There are two kinds
of generations of monarchs...the spring/summer reproductive generations (4-5 of these in Kansas from about May-September) and the migratory generation (only one per year).
The entire life cycle of a monarch from egg to adult is about one month and during spring/summer as adults monarchs live about three-six weeks, which is pretty standard for most insects at this time. In the fall a single migratory generation emerges across the eastern U.S. and into Canada that will make the migration to the overwintering sites in central Mexico west of Mexico City. Some of these butterflies will travel over 2,000 miles and it will take them a lot longer than a few weeks to get to their destination. It turns out that these migratory monarchs may live up to seven-nine months. Wow!
So how do they do it? Well, for starters they are non-reproductive (mating is very energetically expensive or such a small organism). Also, most of their flight during the migration is not powered; that is, they are not actively flapping their wings. Rather, they are doing a lot of gliding and soaring, riding thermals (columns of warm rising air) to climb to an altitide of sometimes several miles and then gliding long distances - just like some birds and glider pilots do.
The location of the overwintering sites offers the perfect microclimate for the monarchs - with temperatures dipping down into the 30s at night and warming up to the 70s or 80s during the day. Millions of monarch butterfllies spend much of their time clustered in the trees, remaining relatively inactive until warming up enough during the day to leave the roosts in search of water and/or nectar. Then it's back to the roosts in the evening.
So if these migratory monarchs are non-reproductive, how do we get new monarchs in the spring, you ask? Good question!
Right around Valentine's Day oddly enough the monarchs become reproductive and begin mating as they leave the roosting areas to head back north. The females will then lay their eggs on milkweeds in the southern part of the U.S. before they die. These offspring will mature into adult butterflies that will continue to move north (reproducing along the way) to eventually repopulate the rest of the continent - all the way up to the northern limit of milkweed in southern Canada.
So, whereas the migration south consists of a single generation, the return trip will take several generations to complete. You can think of the monarchs that make the trip to Mexico then as the great-great-great-grandchildren of the monarchs that were there the year before. There may be one too many or too few "greats" in there but you get the picture - the migratory monarchs are separated by several generations, making their annual trek to the same locations year after year all the more amazing.