The flowers of Arizona queen of the night (aka night blooming cereus) last one night before fading away. Perhaps more amazing, the individuals within a population of plants bloom synchronously on less than 5 nights per year and the flowers are self-incompatible. How, then, do the plants get pollinated given the narrow window of time? A few scenarios are possible. Perhaps a generalist pollinator species could visit the flowers, but presumably a generalist would have a lot to choose from and it would be quite hit-and-miss that they'd visit these flowers often enough (and I suspect most generalists are not active at night). Perhaps a specialist insectoid pollinator species emerges at the same time as Peniocereus blooms. That hasn't been shown to be the case.
The question of the floral biology of Peniocereus greggii was examined by Dr. Robert Raguso and a team of researchers (see Raguso, RA et al.. 2003. Trumpet Flowers of the Sonoran Desert: Floral Biology of Peniocereus Cacti and Sacred Datura. International Journal of Plant Sciences. 164:877-892). The researchers discovered that Peniocereus greggii and the often-growing-nearby and spring-to-fall blooming Datura wrightii (see a related species) shared a few things in common, including pollinators (hawk moths at dusk, bees in the morning), pollination reward, (lack of) UV reflectance and a few benzenoid compounds (components of floral scent). In other words, the pollination of Peniocereus greggii seems to rely on the presence of Datura wrightii to sustain a population of shared hawk moth pollinators that is both specialist enough and in sufficient numbers to visit its flowers when it is in bloom (as for the bees, recall the suggestion that generalists visit many plants and the chances of cross-pollination are reduced).
Read a factsheet about Peniocereus greggii via New Mexico Rare Plants. Two other photographs are available to be viewed via Wildflowers of Tucson, Arizona. Lastly, a legendary tale and a curatorial anecdote offer two different perspectives on the plant via the site of Tohono Chul Park.