Chamaesoma

Chamaesoma is a monotypic genus of millipede in the family Chamaesomatidae, and Chamaesoma broelemanni is the only species in this genus. This millipede is notable as one of only a few species in the order Chordeumatida with only 26 segments in adults (counting the collum as the first segment and the telson as the last), four fewer segments than typically found in adults this order. This species lives on the surface of the soil and is distributed across most of France and Luxembourg. The genus Chamaesoma and its only species were first described by the German zoologist Karl W. Verhoeff in 1913 based on type material found by the French zoologists Henri Brölemann and Henri Ribaut under wet leaf litter near St. Béat in the Pyrenees in France.

Description
The body of this millipede ranges from brown to gray. This millipede is one of only a few species in the order Chordeumatida with only 26 segments in adults, the minimum number recorded in this order. Accordingly, the adult female of this species has only 42 pairs of legs, and the adult male has only 40 pairs of walking legs, excluding the eighth and ninth leg pairs, which become gonopods.

This species arrives at a lower number of segments and legs through a process of post-embryonic development that deviates from the anamorphosis usually observed in the order Chordeumatida. Like other species in this order, C. broelemanni is teloanamorphic, adding segments and legs through a series of molts until the adult stage, when the molting stops and the adult emerges with a final number of segments and legs. This species, however, reaches maturity and stops molting one stage earlier, in the eighth stage rather than in a ninth stage. Furthermore, C. broelemanni conforms to the pattern usually observed in most species of Chordeumatida only through the first five stages, then adds one fewer segment than usual upon entering the sixth and seventh stages. Thus, in this process, C. broelemanni goes through stages of development with 6, 8, 11, 15, 19, 22, and 24 segments, before emerging as an adult with 26 segments in the final stage.

This process is the same as that observed in the species Opisthocheiron canayerensis, one of the other species in the order Chordeumatida with only 26 segments in adults, but with one difference. In C. broelemanni, the transformation of legs into gonopods in the male begins in the sixth stage and continues until the eighth and final stage. In O. canayerensis, however, this process does not begin until the seventh stage.