Guide to Monarch Instars

1st Instar

1st instar (Kristen Kuda)

Body Length 2 to 6 mm
Body width 0.5 to 1.5 mm
Front Tentacles Small bumps
Back tentacles Barely visible
Head Capsule 0.6 mm in diameter

 

Appearance

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. There is a pair of dark triangular patches between the head and front tentacles which contain setae, or hairs. The body is covered with sparse setae. Older first instar larvae have dark stripes on a greenish background.

After hatching, the larva eats its eggshell (chorion). 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. First (and second) instar larvae often respond to disturbance by dropping off the leaf on a silk thread, and hanging suspended in the air. Time in this larval stage is usually 1-3 days, temperature dependent.

Image: One first instar larva.

2nd Instar

2nd instar (Kristen Kuda)

Body Length 6 to 9 mm
Body Width 1 to 2 mm
Front Tentacles 0.3 mm
Back Tentacles Small knobs
Head Capsule 0.8 mm in diameter

 

Appearance

Second instar larvae have a clear pattern of black (or dark brown), 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 triangular spots behing the head do not have the long setae present in the spots on the first instar larvae. The setae on the body are more abundant, and look shorter and more stubble-like than those on first instar larvae. Time in this larval stage is usually 1-3 days, temperature dependent.

Image: Two second instar larvae. 

3rd Instar

3rd ​instar (Kristen Kuda)

Body Length 10 to 14 mm
Body Width 2 to 3.5 mm
Front Tentacles 1.7 mm
Back Tentacles 0.9 mm
Head Capsule 1.5 mm in diameter

 

Appearance

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. Time in this larval stage is usually 1-3 days, temperature dependent.

Third instar larvae usually feed using a distinct cutting motion on leaf edges. Unlike first and second instar larvae, third (and later) instars respond to disturbance by dropping off the leaf and curling into a tight ball. Monarch biologist Fred Urquhart called this behavior "playing possum."

4th Instar

4th instar (Kristen Kuda)
 

Body Length 13 to 25 mm
Body Width 2.5 to 5 mm
Front Tentacles 5 mm
Back Tentacles  2 mm
Head Capsule 2.2 mm in diameter

 

Appearance

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.

Male and female larvae can't be distinguished by the naked eye until the pupal stage. However, male and female respective reproductive organs are visible in dissected, third, fourth, and fifth instars. Time in this larval stage is usually 1-3 days, temperature dependent.

5th Instar

5th instar (Kristen Kuda)

Body Length 25 to 45 mm
Body Width 5 to 8 mm
Front Tentacles 11 mm
Back Tentacles 4 mm
Head Capsule 3.5 mm in diameter

 

Appearance

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. Time in this larval stage is usually 3-5 days, temperature dependent.

Left: 5th instar on milkweed leaf

Right: 5th instar larvae can vary greatly in size; both of these larvae are 5th instars. (Photo: Kip Kiphart)