Abstract
Sex chromosomes have been repeatedly implicated in the process of speciation of black flies and other nemotocerans. Arguments are presented here against the case that frequent differences between species in their sex chromosomes are based on (i) different average rates of differentiation of sex-linked and autosomal loci or (ii) the fact that the X and Y chromosomes are less numerous than autosomal chromosomes and so are more subject to the effects of drift and the random fixation of chromosome rearrangements. The argument is made that speciation in black flies and many other groups is an adaptive process and that differentiated sex-chromosome systems play a role in this process.