The role of pollinators as drivers of morphological diversification has been extensively documented. However, how non-pollinators, such as pollen thieves and nectar robbers, affect flower morphology has received little attention. In this study, we used five species of Penstemon to investigate the relationship between pollinators and non-pollinators, and the flower morphology of the plants they visit. The results suggest that smaller flowers are more likely to be predominantly visited by small and very small pollen-collecting bees, and that larger flowers are more likely to be visited by large bees. Very small and small pollen-collecting bees rarely contacted the stigma and can thus be characterized as illegitimate visitors. However, in the absence of a more efficient pollen vector, these visitors can act as occasional pollinators. On the other hand, nectar robbing by wasps was not correlated with flower size but was associated with premature wilting. Pollinator assemblages varied geographically, but the presence and frequency of non-pollinator visitors did not, and instead they were species-specific. Finally, our results suggest that non-pollinator visitors have a detrimental effect on flower appearance from the pollinators' perspective.

传粉媒介作为形态多样化驱动因素的作用已被广泛记录。然而，非授粉者，如花粉窃贼和花蜜窃贼如何影响花朵形态却很少受到关注。在这项研究中，我们使用五种Penstemon来研究传粉媒介和非传粉媒介之间的关系，以及它们访问的植物的花朵形态。结果表明，较小的花朵更有可能主要被小型和非常小的花粉采集蜜蜂拜访，而较大的花朵更有可能被大型蜜蜂拜访。非常小的和小型的花粉采集蜜蜂很少接触柱头，因此可以被定性为非法访客。然而，在缺乏更有效的花粉载体的情况下，这些访客可以充当偶尔的传粉者。另一方面，黄蜂抢夺花蜜与花朵大小无关，但与过早枯萎有关。传粉媒介组合因地理位置而异，但非传粉媒介访客的存在和频率却没有变化，相反，它们具有物种特异性。最后，我们的结果表明，从传粉媒介的角度来看，非传粉媒介访客对花朵外观有不利影响。