Monarch: 3rd thru 5th instar
Third Instar Head (x6)
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

Body (x6) 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.

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."


Fourth Instar
Head (x5) 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.


Fifth Instar
Head (x2.5) 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 look 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.

MLMP Egg thru 2nd instar