Primrose plants are distylous, bearing flowers of either pin or thrum form. The illustration below shows details of the pin (A) and thrum (B) morphs. Legend (1) corolla, (2) calyx, (3) stamens, (4) pistil.
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Heterostyly was extensively studied by Charles Darwin (more details here) who found that the plants pollinated by the opposite type of flower had much greater reproductive success. He wrote in 1876 that "I do not think anything in my scientific life has given me so much satisfaction as making out the meaning of the structure of heterostyled flowers". This engraving was used to illustrate his book The Different Forms of Flowers of the Same Species.
Here is a pin form in the garden.
And here is a thrum form.
One thing which puzzled Darwin and has puzzled many observers since is the apparent lack of insects visiting primrose flowers - so who or what does the pollinating? The pin and thrum design is thought to discourage self-pollination and encourage cross-pollination so there must be an external agent. F.E. Weiss wrote in his Observations on the Pollination of the Primrose in 1903 that the commonest pollinating insect was Bombylius, or bee-fly, aided by various bees (Anthophora, Andrena and occasional bumblebees). Because the primrose has such a long narrow flower and the nectaries are right at the base, whatever is pollinating it must have a fairly long tongue or proboscis (that rules out honey bees, some solitary bees and some bumblebees). Butterflies and moths are also both said to visit primroses. I'm going to keep a closer eye on mine this spring to see which insects visit them.