotic species of Polydesmidae, e.g. by the tropical African Oxydesmus. There is good reason to think, however, that the animal makes use of its own voided excrement in the formation of these receptacles. A considerable number of Chilognatha of doubtful systematic position have been recorded from beds of the carboniferous formation. The best known are Acantherpestes and Euphoberia. Specimens referred to existing genera have been discovered in amber beds of Oligocene age.
End of Article: CLASS DIPLOPODA
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