The above two monarchs are attempting to mate. You can see the claspers on the end of the one male's abdomen. However, they will eventually find out this does not work., lol.
While the black dots on this male should have pheromones that would alert the other male, it does not always seem to turn off the need to reproduce if at all possible. This is a different set of males and below is another set of males.
I love how these wound up stacked on top of each other. It seemed that most of these that were attempting to mate by landing on one already in the trees, were males. Typically i will see the male knock the female down out of the air onto the ground and then a wrestling match will ensue. If the male is able, he will subdue the female and clasp onto her. He will turn her wings up in the folded position between his wings. Next he will slowly take off and fly up to the trees where they will stay joined for about 16 hours.