The monarch life cycle

Laying the egg

A female monarch lays her egg on a milkweed (Asclepias) leaf. (The eggs are tiny and creamy white. You’re most likely to find one on the underside of a leaf.)

Mother monarch: A mother monarch lays her egg.
A mother monarch lays her egg.
Courtesy dmills @ www.flickr.com

Three to five days later, the caterpillar hatches.

A monarch caterpillar transforms five times, going through five instars, or stages of development, on its way to becoming a butterfly. The caterpillar rests and molts, or sheds its skin (which it often eats), at the end of each instar.


The first instar

In its first instar, the caterpillar is seriously tiny: about 1/8” long. It has a black head, and it’s tough to see the characteristic black, white, and yellow stripes. It eats its eggshell, and then a teeny round hole in the leaf.

Egg and hatchling: An egg and a first instar caterpillar.
An egg and a first instar caterpillar.
Courtesy dubh @ www.flickr.com

The second instar

A second instar caterpillar is still only 1/4” long, but you can clearly see the stripes. In this stage, the caterpillar begins to sprout two pairs of antenna-like tentacles.

Second instar: A second instar caterpillar, eating away.
A second instar caterpillar, eating away.
Courtesy eclectic shoes @ www.flickr.com

The third instar

Third instar caterpillars grow to about 5/8” long and are brightly colored, with longer tentacles.

Third instar: A third instar caterpillar.
A third instar caterpillar.
Courtesy eclectic shoes @ www.flickr.com

The fourth instar

A fourth instar caterpillar is about 1” long, with long tentacles, and a small white dot on each of its prolegs, or fake legs. (Remember that caterpillars are insects, and, like all insects, have only six true legs.) These guys are the proverbial very hungry caterpillars, eating as much as a leaf an hour!

Fourth Instar: A fourth instar caterpillar fattens up.
A fourth instar caterpillar fattens up.
Courtesy Stonebird @ www.flickr.com

The fifth instar

A fifth instar caterpillar is on the move, searching for a place to pupate.

Fifth Instar: A caterpillar in the fifth instar prepares to pupate.
A caterpillar in the fifth instar prepares to pupate.
Courtesy benmcleod @ www.flickr.com

The monarch chrysalis

When it finds a good spot, it hangs upside down, curls its body into a “J” shape, and molts for the last time. Only instead of molting into a new variation on the caterpillar theme, a fifth instar caterpillar becomes…

Pupa: A monarch chrysalis.
A monarch chrysalis.
Courtesy tiswango @ www.flickr.com

…a chrysalis, or pupa.


Rapid change

The change from caterpillar to pupa can happen incredibly fast. Check out this video of a caterpillar forming its pupa!

Inside the chrysalis, the caterpillar has dissolved into goo and its body is reorganizing to become a butterfly. This process takes 10 – 14 days, but depends on temperature—the cooler it is, the longer it takes.

When the adult butterfly is ready to emerge, the chrysalis fades from green to black and you can see the pattern of the wings through it. The chrysalis splits open and the butterfly wriggles out. Its new wings are soft and need a few hours to dry and harden before it can fly. Females begin laying eggs immediately after their first matings, and both sexes mate several times during their lives. Adults that emerge in the summer live for two to five weeks. But the last generation of monarchs to hatch, at the end of the summer or the start of the fall, has to migrate to their overwintering grounds in central Mexico or California. These adults can live as long as nine months!