This pair of monarchs (male on the right and female on the left) is mating on a Lantana (Lantana camara). This is the condition sine qua non to initiate the life cycle of a new generation of monarchs. Male and female back to back put into contact the tips of the their abdomens, where the copulatory organs are. They function like a key-lock system and must match perfectly. Otherwise, the process of recognition, where pheromones and sounds have already proved effective, is interrupted. They remain attached in this position for a very long time till the sperm transference is finished. If they are bothered they move somewhere else without uncoupling.
Monarchs like all butterflies and many other insects are holometabolic. That is, to become a butterfly they undergo four phases: egg, caterpillar, pupa or chrysalis and finally adult or imago, the butterfly.