The monarch butterfly, a creature of vibrant beauty and remarkable resilience, undergoes a fascinating transformation throughout its life. From a tiny egg to a striking butterfly, the monarch's journey is a testament to the wonders of nature. This article delves into the intricate details of the monarch caterpillar's diet and lifecycle, exploring each stage with a comprehensive lens.
The Egg Stage: A Promising Beginning
The monarch's life cycle begins with an egg, a tiny, creamy yellow sphere adorned with narrow longitudinal ridges. These eggs, measuring a mere 0.9 to 1.2 mm long, are laid exclusively on milkweed plants by female monarch butterflies. Milkweed serves as the sole food source for monarch caterpillars, making it a crucial element in their survival.
Female monarchs strategically lay their eggs, usually one per milkweed plant, often gluing them to the underside of a leaf. This placement offers protection from harsh weather conditions and potential predators. A single female monarch can lay an impressive 300 to 400 eggs over a few weeks. In captivity, some have been recorded to lay even more, up to 1,179 eggs!
Each egg is formed inside the female before fertilization. This includes 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.
After about four days, the eggs hatch, revealing the first stage of the monarch's life: the larva, or caterpillar.
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The Caterpillar Stage: An Eating Machine
Upon emerging from its egg, the newly-hatched caterpillar, measuring only 1/16th of an inch long, embarks on a period of voracious eating and rapid growth. Its first meal is its own eggshell, providing vital nutrients for its development.
Exclusive Diet: Milkweed Specialists
Monarch caterpillars are specialists, meaning they feed exclusively on milkweed plants. This dietary restriction makes milkweed a critical resource for monarch populations. The caterpillars consume vast amounts of milkweed, growing at an astonishing rate. In just 7 to 17 days, a caterpillar can increase its weight by 2,700 times!
Instars: Stages of Growth
The caterpillar stage is divided into five phases called instars. As the caterpillar grows, it sheds its skin through a process called molting. This occurs because the caterpillar's skin doesn't grow. When they need a bigger skin suit, they spin a pad of silk, attach their hind legs into it, and walk out of their old suit. Each molt allows the caterpillar to continue growing. The period between each shedding of the skin is called an instar.
The five caterpillars can double in size with each molting.
- First 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.
- Second 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 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.
- Third 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 visible. The first set of thoracic legs is 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.
- Fourth Instar: 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.
- Fifth Instar: 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.
Anatomy of a Caterpillar
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. 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. Like other insects, monarchs obtain oxygen through holes in the sides of their thorax and abdomen called spiracles.
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Preparing for Pupation
Around day 18, the caterpillar is ready to form a chrysalis. 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 a cage they will usually climb to the top and spin a pad of silk. Once there, it weaves a tiny silk pad as an anchor. It then inserts the hooks at the tip of its abdomen into the anchor and assumes a J shape.
The Chrysalis Stage: A Transformation Chamber
Once the 5th instar caterpillars become ready to pupate, they get the urge to wander until they find a suitable, protected place to form their chrysalis. This stage, also known as the pupal stage, is a period of remarkable transformation. The caterpillar sheds its skin one last time, revealing a jade green chrysalis adorned with gold spots.
Camouflage and Development
In the pupal stage, monarchs are amazing camouflage artists! Its turquoise-green colouring, gold spots that reflect light and small size (3 cm) make a pupa very difficult to spot in the wild. It gradually changes from jade green to bluish, until the adult colours show through the transparent case. The chrysalis may look dormant, but much is happening inside. 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.
Emergence
After 8-15 days, the butterfly's beautiful orange and black wings appear the day before it is born. The pupa splits open to let the adult butterfly wriggle free. This is called emergence when the adult butterfly comes out of its pupa case. Clinging onto its pupa case, it then “pumps” hemolymph (insect “blood”) through the vessels in its still-wet wings.
The Adult Stage: Flight and Reproduction
Within about 4 or 5 hours, the wings will have unfolded and stiffened and the butterfly can take flight. The butterfly will eventually take its first flight. First stop: a nectar-bearing flower so that it can stock up on energy. After it has feasted, the butterfly can concentrate on its main job as an adult: reproduction.
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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. Once fertilized, the female heads off in search of milkweed plants to lay her eggs, continuing the cycle.
Anatomy of an Adult Butterfly
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. 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.
Generations and Migration
Most monarchs are born in the spring and summer. They live for only 2-6 weeks! Every summer, 3 or 4 generations of these non-migratory monarchs hatch out, migrating relatively short distances as the weather warms and their summer range expands. They mate and lay eggs along the way, parenting the next generation of monarchs.
Monarchs that are born in late summer are different. These are the monarchs that migrate to Mexico for the winter. This generation heads off on an amazing journey that takes them all the way to the mountains in central Mexico where they overwinter. The final generation of the year, however, has a completely different life in store! As the fall approaches, something - no one is sure exactly what - causes this final generation to delay their reproduction and start migrating south. These are the butterflies that will fly hundreds, perhaps more than two thousand miles, and overwinter in Mexico. As they travel, the nectar they drink from flowers allows them to build up enough fat reserves to spend the winter dormant in Mexico.
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. 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.
*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.
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. The last eggs are laid in late April or early May, farther north. 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. Like all monarchs, this generation begins life on plants in the genus Asclepias (milkweeds).
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 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.
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.
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. Generations 3 and 4 monarch eggs are laid throughout the northern part of their range in July and August. 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. 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. During September, October, and early November, migratory adults fly to overwintering sites in central Mexico, where they remain from November to March.
Distinguishing Males from Females
It is easy to tell if you have a female or a male monarch. 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.
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