Darwin hypothesized that some species of Hydrangea (Hydrangeaceae) and Viburnum (Adoxaceae) enhance their pollination attracting-function by sacrificing the fertility of few flowers. This study reports, by using intact and treated (sterile flowers were removed and mounted to inflorescences which had and lacked them, respectively) samples, how the presence of sterile flowers in seven species of Hydrangea and three species of Viburnum enhances the attraction of their inflorescences. Insect visit observations revealed that halictid bees and syrphid flies are main visitors for most studied plant species (average total visit percentage of 43.3% and 24.8%, respectively). Every studied plant species showed higher visit rates for inflorescences with sterile flowers; among the 8 of them with sterile flowers in nature, only 4 showed significant higher visit rate. Most of the main visitors showed preference for samples with sterile flowers, however significant only for some bee species and 1 syrphid fly specie. Seven plant species showed higher staying time for samples with sterile flowers, however significant only for 1 species. Results suggest that sterile flowers increase the visits to the inflorescences of Hydrangea spp. and Viburnum spp., but not the staying time of floral visitors.