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Eriophyoidea (gall mites) are a megadiverse lineage of worm-like mites that feed on vascular plants. The setae of these mites are sometimes distinctive because of their atypical shape, either bifurcated, angled or swollen, but never many-branched. Our study of eriophyoid setae revealed that bifurcated and angled setae are widely distributed across Eriophyoidea. The group of worm-like soil mites (Nematalycidae) with which they are affiliated have bifurcated and trifurcated setae. The plesiomorphic and also most common state of all mites is hyper-furcating setae (more than three branches), which are almost always represented in rows (seri-furcating). The likely explanation for the filiform and unbranched setae of Eriophyoidea is the gradual loss of branches, one by one, with the bifurcated state that is shared with Nematalycidae being an ancestral state. Angled setae are an intermediary state because they probably represent a bifurcating seta with only a single branch; the other one is completely diminished. Accordingly, hypo-furcating setae (three or fewer branches) are a synapomorphy that unites Eriophyoidea with Nematalycidae. Our phylogenetic analyses also showed that Leipothrix, the largest genus with a bifurcated seta on the palps, is monophyletic once Cereusacarus juniperensis is excluded and five species of Epitrimerus have been transferred into this genus. The setae in Eriophyoidea are filiform, slightly bent and thickened near the base. Confocal microscopy indicates that their proximal and distal parts differ in light reflection and autofluorescence. Approximately 50 genera have atypically shaped setae: bifurcated, angled or swollen. These modifications are known in the basal part of prosomal setae u′, ft′, ft″, d, v, bv, ve, sc and caudal setae h2. We assessed the distribution of atypically shaped setae in Eriophyoidea and showed that they are scattered in different phylogenetic lineages. We hypothesized that the ancestral setae of eriophyoid mites were bifurcated before later simplifying into filiform setae. We also proposed that hypo-furcating setae are a synapomorphy that unites Eriophyoidea with Nematalycidae. We analyzed four new mitochondrial genomes of Leipothrix, the largest genus with bifurcated d, and showed that it is monophyletic and has a unique mitochondrial gene order with translocated trnK. We exclude Cereusacarus juniperensisn. comb. Xue and Yin, 2020 from Leipothrix and transfer five Epitrimerus spp. to Leipothrix: L. aegopodii (Liro 1941) n. comb., L. femoralis (Liro 1941) n. comb., L. geranii (Liro 1941) n. comb., L. ranunculi (Liro 1941) n. comb., and L. triquetra (Meyer 1990) n. comb.