Hydrobiologia 360: 25–32, 1997. 25 A. Brancelj, L. De Meester & P. Spaak (eds), Cladocera: The Biology of Model Organisms. c 1997 Kluwer Academic Publishers. Printed in Belgium. Structure of thoracic limbs in Bosminopsis deitersi Richard, 1895 (Anomopoda, Branchiopoda) Alexey A. Kotov A. N. Severtsov Institute of Ecology and Evolution of Russian Academy of Sciences, Leninsky Prospekt 33, 117071 Moscow, Russia Key words: Bosminopsis deitersi, Bosmina, thoracic limbs, comparative morphology, Bosminid monophyly Abstract A study of the structure of the limbs of Bosminopsis deitersi Richard, 1895 was performed. The structure is similar in populations from different regions. According to Meissner (1903), limbs of Bosminopsis differ from those of Bosmina strongly enough to separate Bosminopsis in a special family. However, our observations indicate that the structure of the limbs of Bosmina and Bosminopsis is similar, and conform to the general plan of Bosminid limbs. The first limb is strongly simplified, and the first limb of males is unique among anomopods. Limbs III–V are less oligomerized than limbs I–II, and are organized according to the chydorid (and macrothricid) type. The degree of oligomerization is different in the two genera. Comparative information on the limb structure in one genus may contribute to the reconstruction of the evolution of a corresponding limb in the other genus. The uniformity of limb structure within each genus, and their similarity between both genera is thought to be a reflection of the monotonous life conditions in open water. Abbreviations dag – distal armature of gnathobase; db – distal brush of setae of limb I; dse – distal setae of exopodite; end – endopodite; eh – ejector hooks of limb I; ep – epipodite; ex – exopodite; fp – filter plate of gnathobase; fpd – flat projection of endopodite; fpe – flat projection of exopodite; frs – fringe of setules; fs – field of setules; gn – gnathobase; hls – hook-like spines; hok – hook of male clasper; ies – innermost spines of endopodite III; ise – innermost seta of endopodite I; lee – long element of exopodite; lse – lateral setae of exopodite; mp – maxillar process of limb I; odl – outer distal lobe of limb I; ofp – outer filter plate; ose – outermost soft setae of limb II; pep – preepipodite; pws – powerful seta; rofp – rudimentary outer filter plate of Bosmina; sdl – inner subdistal lobe of limb I; seg – sawform element of distal armature of gnathobase II; sen – sensilla; sss – solitary shaggy seta; sis – sawlike innermost seta of limb II; ths – thick setules Introduction Earlier, a comparative study on the limb structure of females of several Bosmina species was performed, and a uniformity in morphology in all investigated rep- resentatives was demonstrated (Kotov, 1995). So far, however, the limbs of Bosminopsis have not been in detail. There was only one attempt at description of the limbs in Bosminopsis zernovi Linko, 1901, by Meis- ner (1903) (at present, B. zernovi is considered to be a subspecies of B. deitersi Richard, 1895). According to Meissner (1903), the limbs of Bosminopsis differ from those of Bosmina very strongly, and he proposed to assign the genus Bosminopsis to a different family. Meisner’s publication remained unknown to western investigators, but in Russian literature, this question was recognized as unsolved (Rylov, 1950; Manujlo- va, 1964; Korovchinsky, 1992). The ‘na¨ ıve’ (in the words of N. N. Smirnov) drawings and descriptions of Meissner do not comply with the recent standard