Different types of Primrose flowers - pin-eyed and thrum-eyed

Primroses are unusual in having two almost identical, yet slightly different types of flowers. Look carefully at the centre of the flowers pictured below and you will see that in one type, there is a greenish disc (pin-eyed), while in the other (thrum), there is instead a cluster of bright yellow anthers.


Pin-eyed Primrose Flower	Thrum-eyed Primrose Flower

The petals of Primroses are joined together at the base to form a long tube. Inside this tube are the stamens, style and stigma. (For information on basic flower parts see Flower Structure). In pin-eyed flowers, the stigma is at the top of the flower tube and can be seen in the centre of the flower looking like a small green pin head. In this type of flower, the anthers are halfway down the central flower tube, in a ring around the style. In the thrum-eyed type, the style reaches only halfway up the flower tube, so that the stigma is also positioned halfway up inside the tube. The anthers in this type are at the top of the flower tube and can be seen as a yellow/orange mass in the centre of the flower.

Diagram of a pin-eyed flower cut from top to bottom (longitudinal cross section)	Diagram of a thrum-eyed flower cut from top to bottom (longitudinal cross section)
Insects visit the flowers in search of nectar, which is located at the bottom of the flower tube. This means that only long-tongued insects can actually reach the nectar in the base. An insect such as a Brimstone butterfly (Gonepteryx rhamni), visiting a pin-eyed flower, gets pollen stuck to the middle of its proboscis from the anthers half-way down the flower tube. If it then goes to visit a thrum-eyed flower, the pollen is perfectly positioned to be wiped off on the stigma, in this case, halfway down the flower tube. The reverse is also true. If the butterfly first visits a thrum-eyed flower, pollen is wiped off onto the top of its proboscis as it searches for nectar. This is then ideally placed to be transferred onto the stigma of the next pin-eyed flower which it visits. If it is spring time, why not go out and see how many Primroses you can find near you. Are the flowers thrum or pin-eyed? Can you see any insects in them? Why not watch for a while to see if you can catch a glimpse of some of the year's earliest butterflies coming to take advantage of their nectar. Do any other insects visit the flowers? Just sitting quietly and watching can often reveal fascinating things!