THE FERTILISATION OF THE BUCKET ORCHID

This plant is fertilized by an amazing relationship with small insects etc. The insect is attracted to the plant, falls into the bucket, climbs out through a narrow passage where it is held at the end while two sacs are stuck on its back. When the glue is dry it flies off to another Bucket Orchid that it finds attractive. As it emerges through the narrow passage, this time two hooks neatly pull the sacs off its back and the plant has cross fertilised with another orchid!
How could such a sequence of events have come about by a process of many separate stages arising by pure chance over millions of years?