Orchid pollen is not used by honeybees as a source of food – instead they are attracted mainly by the plant’s nectar.
Those orchids that have most clearly developed this adaptation have a spur at the base of their lip. This means that the insect must have an appendage in order to reach the nectar. When it does so, it brushes against the pollinia, masses of orchid pollen.
The most striking example is Angraecum sesquipedale, which drew Darwin’s attention. He even predicted that a moth would be found with a proboscis long enough to reach the nectar at the end of the spur. In fact, an insect exactly matching Darwin’s description was found many years later. This type of pollination is often described as “lock and key,” because the insect’s organ fits into the flower just as a key fits into a lock.
In the case of slipper orchids (Cypripedium, Paphiopedilum), an insect in search of nectar slips in under the column and leaves only once it has made its way along the side of the gynoecium, carrying with it pollinia stuck to its back. This step prevents it from pollinating the same flower. When it visits another flower, it will deposit the pollinia on the stigmas before leaving, taking a new load with it. This is called “trap” pollination.
The insect-orchid association is so closely established in many orchids that it can be described as “faithful” or “restrictive,” since only the type of insect capable of pollinating a particular orchid effectively will be able to visit the flower and obtain its nectar reward.
In addition, only orchids have this adaptation whereby pollen is clustered in pollinia, small hard masses easily “carried” on an insect’s back or a bird’s beak, especially when they have an adhesive disk (the viscidium).