Monarchs, like other butterflies and moths, undergo complete metamorphosis, meaning that they have an egg, larva (caterpillar), pupa (chrysalis), and adult stage. The egg and caterpillar stages occur only on species of milkweed (genus Asclepias), whereas adults survive by nectaring on a variety of flowering plants.
Female monarchs only lay eggs on milkweed plants since monarch caterpillars only eat milkweed. The milkweed plant provides both food and shelter for a caterpillar for approximately two weeks (dependent on temperature) while it eats almost constantly, pausing only to shed its skin. The period between each shedding of the skin, or molt, is called an instar. Monarchs have five larval instars and grow to almost 2,000 times their original mass.
A late fifth instar monarch will generally crawl away from the milkweed plant it was feeding on to find a secure location where it forms a silk pad and hangs upside down in a J shape before shedding its skin one last time to expose the bright green chrysalis. In 8 to 15 days, an adult emerges, pumps fluid to its wings to give them shape, and spends several hours drying before it is ready to venture off to find nectar or a mate.
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 300-500 eggs over two to five weeks of egg laying, with a record in captivity of 1,179!
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 eggshell 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: 1.2mm Width: 0.9mm
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. The entire larval stage in monarchs lasts from nine to fourteen days under normal summer temperatures.
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.
Guide to Monarch Instars
1st Instar
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
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
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
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 behind 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
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
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 visible. The first set of thoracic legs is 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
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
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
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
The body pattern and colors of the 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. Time in this larval stage is usually 3-5 days, temperature dependent.
Pupa
Just before they pupate, monarch larvae spin a silk mat from which they hang upside down by their last pair of prolegs. 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.
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 eight to fifteen days under normal summer conditions.
Adult
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.
Photos (left to right): female monarch abdomen, female monarch wings, male monarch abdomen, male monarch 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 give 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.
Photos (left to right): monarch compound eyes (Michelle Solensky), adult tarsus (Michelle Solensky), monarch proboscis (Sonia Altizer), monarch wings scales (Michelle Solensky)
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.
Photos (left to right): Monarch aposematic coloration, Monarch nectaring, Monarch on coneflower
Annual Life Cycle
The monarchs that spend the winter in the mountains of central Mexico are the final generation of a cycle that begins anew each year. Most of the butterflies in this final generation begin their lives in the northern US or southern Canada and then migrate thousands of kilometers to mountaintops that neither they nor their parents (and likely their grandparents) have ever seen before. After spending several months in Mexico, they return north beginning in March, starting the cycle again as they lay eggs in northern Mexico and the southern US. Their parents, grandparents, and great-grandparents have very different lives. These summer monarchs live only about a month as adults and begin laying eggs when they are only a few days old.
In most years, the total number of monarchs probably increases with each generation. Because the winter generation must live for such a long time before reproducing, the entire population shrinks as some of these individuals die during the fall migration and overwintering period. The population then grows over the spring and summer.
Table 1: Summary of Monarch Annual Life Cycle
| Generation # | Timing of immature stages* | Timing of adult stages* | Migrates? | Overwinter? |
| 1 | March-May | April-June | Yes, north in spring | No |
| 2 | May-July | June-July | North, through early June | No |
| 3 | July-August | July-August | Some movement south | Some |
| 4 | July-October | August-April | Yes, south in fall and north in spring | Yes |
*Months during which each generation exists.
Information on the monarch migration are taken from papers by S.B. Malcolm, B.J. Cockrell, and L.P. Brower and from data collected by the Journey North website.
Generation 1
Monarchs in Generation 1 are the offspring of the monarchs that overwinter. They are laid from late March through April in the southern United States and northern Mexico and fly north as adults. They do not undergo reproductive diapause.
Generation 1 Life Cycle Summary
The first monarch generation of the year begins when females who have spent the winter in Mexico lay eggs in northern Mexico and the southern U.S., beginning in late March. The last eggs are laid in late April or early May, farther north. Since it is often cool when Generation 1 larvae are developing, it may take them up to 40 or 50 days, or even more, to develop from eggs to adults.
Generation 1 adults emerge from late April to early June. They mate and begin to lay eggs about four days after emerging. They then continue the journey north that their parents began, laying eggs along the way. They begin to arrive in the northern US and southern Canada in late May.
Like all monarchs, this generation begins life on plants in the genus Asclepias (milkweeds). The most important host plants for Generation 1 monarchs in the southern US are Asclepias oenotheroides, Asclepias viridis, and Asclepias asperula.
Generation 2
Monarchs in Generation 2 are the grandchildren of the overwintering monarchs. They are laid throughout much of eastern North America from late April through June. They do not undergo reproductive diapause.
Generation 2 Life Cycle Summary
Generation 2 larvae are widely distributed throughout the eastern United States, first beginning to appear in the south in early May and in the north in mid to late May. Eggs that become generation 2 may be laid as late as July in the north. These larvae also eat milkweed species; a few of the main species that they use include A. syriaca (common milkweed), A. incarnata (swamp milkweed), and A. tuberosa (butterfly milkweed).
Generation 2 adults emerge in June and July and mate and lay eggs soon after emerging. Most of those who begin their lives in the south move north as adults since the southern summers are too hot and dry for their offspring. Those laid farther north probably do not move far and can use all of their energy to produce as many offspring as possible.
Generations 3 & 4
Monarchs in Generations 3 and 4 are the great- and great-great-grandchildren of the overwintering monarchs. They are laid throughout the northern part of the range of eastern migratory monarchs from late May through July (Generation 3) and late June through August (Generation 4). Some generation 3 individuals emerge early enough to reproduce in the northern part of their breeding range or after moving south. However, Generation 3 individuals who emerge late in August will undergo reproductive diapause and migrate to Mexico, as will most Generation 4 individuals.
Generations 3 & 4 Life Cycle Summary
Generations 3 and 4 monarch eggs are laid throughout the northern part of their range in July and August. Some adults move south in late July and August and may lay eggs as late as October in the southern part of the US.
Some generation 3 monarchs emerge early enough to produce another summer generation. However, those that emerge later differ from other monarchs in two important ways. First, they will migrate to and from the overwintering sites in Mexico. Second, they do not reproduce right after they emerge. In response to decreasing temperatures, shortening daylengths at the end of the summer, and aging milkweed, their reproductive organs remain in an immature state. Instead of mating and laying eggs, they spend their time drinking nectar and clustering together in nighttime roosts in preparation for their long journey south. This delayed maturity is called reproductive diapause. Most of the monarchs will remain in this condition until the following spring, when they begin to mate in the overwintering colonies.
However, if milkweed is present year-round, such as non-native tropical milkweed (Asclepias curassavica), monarchs may become reproductive or fail to enter diapause entirely and lay eggs on the milkweed. Winter larvae feeding on tropical milkweed face multiple threats, including a higher chance of becoming infected with Ophryocystis elektroscirrha (OE) parasites, which are harmful to monarchs.
During September, October, and early November, migratory adults fly to overwintering sites in central Mexico, where they remain from November to March. In March, they begin to journey north, laying the eggs that will become the new Generation 1 along the way.
