| Summary: | Potamonectes depressus (F. ) and P. elegans (Panzer) are allopatric over most of their Palaearctic range but intermediates occur where their ranges coincide. This study investigates the relationship between the two forms and assesses their taxonomic status. The history of the confusion between the two is reviewed and the types are examined. Specimens from Fennoscandia and from southern England are selected as being typical of P. depressus and P. elegans respectively, and the morphological differences between them are evaluated. Although the aedeagus varies in both forms it remains the most reliable means of distinguishing them. Most other characters differ significantly, and the two forms give the impression of being good species. The status of other populations is then determined, and the existence in northern Britain of a continuous range of variation between the two extremes is confirmed. Most Scottish populations show evidence of introgression, though populations intermediate in all respects are rare. Each population has a distinctive and stable set of characters, even though limited interchange occurs. This evidence is supported by the isoelectric focusing of whole proteins. The life history is described from laboratory-laid eggs reared through to adult. The use of spermatophores is demonstrated, and the larvae are compared. The production of fertile P. depressus x P. elegans hybrids supports a conspecific status for the two forms. Pleistocene fossils are described which suggest that a wide range of aedeagus forms has existed in Britain for some 40 000 years. The nature of the hybrid zone is discussed, and it is concluded that it owes its stability to the individuality and stability of local populations. The complex in North America is also reviewed, and the Holarctic distribution of P. depressus is confirmed. Southern forms, however, are not conspecific with P. depressus-ele ans, the morphological differences being supported by breeding experiments and isoelectric focusing. They are themselves split into two species, P. rotundatus (LeConte) in the east and an undescribed species in the west. R. H"C" L ,
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