Pin-eyed and thrum-eyed primroses.

In the pin-eyed flowers, the stigma is at the top and the anthers, which carry the pollen, are half-way down the flower tube;  while in the thrum-eyed ones, it is the anthers that are at the top and the stigma is lower down the top of an insect’s proboscis, and this is then deposited onto the stigma of a pin-eyed flower..
When a butterfly visits a pin-eyed flower to feed on the nectar, it gets pollen stuck on the middle of its proboscis from the anthers that are half-way down the tube.  Then, when it visits a thrum-eyed flower, the pollen is perfectly placed to be wiped onto the stigma.  This also happens the other way around:   pollen from the anthers of a thrum-eyed flower sticks to