The word larva refers to the growth stage of all insects with complete metamorphosis. Caterpillar refers only to a butterfly or moth in this stage. Either word is correct, but most scientists say larva. It is during this stage that monarchs do all of their growing; in fact, this is just about all that they do. These “eating machines” take few breaks even for resting. A monarch caterpillar will grow 3,000 times its birth weight size in under a month’s time.

The monarch larva (caterpillar) has five stages of development which is known as “instar.” At the end of each stage (instar) the caterpillar molts. By shedding its skin, the caterpillar continues growing. With each stage, the caterpillar becomes larger and undergoes slight changes in appearance. The larva stage is 11-18 days, depending on temperature.

1st Instar

A newly-hatched monarch larva is pale green or grayish-white, shiny, and almost translucent. It has no stripes or other markings. The head looks black, with lighter spots around the antennae and below the mouthparts, and may be wider than the body.After hatching, the larva eats its eggshell (chorion). There is a pair of dark triangular patches between the head and front tentacles which contain setae, or hairs. It then eats clusters of fine hairs on the bottom of the milkweed leaf before starting in on the leaf itself. It feeds in a circular motion, often leaving a characteristic, arc-shaped hole in the leaf.

2nd Instar

Second instar larvae have a clear pattern of black (or dark brown), and yellow and white bands, and the body no longer looks transparent and shiny. An excellent characteristic to use in distinguishing first and second instar larvae is a yellow triangle on the head and two sets of yellow bands around this central triangle. The hairs (setae) on the body are more abundant, and look shorter and more stubble-like than those on first instar larvae.

3rd Instar

The black and yellow bands on the abdomen of a third instar larva are darker and more distinct than those of the second instar, but the bands on the thorax are still indistinct. The triangular patches behind the head are gone, and have become thin lines that extend below the spiracle. The yellow triangle on the head is larger, and the yellow stripes are more visable. The first set of thoracic legs are smaller than the other two, and is closer to the head. Third instar larvae usually feed using a distinct cutting motion on leaf edges.

4th Instar

Fourth instar larvae have a distinct banding pattern on the thorax which is not present in third instars. The first pair of legs is even closer to the head, and there are white spots on the prolegs that were less conspicuous in the third instar.

5th Instar

The body pattern and colors of fifth instar larvae are even more vivid than they were in the fourth instar, and the black bands looks wider and almost velvety. The front legs look much smaller than the other two pairs, and are even closer to the head. There are distinct white dots on the prolegs, and the body looks quite plump, especially just prior to pupating.

Fifth instar monarch larvae often chew a shallow notch in the petiole of the leaf they are eating, which causes the leaf to fall into a vertical position. They move much farther and faster than other instars, and are often found far from milkweed plants as they seek a site for pupating. 

All 5 Stages of Instars