- This amazing information was compiled and written by the University of Minnesota
All insects change in form as they grow; this process is called metamorphosis. Butterflies and moths undergo complete metamorphosis, in which there are four distinct stages: egg, larva (caterpillar), pupa (chrysalis), and adult. It takes monarchs about a month to go through the stages from egg to adult, and it is hormones circulating within the body that trigger the changes that occur during metamorphosis. Once adults, monarchs will live another two to six weeks in the summer. Monarchs that migrate live all winter, or about six to nine months.
While butterflies and moths do not care for their young after hatching, they do lay their eggs on the appropriate host plant, which will be food for the newly hatched caterpillars. For monarchs, that’s milkweed! Monarch females usually lay a single egg on a milkweed plant, often on the bottom of a leaf near the top of the plant. Eggs are only about the size of a pinhead or pencil tip and are off-white or yellow, characterized by longitudinal ridges that run from the tip to the base. The eggs hatch about four days after they are laid.
As females lay their eggs, they secrete a small amount of glue to attach the eggs directly to the plant. Female monarch butterflies lay 300-500 eggs over two to five weeks of egg laying, with a record in captivity of 1179!
Each egg is formed inside the female prior to fertilization, including the hard outer shell, called the chorion, which protects the developing larva inside. The shell is lined with a layer of wax, which helps keep the egg from drying out. The eggs have tiny funnel-shaped openings at one end, called micropyles. These holes penetrate all the way through the shell allowing sperm to enter, since eggs form their hard shell prior to fertilization. The raised areas on the egg shell are called ridges, they are also formed before the egg is laid. The dark head of the developing caterpillar can be seen near the top of the egg prior to emergence.
Monarchs remain in the egg stage of their life-cycle for 3-5 days, depending on the temperature.
Height 1.2 mm Width 0.9 mm
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.
Larvae, just like all other insects, have three distinct body parts: the head, thorax, and abdomen. The head has a pair of short antennae, mouthparts (upper lip, mandibles, and lower lip), and six pairs of simple eyes, called ocelli. Even with all of these eyes, the caterpillar’s vision is poor. The antennae help to guide the weak-eyed caterpillar as it moves around, and the maxillary palps (sensory organs), help direct food into its jaws. The spinneret produces silk, which the caterpillar uses to anchor itself when needed and to create the silk pad it uses to hang from when it pupates.
Each thoracic segment has a pair of jointed, or true legs, while some of the abdominal segments have false legs, or prolegs. Monarchs have five pairs of prolegs. The prolegs have tiny hooks on them that hold the larva onto its silk mat or leaf. The fleshy tentacles at the front and rear ends of monarch larvae are not antennae, but they do function as sense organs.
Like other insects, monarchs obtain oxygen through holes in the sides of their thorax and abdomen called spiracles. The spiracles are connected to a network of long air tubes called tracheae, which carry oxygen throughout the body.
As the caterpillar grows and becomes too large for its skin, it molts, or sheds its skin. The head capsule is the first part of the old skin to come off during the molting process. Then the old skin peels back from the front of the caterpillar. At first, the new skin is very soft, and provides little support or protection. This new skin soon hardens and molds itself to the caterpillar.
The shed skin is often eaten before the caterpillar ingests more plant food! The intervals between molts are called instars. The entire larval stage in monarchs lasts from nine to fourteen days under normal summer temperatures.
- 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. 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.
- 2ND INSTAR
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.
- 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. 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."
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.
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.
➡️ This amazing information was compiled and written by the University of Minnesota