Post-embryonic development of amphipod crustacean pleopods and the patterning of arthropod limbs

Morphology and post-embryonic development of the pleopodal rami of the amphipod crustacean Gammarus roeselii Gervais, 1835 are described, with particular reference to the differentiation of limb articles. An article is the part of a limb between two complete arthrodial membranes and, in the pleopodal rami, most arthrodial membranes are accompanied by two plumose setae on the opposite faces of the article. During post-embryonic development, the rami acquire new plumose setae and new arthrodial membranes proximal to the already differentiated part; this implies that new articles are produced by the division of the most proximal article only. On the proximal article of the inner ramus there are also some bifid setae, whose number increases post-embryonically, most likely by proximal addition of new setae. Data from different limb types of different arthropod groups suggest that during post-embryonic development new articles are also produced from one or more specific zone(s) of the limbs. General aspects of these “growth zones” in arthropod limbs are discussed and their distribution, in respect to both different limb types and different taxa, is reviewed. The available data do not allow unambiguous conclusions about the evolution of growth zones and limb patterning, but the most relevant questions are nevertheless outlined.