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The genus Actenosigynes includes two species, A. fulvoniger (Michener, 1989) and A. mantiqueirensis Silveira, 2009, both oligolectic on flowers of Blumenbachia (Loasaceae) in southern Brazil. We describe a third species, Actenosigynes silveirai Siriani-Oliveira, sp. n., and provide additional evidence to the suspected narrow host-plant specificity between bees of this genus and Loasaceae. This new species was only recorded to collect resources on flowers of Aosa, a genus closely related to Blumenbachia in the subfamily Loasoideae. We illustrate female and male specimens of the three species to offer a complete summary of the morphological variation within this modestly sized genus of Neopasiphaeinae, including photographs of male genitalia and associated metasomal sterna. Moreover, we provide an identification key for the three species of Actenosigynes and the first phylogenetic and dating estimate for these taxa. The genus diversified in southern South America during the Miocene-Pliocene, following a more ancient divergence associated with the orogenic events that separated its sister-genus, Torocolletes, west of the Andes. We dedicate this newly described species to Fernando A. Silveira for his contributions to research on Brazilian bee taxonomy and biology.

Analyses of resource presentation, floral morphology and pollinator behaviour are essential for understanding specialised plant-pollinator systems. We investigated whether foraging by individual bee pollinators fits the floral morphology and functioning of Blumenbachia insignis, whose flowers are characterised by a nectar scale-staminode complex and pollen release by thigmonastic stamen movements. We described pollen and nectar presentation, analysed the breeding system and the foraging strategy of bee pollinators. We determined the nectar production pattern and documented variations in the longevity of floral phases and stigmatic pollen loads of pollinator-visited and unvisited flowers. Bicolletes indigoticus (Colletidae) was the sole pollinator with females revisiting flowers in staminate and pistillate phases at short intervals, guaranteeing cross-pollen flow. Nectar stored in the nectar scale-staminode complex had a high sugar concentration and was produced continuously in minute amounts (~0.09 μl·h -1 ). Pushing the scales outward, bees took up nectar, triggering stamen movements and accelerating pollen presentation. Experimental simulation of this nectar uptake increased the number of moved stamens per hour by a factor of four. Flowers visited by pollinators received six-fold more pollen on the stigma than unvisited flowers, had shortened staminate and pistillate phases and increased fruit and seed set. Flower handling and foraging by Bicolletes indigoticus were consonant with the complex flower morphology and functioning of Blumenbachia insignis. Continuous nectar production in minute quantities but at high sugar concentration influences the pollen foraging of the bees. Partitioning of resources lead to absolute flower fidelity and stereotyped foraging behaviour by the sole effective oligolectic bee pollinator., (© 2019 German Society for Plant Sciences, Royal Botanical Society of the Netherlands.)