Abstract
The number of stadia during post-embryonic development is supposed to be fixed in most species of the millipede order Polydesmida. For the first time since 1928, additional moults were observed in two males of Polydesmus angustus Latzel, 1884 reared in the laboratory. These ‘elongatus’ males sensu Verhoeff reached stadium IX instead of stadium VIII, with addition of a further podous ring (32 pairs of legs). One male had well-developed gonopods at stadium VIII, which regressed at stadium IX; the other had no gonopods at stadium VIII, which developed at stadium IX. The two cases correspond to the ‘regressionis’ and ‘progressionis’ forms described by Verhoeff in Polydesmus complanatus (Linnaeus, 1761), which confirms entirely his results. Additional moults appear to be associated with small body sizes and possible underlying mechanisms are discussed. Comparisons between millipede orders indicate that post-embryonic development is less strictly canalized in Polydesmida than in Chordeumatida. This implies that the adult number of body rings is of limited taxonomic value in Polydesmida and should not be viewed as a character of generic importance.
Highlights
IntroductionIn millipedes (Diplopoda), post-embryonic development occurs basically by anamorphosis, i.e. the number of body rings is small at birth (e.g., four podous rings bearing three pairs of legs, two apodous rings, plus the telson) and increases progressively at each moult
In millipedes (Diplopoda), post-embryonic development occurs basically by anamorphosis, i.e. the number of body rings is small at birth and increases progressively at each moult
Adults are in stadium VIII, with the head, 18 podous rings, 1 apodous ring and the telson; in other species, adults are in stadium VII (17+1+T); in a few species, the stadium and ring number of adults differ between the sexes, but with no intra-sex variation
Summary
In millipedes (Diplopoda), post-embryonic development occurs basically by anamorphosis, i.e. the number of body rings is small at birth (e.g., four podous rings bearing three pairs of legs, two apodous rings, plus the telson) and increases progressively at each moult. Adults occur in several stadia within a species, either because the stadium at which maturity is reached varies among individuals, or because adults undergo further moults. Development is much more canalized in the order Polydesmida, in which there is generally only one adult stadium in each species, with a fixed number of body rings (Mesibov 2011). In spite of obvious exceptions, e.g. in the genus Devillea Brölemann, 1902 (Xystodesmidae), in which adults probably occur in several stadia (Enghoff et al 1993), the number of body rings in the adult is considered as fixed in most Polydesmida and is sometimes used in taxonomy In spite of obvious exceptions, e.g. in the genus Devillea Brölemann, 1902 (Xystodesmidae), in which adults probably occur in several stadia (Enghoff et al 1993), the number of body rings in the adult is considered as fixed in most Polydesmida and is sometimes used in taxonomy (e.g. Djursvoll et al 2000)
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