Monarch: 3rd thru 5th instar
Third Instar
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.
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
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
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.