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Simple Summary: The pollen wasps are a fascinating group of insects that live similarly to solitary bees: females provision their brood cells with a mixture of pollen and nectar instead of insect prey. The relationships of these wasps with flowering plants are of special interest, since most species of the masarine wasps are specialized for particular groups of plants and/or flower types. Within the genus Celonites, trophic preferences are better known for the Afrotropical and the Mediterranean species, while they are nearly unknown for representatives from Central Asia. We studied two closely related species from the Altai Mountains: Celonites kozlovi and C. sibiricus. The first species is a generalist, which is not typical of the genus. Due to behavioral plasticity, it is able to use flowers of various plant species from at least five families (Asteraceae and Lamiaceae predominate) as pollen sources. The second species has specialized pollen-collecting structures that allow females to use flowers of Lamiaceae efficiently, while other plants are only occasionally visited. These adaptations are similar to those of some Mediterranean species of Celonites, but they evolved independently. The existence of two different foraging strategies by the closely related C. kozlovi and C. sibiricus leads to niche segregation and allows them to coexist in the same extreme habitats at the northernmost border of the range of the pollen wasps. Celonites kozlovi Kostylev, 1935, and C. sibiricus Gusenleitner, 2007, coexist in semi-deserts of the Altai Mountains. The trophic relationships of these pollen wasp species to flowers are largely unknown. We observed the flower visits and behaviors of wasps on flowers; pollen-collecting structures of females were studied using SEM; the taxonomic position of these two species was ascertained with the barcoding sequence of the mitochondrial COI-5P gene. Celonites kozlovi and C. sibiricus form a clade together with C. hellenicus Gusenleitner, 1997, and C. iranus Gusenleitner, 2018, within the subgenus Eucelonites Richards, 1962. Celonites kozlovi is polylectic in the narrow sense, collecting pollen from flowers of plants belonging to five families (with the predomination of Asteraceae and Lamiaceae) using diverse methods for both pollen and nectar uptake. In addition, this species is a secondary nectar robber, which has not been observed in pollen wasps before. The generalistic foraging strategy of C. kozlovi is correlated with an unspecialized pollen-collecting apparatus on the fore-tarsi. In contrast, C. sibiricus is broadly oligolectic, predominantly collecting pollen from flowers of Lamiaceae. Its specialized foraging strategy is associated with apomorphic behavioral and morphological traits, particularly specialized pollen-collecting setae on the frons, which enable indirect pollen uptake using nototribic anthers. These adaptations in C. sibiricus evolved independently of similar specializations in the Celonites abbreviatus-complex. Celonites kozlovi is re-described, and males are described for the first time. [ABSTRACT FROM AUTHOR]