A little bit of Monarch biology

I have mentioned in previous posts that Monarchs migrate over 2,000 miles to Mexico. They are unique in that they are the only insect to undergo such a spectacular journey. But I have failed to give you more information. The Monarch butterfly is a FASCINATING insect. First, it transforms in a magical process called metamorphosis. It begins with the female butterfly who lays a single egg on the underside of a milkweed leaf. Females can lay 200-400 eggs during her life! And given she weighs only half a gram, or the weight of a paper clip, she will lay more eggs than her own weight!

Each egg is only the size of the head of a pin. In 3 days, the egg turns black and an itsy bitsy caterpillar hatches. Its first meal is the egg shell. After that, the Monarch caterpillar eats ONLY milkweed. It eats and eats and eats and grows over 3,000x during a two week period. It is hard to imagine that much change in such a short period. Think about this: an 8 lb child growing 3,000x in two weeks will be 24,000 lbs, or about two African Elephants. Wowsers.

Each caterpillar goes through 5 instars, or it sheds its skin 5 times. Shedding skin allows the caterpillar to keep growing!

Once the caterpillar decides it is big enough, it forms a little silken knot and hangs in a “J”. The skin splits open and the caterpillar wriggles into a shiny green chrysalis, complete with gold spots.

It hangs out in the chrysalis stage for a bit (9-12 days) before the chrysalis becomes clear and you can see the dark black and orange wings. The butterfly emerges in less than 30 mins and hangs from the chrysalis, pumping fluid through the veins of its wings.

And that whole life cycle takes ~30 days, depending on temperature. This whole process is mind-blowing. It is as crazy to me as if we humans started out as a fish, became a sloth for a bit, and then finally ended as humans.

But the Monarchs are even more incredible! In one year, there are on average 5 generations of Monarchs. The Monarchs we see flying around the point today are what I like to call the “Super” generation. They are living 8-9 months. The “normal” generation lives ~6 weeks. The “Super” generation migrates to Mexico every fall where it overwinters in the Oyamel fir trees of the Transvolcanic Mountains at 10,000 – 12,000 feet elevation. In the spring, this “Super” generation (the same butterflies we see flying around Cape May today) migrate from Mexico to the Gulf Coast where the first wave of milkweed is popping up. These Monarchs will finally mate and reproduce and then die. Their offspring will migrate farther north, mate and reproduce and then die. This continues so that we witness about four generations of Monarchs during the spring/summer months. But it is always this last generation during September and October that begins the migration to Mexico. Why do they migrate? How do they know to get there? This butterfly has never been to Mexico before! Perhaps it is a variety of environmental cues—angle of the sun, dying milkweed and nectar, colder nights—that signal Monarchs it is time to leave Canada and the US and head south. It is incredible that such a tiny creature, weighing only half a gram, with a brain the size of a poppy seed, can undertake such a journey.

Photos by Michael O'Brien