This roport contains descriptions of the karyotypes of 60 species ofSyrphinaecollected in South America, including five species for which the karyotypes have been deseribed previously. The 55 newly added South American species represent the tribesSyrphini(11),Bacchini(14),Melanostomini(11) andToxomerini(19) with chromosome numbers ranging from 2n=6 to 18, including microchromosomes. The new results are integratod with our earlier observations on the karyotypes of over three hundred species ofSyrphinae. With a few excoptions, the chromosome numbers of spocies are similar in frequencies in the two samples but TCL is lower in the South American sample of spocies. Most South American spocies have a pair of short, heteromorphic and apparently telocentric sex chromosomes but a few exceptional types of sex-chromosome mechanisms were found. Reductions in chromosome number can be explained by end-to-end fusions in some cases though reciprocal translocations may also be occurring. It would seem that functional dicentric chromosomes are produced, by either process, in which one centromere suppresses the other. Karyological evolution inSyrphinaetribes seems to have involved a progressive reduction of chromosome numbers, pericentric inversions and the accumulation of microchromosomes. The role of such processes in the karyological evolution of seven tribes (including thePipizini) is considered briefly.
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