Monarch caterpillar instars

After hatching from an egg, monarch caterpillars live about 10-14 days.

(Read about the monarch butterfly life cycle here)

However, a lot happens in that time.

The larval stage of a monarch is called a caterpillar. When the caterpillar first hatches, it is 2-6 mm long. As it grows, it molts — sheds its skin — and ends up to be 25-45 mm when it is full grown. The time between molts are called instars, and monarch caterpillars go through several different instars, because they grow 2,700 times larger between hatching from an egg before turning into a chrysalis.

During the larval stage, caterpillars eat nothing but milkweed leaves. As milkweed is the monarch host plant, it is the only plant that its caterpillars will eat.

First instar

In the first stage of life as a caterpillar, called the first instar, the larva is 2-6 mm long and does not have its characteristic stripes right awa. Monarch caterpillars begin a pale green or grayish-white color with a black head. By the end of the first instar, dark stripes begin to appear.

When startled, caterpillars at this stage might drop off of a milkweed leaf on a silk thread, suspending themselves in mid-air.

This instar lasts one-three days, dependent on temperature.

Second instar

After its first molt, a monarch caterpillar enters the second instar and grow 6-9 mm long. This is the point when you will see the typical striped pattern of black, yellow and white bands.

This instar lasts one-three days, dependent on temperature.

Third instar

The caterpillar again molts and grows to 10-14 mm long in its third stage. The first set of thoracic legs are smaller than the other two at this stage and are closed to the head.

In the third instar, the caterpillar changes the way it reacts when startled. At this point, it will respond by dropping off of a milkweed leaf and curling into a tight ball.

This instar lasts one-three days, dependent on temperature.

Fourth instar

The caterpillar molts and grows to 13-25 mm in this stage. The first pair of legs is now closer to the head, and the prolegs farther back on the body have white spots that were not noticeable before.

This stage lasts one-three days, dependent on temperature.

Fifth instar

The final molt happens, and then the caterpillar grows to its final length of 25-45 mm long. The pattern and color of the caterpillar are most vivid at this stage, and the front legs look much smaller compared to the others.

Time in this instar is a little bit longer at three-five days, dependent on temperature.

Monarch caterpillar