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In addition to five pairs of euchromatic chromosomes common to all individuals, three different morphological types of heterochromatic chromosomes were found in Hydrotaea meridionalis: Two acrocentric chromosomes of different length and a smaller submetacentric chromosome that occur in different combinations. The most common type, the larger of the acrocentric chromosomes, occurred as a pair in 149 out of 197 larvae. Mother‐offspring combinations revealed that the larvae progeny of several mothers carried morphologically only the common karyotype. Despite existing morphological variation in karyotypes the same karyotypes were found in male and female adult flies. This observation is interpreted by an autosomal sex‐determining mechanism. As there was also found a sex‐specific karyotype in males, a simple hypothesis to explain the observed distribution of karyotypes among sexes is suggested: one male locus located on one of the autosomes with a dominant heterogametic sex determination mechanism. Only in homozygous recessives for that locus is sex determined by the independent XX/XY sex‐determining system.

Allele frequencies at 12 allozyme loci are presented together with the karyotypes of six Hydrotaea species, H. irritans, H. meridionalis, H. velutina, H. pellucens, H. pandellei, and H. meteorica. For these six, and two further species, H. albipuncta and H. borussica, the allozyme data are used to construct phylogenetic trees. Relatedness as estimated on basis of these trees was then compared to similarity in karyotype and morphology. Conclusions based on allozymes, karyotype, and morphology, were mainly qualitatively similar. Genetic analyses and morphology coincided to the conclusion that H. irritans, H. pandellei, and H. pellucens belong to one group of closely related species, and that H. albipuncta and H. meteorica were least related to the other species analysed and also rather distinct from each other. Finally, the three species, H. meridionalis, H. borussica, and H. velutina were equally different from each other and the irritans-subgroup when judged by allozyme data, but based on karyotype and morphology, H. meridionalis and H. borussica, but not H. velutina, could be considered as part of the irritans subgroup.

Allozyme variation in the sheep headfly Hydrotaea irritans was studied on two spatial scales. Geographic variation among seven Danish and one Dutch population revealed significant but rather low genetic differentiation with FST = 0.01 over all loci. The Dutch population was on average not more different from the Danish populations than the Danish populations from each other. Allele frequencies were very skewed with the most common allele always exceeding 0.85 and usually 0.9 in frequency, but with many rare alleles at some loci. Tests for neutrality of the variation at the nine polymorphic loci revealed highly significant deviations from expected homozygosity in this species, which was not found in a comparative analysis of allozyme variation at similar loci of seven other Hydrotaea species. To explain the peculiar observed pattern of allozyme variation in H. irritans, it is suggested that this species has successfully expanded its range and spread through northern and central Europe in the recent past. Alternatively, H. irritans may have recently invaded a new niche, resulting in increased abundance of the species and subsequent dispersal to former areas of the species distribution. Zusammenfassung Allozymvariation in der Kopffliege Hydotaea irritans wurde auf 2 raumlichen Ebenen untersucht. Geographische Variation zwischen 7 danischen und einer hollandischen Population zeigte signifikante aber geringe genetische Differenzierung uber alle Genorte (FST = 0.01). Die hollandische Population war im Durchschnitt nicht mehr unterschiedlich von den danischen Populationen als die danischen Populationen untereinander. Allelhaufigkeiten waren sehr schief, wobei das haufigste Allel immer in seiner relativen Haufigkeit 0.85 und normalerweise auch 0.9 uberstieg, jedoch mit vielen seltenen Allelen an einigen Genorten. Tests hinsichtlich der Neutralitat der Variation an 9 polymorphen Genorten zeigte hochsignifikante Abweichungen von der erwarteten Homozygositat in dieser Art im Unterschied zu einer vergleichenden Analyse der Allozymvariation an ahnlichen Genorten von sieben anderen Arten innerhalb der Gattung Hydrotaea. Um dieses eigenartige Muster der Allozymvariation in der Kopffliege zu erklaren, wird vorgeschlagen, dass diese Art sich in der letzten Zeit erfolgreich durch Nord- und Zentraleuropa verbreitet hat. Alternativ konnte das gefundene Muster an genetischer Variation auch dadurch erklart werden, dass diese Art in letzter Zeit eine neue Nische gefunden hat, die zu erhohter Haufigkeit und nachfolgender Verbreitung in Gebiete der ursprunglichen Verteilung der Art gefuhrt hat.

A consequence of sewage discharge into a shallow marsh pool was large-scale hatching of the biting midge Culicoides nubeculosus from the recipient and mass attacks by this blood-sucking insect on grazing cattle. Culicoides nubeculosus is a great nuisance due to the painful bite and in Denmark it is a potential vector of pathogenic agents. In the future, these aspects of sewage discharge must be borne in mind.