Snapdragons have a specialist pollination strategy. Their corolla tube, which contains nectar and pollen, is closed by two "lips" which prevent most insects from entering and stealing their nectar. Only bumblebees – their preferred pollinators – and a few other insects can trip the trapdoor mechanism and gain access to the tube.
I've spent more time than I care to admit watching honey bees trying and (and failing) to enter our snapdragons.
And here is a common carder bee – a type of bumblebee – showing you how it's done:
