We made 28 collections of black flies (Diptera: Simuliidae) at 24 locations in central and northeastern Washington state, United States of America, and identified 10 species in three genera, including Simulium arcticum Malloch, which we studied cytogenetically. We analysed 745 larvae of S. arcticum cytogenetically from nine of the 11 sites where it occurred; five sites had small sample sizes. For the collections with large sample sizes, the distribution of S. arcticum may have a geographic pattern. Larvae in western tributaries of the Columbia River have the sex-linked IIL-2 inversion and heterozygotes for the IS-1 autosomal polymorphism in abundance but lack the IIL-21 sex-linked inversion, whereas larvae in eastern tributaries of the Columbia River possess the IIL-21 inversion but lack IIL-2 and the IS-1 inversions. A cytotype new to science, S. arcticum IIL-81, occurs in some larvae at the Methow River in the eastern Cascades region. All females, regardless of location, possess enhanced (Ce Ce) centromere bands in their IIL-chromosomes, whereas all males possess the enhanced, thin (Ce Ct) centromere band dimorphism. The Methow River had nine types of chromosomally identified males in 2019 and eight types in 2020.

我们在美国华盛顿州中部和东北部的 24 个地点收集了 28 个黑蝇（双翅目：拟蝇科），并确定了三个属的 10 个物种，包括我们通过细胞遗传学研究的Simulium arcticum Malloch。我们从 11 个发生地点中的 9 个地点对 745 只S. arcticum幼虫进行了细胞遗传学分析；五个地点的样本量较小。对于样本量较大的集合，S. arcticum的分布可能有地理模式。哥伦比亚河西部支流中的幼虫具有 IIL-2 性连锁倒位和 IS-1 常染色体多态性杂合子，但缺乏 IIL-21 性连锁倒位，而哥伦比亚河东部支流中的幼虫具有IIL-21 倒置但缺乏 IIL-2 和 IS-1 倒置。一种新的科学细胞型，S. arcticum IIL-81，出现在喀斯喀特东部 Methow 河的一些幼虫中。所有雌性，无论位置如何，在其 IIL 染色体中都具有增强的 (Ce Ce) 着丝粒带，而所有雄性都具有增强的薄 (Ce Ct) 着丝粒带二态性。梅索河在 2019 年有九种染色体鉴定的雄性，在 2020 年有八种。