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Monarch Life Cycle

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.

Egg
Larva
Pupa
Adult

Egg

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 an average of about 700 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. 

Height 1.2 mm
Width 0.9 mm

Larva

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.

5th instar monarch caterpillar on common milkweed (Asclepias syriaca) (Photo: Carrie Benham)

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. If you'd like to distinguish between the different larval instars, we have developed A Field Guide to Monarch Caterpillars. You can order this field guide from our store or use the following information on each instar:

1st Instar

1st Instar

Specifications

1st Instar body and head (x20) (Photo: 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.

2nd Instar

2nd Instar

Specifications

2nd Instar Body and Head (x12.5) (Photo: 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.

3rd Instar

3rd Instar

Specifications

3rd instar (Photo: 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.

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

Specifications

4th instar (Photo: 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.

5th Instar

5th Instar

Specifications

5th instar monarch larva (Photo: 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.

Pupa

Just before they pupate, monarch larvae spin a silk mat from which they hang upside down. The silk comes from the spinneret on the bottom of the head. As it sheds its skin for the last time, the caterpillar stabs a stem into the silk pad to hang. This stem extends from its rear end and is called the cremaster.

Fresh monarch pupa (Photo: Shaun Bevins)

While the process of complete metamorphosis looks like four very distinct stages, continuous changes actually occur within the larva. The wings and other adult organs develop from tiny clusters of cells already present in the larva, and by the time the larva pupates, the major changes to the adult form have already begun. During the pupal stage this transformation is completed. Many moth caterpillars (but not all) spin a silken cocoon to protect them as pupae. Butterflies do not do this, and their pupa stage is often called a chrysalis. While it is fine to refer to the previous stage as either larva or caterpillar, it is not correct to call a butterfly pupa a cocoon, since it does not have a silken covering.

Just before the monarchs emerge, their black, orange, and white wing patterns are visible through the pupa covering. This is not because the pupa becomes transparent; it is because the pigmentation on the scales only develops at the very end of the pupa stage. This stage of development lasts ten to fourteen days under normal summer conditions.

Adult

Monarch mating pair. The male grasps the female and they stay paired for many hours, sometimes overnight. (Photo: Wendy Caldwell)

The primary job of the adult stage is to reproduce—to mate and lay the eggs that will become the next generation. Monarchs do not mate until they are three to eight days old. When they mate they remain together from one afternoon until early the next morning—often up to 16 hours! Females begin laying eggs immediately after their first mating, and both sexes can mate several times during their lives. Adults in summer generations live from two to five weeks.

Each year, the final generation of monarchs, adults that emerge in late summer and early fall, has an additional job. They migrate to overwintering grounds, either in central Mexico for eastern monarchs or in California for western monarchs. Here they spend the winter clustered in trees until weather and temperature conditions allow them to return to their breeding grounds. These adults can live up to nine months.

Male and female monarchs can be distinguished easily. Males have a black spot on a vein on each hind wing that is not present on the female. These spots are made of specialized scales which produce a chemical used during courtship in many species of butterflies and moths, although such a chemical does not seem to be important in monarch courtship. The ends of the abdomens are also shaped differently in males and females, and females often look darker than males and have wider veins on their wings.

The body of an adult butterfly is divided into the same major parts as the larva: head, thorax, and abdomen. There are four main structures on the adult head: eyes,antennae, palpi, and proboscis. A butterfly’s relatively enormous compound eyes are made up of thousands of ommatidia, each of which senses light and images. The two antennae and the two palpi, which are densely covered with scales, sense molecules in the air and gives butterflies a sense of smell. The straw-like proboscis is the butterfly’s tongue, through which it sucks nectar and water for nourishment. When not in use, the butterfly curls up its proboscis.

The thorax is made up of three segments, each of which has a pair of legs attached to it. The second and third segments also have a pair of wings attached to them. The legs end in tarsi (singular, tarsus), which grip vegetation and flowers when the butterfly lands on a plant. Organs on the back of the tarsi "taste" sweet liquids. Monarchs and other nymphalid butterflies look like they only have four legs because the two front legs are tiny and curl up next to the thorax.

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