Growth and regeneration of the second antennae of Asellus aquaticus (Isopoda) in the context of arthropod antennal segmentation

The production of new articles in the flagellum of the second antennae of Asellus aquaticus was studied in both undamaged and regenerating antennae. Segmentation is an iterative process in two phases: a) the first proximal flagellomere (the meristematic article) repeatedly gives off distally a new primary article; b) each primary article divides into four secondary articles (a quartet). To a certain extent, production and development of different quartets are independent processes. Evidence is provided that the formation of new articles and their setae are partly decoupled. During regeneration from the preferred breakage point (the so-called ‘autotomy plane’), the flagellum is generated by the same mechanism of two-phase segmentation. The regenerated flagellum has a normal segmental composition, except for the tip (the apical complex), which has four flagellomeres rather than the normal five. The similar segmental pattern observed also in other malacostracan crustaceans and in insects, supports a close phylogenetic relationship among the two groups; if the latter proves not to hold, that similarity would provide an example of parallel evolution of developmental mechanism. The difference between ‘true’ articles and annulations, defined on the structure of the muscular system, is discussed on the basis of comparative developmental data. In general, annulations are produced more sequentially, compared with the almost simultaneous emergence of true articles.