Decapauropus

Decapauropus is a large genus of pauropods in the family Pauropodidae that includes more than 300 species. This genus was originally described by Paul Remy in 1931 to contain the newly discovered type species Decapauropus cuenoti. As the name of this genus suggests, this genus is notable for including females with ten pairs of legs instead of the nine leg pairs usually found in adult pauropods in the order Tetramerocerata. Before the discovery of D. cuenoti, adult pauropods were thought to have invariably nine pairs of legs.

Description
In 1957, Remy demoted Decapauropus from a genus to a subgenus within the genus Allopauropus, but Ulf Scheller restored Decapauropus as a separate genus in 2008. Pauropods in both genera have five-segmented legs for the first and last leg pairs and six-segented legs for the pairs in between. The two genera can be distinguished, however, by the setae on the pygidial sternum: Whereas Decapauropus has two pairs of setae, Allopauropus has three pairs. Furthermore, the two genera differ in the subadult (fourth) stage of post-embryonic development in terms of the setae on the pygidial tergum: Whereas Decapauropus has only one pair of setae, Allopauropus has two pairs.

Despite the name of the genus, only some species in this genus are known to feature females with ten leg pairs. The species known to feature these females include not only D. cuenoti but also other species later found to feature females with ten leg pairs, for example, D. gracilis, D. vulgaris, and D. productus. Even among adult females of these species, however, most have the usual nine leg pairs, and only a minority have the unusual tenth leg pair. These unusual females add the tenth pair in an additional adult stage beyond the five stages of post-embryonic development usually observed in the order Tetramerocerata. This extra stage features the twelve trunk segments usually found in adults in this order but adds an extra pair of legs. Although this extra adult stage is rare in warm and moist regions with favorable conditions for survival, females with ten leg pairs are generally more common where the environment is less hospitable.

Distribution
This genus has a subcosmopolitan distribution. Some individual species in this genus are also widely distributed: The species D. cuenoti, D. gracilis, and D. vulgaris, for example, are subcosmopolitan. The species D. cuenoti is common in Europe but also found in North Africa, the Mascarene Islands, and the United States, D. gracilis is common in Europe but is also found in Africa, South Asia, and the Americas, D. vulgaris is common in Europe but also found in Africa, Sri Lanka, and North America, and D. productus occurs mainly around the Mediterranean but is also found in Iraq and the United States.

Sex ratios and reproduction
The species D. cuenoti, D. gracilis, D. productus, and D. vulgaris are among the pauropod species with remarkably low ratios of males to females. In some regions, these sex ratios are so low as to suggest reproduction by parthenogenesis. Males appear to be especially scarce in regions where the environment is less favorable, and the extra adult stage for females with ten leg pairs may be an adaptation to parthenogenetic reproduction in some species.

Studies of sex ratios in D. cuenoti, D. gracilis, D. productus, and D. vulgaris in Europe and North Africa find some geographic variation in the scarcity of males. Although males of the species D. gracilis are almost uniformly scarce in Europe and North Africa (with about twice as many females as males), there are notable differences among regions in the sex ratios observed in the other three species: Males of the species D. cuenoti are entirely absent in northern Europe and North Africa, but less scarce in central and southern Europe (where the male/female ratio is 0.3); D. vulgaris exhibits a similar geographic pattern over the same range. Males of the Mediterranean species D. productus are more scarce in North Africa and the Canary Islands (where the male/female ratio is 0.3) than in Europe (where the male/female ratio is 0.8).

A study of D. cuenoti, D. gracilis, and D. vulgaris finds that the fraction of females with ten pairs of legs varies not only among species but also among regions in Europe and North Africa. These females are present in populations of D. gracilis from northern Europe to North Africa, representing between 3 and 10 percent of adults in that range. Females with ten pairs of legs are absent in populations of D. vulgaris in southern Europe and North Africa, however, and rare in populations of that species in northern and central Europe, where they represent less than 1 percent of adults. Populations of D. cuenoti exhibit the most striking geographic variation: Females of this species with 10 pairs of legs are absent in southern Europe, but nearly half (47 percent) have ten pairs of legs in northern Europe, where males are entirely absent. Females with ten pairs of legs are present elsewhere in D. cuenoti populations but represent a small minority of the adults in central Europe (9.6 percent) and North Africa (4.5 percent).