The paper proposes standard polytene chromosome maps for species groups in the subgenus Simulium. In the S. venustum/verecundum complex we distinguish by their chromosomes a minimum of seven sibling species designated by their IIS sequences, as follows: EFG/C (S. truncatum?) holarctic; EFG, Germany; CC (S. venustum?) North America (with probably CC 1 a separate sibling); A/C venustum, New Hampshire (with AC(gB) venustum Newfoundland probably distinct); CC 2 Ontario, Newfoundland; AA verecundum Newfoundland (with possibly A/C verecundum, Ontario distinct); and ACD (S. verecundum? = sublacustre) holarctic. Three of these taxa (EFG/C, ACD, and CC) were probably recognized by taxonomists, though the circumpolar distribution of the first two was not reliably recorded. The remaining species were not previously resolved. The chromosomal characteristics of all species are described and notes are given on available details of their biology and distribution. Some broad aspects of the relationship between cytological and taxonomic methodology in the group are discussed.

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Neuroblast and salivary gland chromosomes were studied in Feulgen squash preparations of Simulium vittatum larvae. The haploid complement consists of three metacentric chromosomes; the arm ratio of the longest (1) and shortest (3) is approximately as 1: 1, that of the second as 1: 2. Centromere regions of salivary gland chromosomes are expressed as characteristic expanded drums with large disorganized chromomeres. The third salivary gland chromosome bears the single large nucleolus and a smaller granular structure comparable to the Ring of Balbiani of chironomids. Pairing of the two constituents of salivary gland chromosomes is quite loose regardless of the degree of polyteny. Photographic maps are given of the standard band sequence of the entire complement. Relatively short inversions are the commonest type of structural rearrangement. From preliminary observations on further black fly species and an eclectic treatment of the nematocerous literature, morphological characteristics of salivary gland chromosomes are discussed with respect to their value in delimiting taxonomic segregates and determining phylogenetic relationships.

In many Simuliidae, patterns of spatial and temporal relationships among the most closely related species are more readily interpreted in terms of sympatric speciation than of allopatric speciation. Specific examples are (i) the allotriploid taxa in Gymnopais and other genera, (ii) the black fly faunas of geologically recent islands (Tahiti), and (iii) species in Prosimulium onychodactylum, a prototype of a continental multisibling species complex. A model of sympatric speciation is presented based on coadaptation of polymorphic sex chromosomes in pairs reinforced by progressive development of assortative mating. This model predicts that (i) populations should frequently exhibit sex-chromosome polymorphism, (ii) these sex-chromosome polymorphisms, and autosomal ones, should in some cases display linkage or association disequilibria, (iii) species pairs or complexes should be incurred that differ only in sex chromosomes and that share extensive ancestral autosomal polymorphisms, and (iv) such species should differ in their biology and perhaps their present-day distribution. Recent publications and observations are in accordance, in general, with predictions from the model. Genetic control, e.g., of diapause, larval developmental timing, and niche preference or ethology, could substitute as a basis of incipient cleavage. The evidence for sympatric speciation is purely inferential, but this is equally true for the allopatric interpretation, and in black flies the circumstantial evidence for prevalence of sympatric speciation appears more compelling. This is not to deny the efficacy of allopatry and founder effect in the origin of some species complexes.Key words: sympatric speciation, black fly, evolution.