The divergent sympatric mitochondrial lineages within traditionally recognized species present a challenge regularly faced by taxonomists and evolutionary biologists. We encountered this problem when studying the Siberian geometrid moths, Alcis deversata and Thalera chlorosaria. Within each of these species we found two deeply diverged mitochondrial lineages that demonstrated a level of genetic differentiation exceeding the standard interspecific DNA barcode threshold. Using analyses of nuclear genes, morphology, ecological preferences and Wolbachia endosymbionts, we tested five hypotheses that might explain the mitochondrial pattern observed: cryptic speciation, ancestral polymorphism, interspecific mitochondrial introgression, secondary admixture of allopatrically evolved populations and an effect of intracellular Wolbachia endosymbionts. We demonstrate that in A. deversata and Th. chlorosaria the mitochondrial differences are not correlated with differences in nuclear genes, morphology, ecology and Wolbachia infection status, thus not supporting the hypothesis of cryptic species and an effect of Wolbachia. Mitochondrial introgression can lead to a situation in which one species has both its own mitochondrial lineage and the lineage obtained from another species. We found this situation in the species pair Alcis repandata and Alcis extinctaria. We conclude that the mitochondrial heterogeneity in A. deversata and Th. chlorosaria is most likely to be attributable to the secondary admixture of allopatrically evolved populations.