RESUMEN
Species of the Simulium varicorne group in Thailand have veterinary significance as vectors of haemosporidian parasites. Accurate identification is, therefore, critical to the study of vectors and parasites. We used morphology and molecular markers to investigate cryptic genetic lineages in samples identified as Simulium chumpornense Takaoka & Kuvangkadilok, 2000. We also tested the efficiency of the nuclear internal transcribed spacer 2 (ITS2) marker for the identification of species in this group. Morphological examinations revealed that S. chumpornense lineage A is most similar to S. khelangense Takaoka, Srisuka & Saeung, 2022, with minor morphological differences. They are also genetically similar based on mitochondrial cytochrome c oxidase I (COI) sequences. Geographically, the sampling site where paratypes of S. khelangense were originally collected is <50 km from where S. chumpornense lineage A was collected. We concluded that cryptic lineage A of S. chumpornense is actually S. khelangense. COI sequences could not differentiate S. kuvangkadilokae Pramual and Tangkawanit, 2008 from S. chumpornense and S. khelangense. In contrast, ITS2 sequences provided perfect accuracy in the identification of these species. Molecular analyses of the blood protozoa Leucocytozoon and Trypanosoma demonstrated that S. khelangense carries L. shoutedeni, Leucocytozoon sp., and Trypanosoma avium. The Leucocytozoon sp. in S. khelangense differs genetically from that in S. asakoae Takaoka & Davies, 1995, signaling the possibility of vector-parasite specificity.