Odonata (Insecta) Larvae as the Second Intermediate Hosts of the Trematodes of Genus Plagiorchis in the Basin of Chany Lake, Western Siberia

— Trematodes of the genus Plagiorchis are widespread endoparasites with a life cycle involving several hosts. The second intermediate hosts of the trematodes of genus Plagiorchis are studied for the first time in the basin of Lake Chany in the forest-steppe zone of Western Siberia, which is crossed by the migration way of many species of aquatic and near-water birds—final hosts of these trematodes. This study was conducted in 2014–2015 in the reed beds of Lake Fadikha, which is a habitat of the first intermediate hosts of plagiorchiids: snails. Invertebrates from classes Insecta, Malacostraca, and Gastropoda, as possible second intermediate hosts of the Plagiorchiidae trematodes, are studied for the prevalence and intensity of trematode infection. Metacercariae (larvae inhabiting the second intermediate hosts) of genus Plagiorchis ( P. elegans and P. multiglandularis ) are found only in the insects from order Odonata. The largest portion among infected larvae is comprised of larvae of Sympetrum vulgatum (68%), followed by S. flaveolum (18%), S. sanguineum (9%), and Aeshna serrata (5%). The prevalence of the metacercariae of the detected trematode species for the four Odonata species during the study years varied from 3.3 to 45.5%; the intensity of infection varied from 2 to 4 trematodes per 1 odonate larva. Infection with metacercariae increases with the age of odonate larvae. A trend towards a positive correlation between the infection (prevalence) of the first (snails) and the second (odonate larvae) intermediate hosts is identified. A significant relationship is identified between the prevalence of metacercariae of the odonate larvae and their population density, which varies throughout the season. Seasonal changes in the infection of odonates with metacercariae of the trematodes of genus Plagiorchis are associated with the phenology of these insects. Periods of increased infection are registered just prior to the mass emergence of odonates, when the abundance of odonate larvae in the water body is the highest, and vice versa, periods of decline in infection are registered after the mass metamorphosis of odonates.