^{(Photograph = Echniopsis pachanoi, Michael Whitehead)}

Introduction

From Wikipedia: 

Pollination syndromes are suites of flower traits that have evolved in response to natural selection imposed by different pollen vectors, which can be abiotic (wind and water) or biotic, such as birds, bats, insects, and so forth.

The Details

Cactus flowers have to work fast. They often don’t last long, a day, or 3 days at the most. They need to find pollinators to carry their pollen to another plant and they need to hurry. Also, they do not need competition from other flowers. Insects that fly all over might or might not carry their pollen to the right flowers.

One way to avoid competition for pollinators is to bloom at night. The pollinators of the night are special, often moths or bats. One way to ensure that pollinators find flowers in the dark is to give off a perfume or odor that is attractive to a certain species of moths. The pollinator will visit one plant and then another  of the same species–transferring the pollen in the process.

Another adaptation of some night-blooming flowers is to have long tubes so that pollinators must have long probosci to reach down to the nectar.  Such a long tube discourages some of the unwanted visitors that don’t have long probosci. Finally, another way to help the right pollinator is to have a large flower that faces sideways or even upwards. Pollinators like bats and large moths can easily fit into the opening of the flower and don’t have to pollinate from the uncomfortable position of being below a flower. 

Additional Reading: Candy Barrel Cactus