CHAPTER 10

DIPLOPODA — REPRODUCTION

BY

ALESSANDRO MINELLI AND PETER MICHALIK

KARYOLOGY [ALESSANDRO MINELLI] Chromosome numbers are available for about seventy species of millipedes, unequally distributed among the major taxa of this myriapod class. The list includes ca. 15 species of the sphaerotheriid genus Arthrosphaera and several members of the Pachybolidae and the Harpagophoridae, as a result of investigations by Indian authors (largely summarized in Achar, 1987); and a number of Brazilian Spirostreptida, in particular several species of Pseudonannolene and a number of Spirostreptidae, studied by Fontanetti and asso- ciates. Both schools have also contributed to the study of chromosome numbers in the Polydesmida (ten karyotypes available thus far, including four Japanese Xystodesmidae studied by Tanabe, 1992). The most recent overview by Fontanetti et al. (2002) is the main source for the information summarized in the next lines. Unfortunately, only fragmentary and in part very old reports are available for some major groups, especially Polyxenida (2n = 16 in Polyxenus sp.; Sokoloff, 1914), Callipodida (2n = 12 in Acanthopetalum richii; Vitturi et al., 1997), Chordeumatida (2n = 24ina“Melogona (Microchordeuma)” sp.; Chowdaiah & Kanaka, 1979) and Julida (2n = 22 in Ommatoiulus oxypygus; Vitturi et al., 1997, plus a couple of problematic reports for other species). No data for Colobognatha or Stemmiulida. Warchałowska-Sliwa´ et al. (2004) confirmed for Glomeris hexasticha and G. connexa the diploid number 2n = 20 tentatively suggested for G. annulata by Bessière (1948). In the Sphaerotheriida, most Arthrosphaera species have 2n = 30, others however 28 or 26. In Polydesmida, the most frequent value is 2n = 12, as found in representatives of dif- ferent families (Chelodesmidae, Polydesmidae, Xystodesmidae and Paradoxosomatidae); however, Tanabe (1992) reported 2n = 16 for Riukiaria semicircularis and Chowdaiah (1966) gave 2n = 24 for a “Strongylosoma”sp. Chromosome numbers are more diverse in Spirobolida (2n = 12, 16, 20, 24, 26, 28) and Spirostreptida (2n = 12, 14, 16, 20, 22, 24, 26, 28). Some variation has been found

© Koninklijke Brill NV, Leiden, 2015 Myriapoda 2 (10): 237-265 238 A. MINELLI & P. MICHALIK even among members of one genus, e.g. Rhinocricus (2n = 20, 24, 28; Fontanetti, 1998a) and Pseudonannolene (2n = 12, 14, 16, 20; summarized in Fontanetti et al., 2002). An XY sex determination system has been suggested in the vast majority of cases; the few putative exceptions deserve further investigation.

SEXUAL DIMORPHISM [ALESSANDRO MINELLI] Male millipedes are generally smaller than the conspecific females; in the groups where the number of rings varies intraspecifically, this is lower in males, but in some groups with fixed number of rings the males have a higher number of leg pairs than the females: in the Glomerida, leg pairs are 17 in females, 19 in males; in Sphaerotheriida, 21 in females, 23 in males. Millipede species sexually dimorphic for the number of body rings are known among the Polydesmida (female with 18 + 1 rings, male with 17 + 1, es. Polydesmus spp., Prosopodesmus panporus, Eutrichodesmus peculiaris, Galliocookia spp., Occitanocookia hirsuta, and several Fuhrmannodesmidae; also dimorphic, but with higher numbers of diplosegments, are the species of the genus Devillea species: see p. 285) and the Chordeumatida (male with 25 ‘rings’, female with 29 in Buotus carolinus; male with 25 ‘rings’, female with 27 in some Tingupidae; male with 27, female with 29 e.g. in Xystrosoma beatense and Peterjohnsia spp. (Mauriés, 1987)). In taxa such as Colobognatha or Julida, where intraspecific variation is generally high and, to some extent, ‘open’, the range of segment numbers found in the males is lower than the corresponding range for the females, partly as a consequence of a lower number of post-embryonic moults, for example a maximum of 11 and 14 postembryonic stadia in males and females of Ommatoiulus sempervirilis, respectively (Akkari & Enghoff, 2011); in millipedes with a fixed number of moults, identical in both sexes, the males may develop anyway faster than the females, as in Pycnotropis tida (Vohland & Adis, 1999). In the Polyxenida, sexual dimorphism is mainly expressed as differences in the size, shape and density of some groups of trichomes. For example, in Unixenus aff. broelemanni, the caudal pencil of the female, including nidamental trichomes, is short, homogeneous, light coloured and furnished with hooked trichomes only; in the male, the pencil is one and half to twice longer, dark coloured in part, and furnished with barbate and serrate trichomes only (Condé & Nguyen Duy-Jacquemin, 1992). Except for the first post-embryonic stadia, the sex of a millipede can be easily determined by the presence of reduced or modified pairs of appendages in the male. Most conspicuous are the male appendages involved in sperm transfer and/or in grasping the partner during copulation, i.e. the telopods of the Pentazonia and the gonopods of the Helminthomorpha. But other pairs of appendages can be modified, too. The first pair of legs of male Julida are almost always modified, enlarged in Parajuloidea but reduced and more or less hooklike in the other groups: shortened in Blaniulidae, reduced to three articles in some Julidae, to two articles only in Physiostreptus. The first pair of legs are also modified in millipedes other than Julida: these appendages lack the pretarsus