Monarch Caterpillar Stages: All 5 Instars Explained

A monarch caterpillar hatches looking almost nothing like the striped creature you picture when someone says “caterpillar.” It starts out tiny, pale, and nearly invisible. Over roughly 10 to 14 days, it passes through five distinct growth stages called instars, and by the end it has transformed into something completely unrecognizable from that first tiny hatchling.

Each instar ends when the caterpillar sheds its skin, a process called molting. Between molts, the caterpillar does almost nothing but eat. The amount of milkweed it consumes increases dramatically with each stage, and the pattern of yellow, white, and black stripes becomes more pronounced at every step.

What Is an Instar?

An instar is simply the developmental stage between two molts. All caterpillars go through instars, but the number varies by species. Monarchs consistently go through exactly five. The word comes from Latin, meaning “form” or “likeness,” and each instar does look noticeably different from the one before it.

Before the first instar, the caterpillar is still inside the egg. The egg itself is tiny, about 1.2mm tall, and shaped like a ridged football. If you look closely at the side of a milkweed leaf, you might spot it glinting in the light. The egg stage lasts 3 to 5 days before a first instar larva chews its way out.

First Instar: The Tiny Beginning

The first instar caterpillar measures just 2 to 6mm long. It is mostly pale green or grayish-white with a faint banded pattern. The stripes that will become so recognizable are barely visible at this stage. The head capsule appears dark, almost black, which can help you spot the caterpillar if you look carefully.

Right after hatching, the caterpillar typically eats its own eggshell before moving on to the leaf surface. This is normal behavior and is thought to give it some early nutrition. First instar caterpillars tend to feed from the underside of milkweed leaves, which offers some protection from predators and weather.

This stage lasts roughly 2 to 3 days. The caterpillar eats almost continuously, growing until its skin can no longer stretch. When it is ready to molt, it stops eating, attaches itself to a surface with silk, and splits its old skin to reveal the slightly larger second instar beneath.

Second Instar: Stripes Start to Show

The second instar caterpillar is 6 to 9mm long and the characteristic yellow, white, and black banding starts to become more visible. The body is still mostly white with yellow and black bands developing along the sides. Two pairs of black tentacle-like filaments begin to appear, one pair near the head and one pair near the tail.

These filaments are not legs or antennae. They are soft, flexible appendages that wave around as the caterpillar moves. Their exact function is not fully understood, but they may help ward off predators or play a role in sensory perception. By the third instar, these filaments will be much more prominent.

The second instar lasts about 2 days. Feeding continues at a rapid pace, and the caterpillar may begin consuming larger portions of the leaf rather than just scraping the surface as it did in the first instar. You can start to see the caterpillar making real progress on a milkweed plant at this stage.

Third Instar: Bold and Recognizable

By the third instar, the caterpillar measures 10 to 14mm and the yellow, white, and black stripes are fully visible and bold. This is the stage where most people recognize a monarch caterpillar for what it is. The banding is sharp and clear, running in alternating bands around the entire body.

The black tentacle filaments near both ends of the body are now much more pronounced and can be seen moving constantly as the caterpillar feeds and walks. The caterpillar’s movements are also more active at this stage, and it will travel farther across a plant in search of fresh leaves.

Third instar caterpillars last about 2 days before molting again. If you are raising monarchs, this is a good time to make sure you have plenty of fresh milkweed available, because appetite increases significantly between the third and fourth instars. A shortage of food at this stage can stress the caterpillar and affect its development.

For guidance on raising caterpillars through each stage, our monarch caterpillar care guide walks through housing, feeding, and handling at every instar.

Fourth Instar: The Feeding Surge

The fourth instar caterpillar grows to 25 to 35mm. Appetite skyrockets during this stage, and the caterpillar can devour an entire milkweed leaf in a matter of hours. The stripe pattern remains the same basic yellow, white, and black arrangement, but everything looks more vivid and defined as the caterpillar grows larger.

At this stage, the caterpillar is building significant body mass. Much of what it eats goes into storing energy and nutrients for the coming transformation into a chrysalis. Cardenolides from the milkweed it consumes accumulate in its body, making it toxic to many predators, which is part of why the stripes serve as a warning signal.

The fourth instar lasts 2 to 3 days. During this time you will likely notice the caterpillar producing more frass (excrement) than in earlier stages, which is a direct result of its increased food consumption. A healthy fourth instar caterpillar that has plenty of milkweed is a good sign the fifth instar is right around the corner.

Fifth Instar: The Final Stage

The fifth and final instar is the largest and most impressive. The caterpillar reaches 45 to 55mm, roughly 2 inches long. The yellow, white, and black stripes are now extremely bold and the caterpillar has a thick, chunky appearance. This is the stage most butterfly gardeners are familiar with and the one that makes a dramatic exit when it prepares to pupate.

The fifth instar lasts 5 to 7 days, longer than any previous stage. During this time the caterpillar is still eating heavily, but near the end it will purge its gut by releasing a large amount of liquid waste. This gut purge signals that it is almost ready to form a chrysalis.

After the gut purge, the caterpillar begins wandering. It may leave its host plant entirely and travel several feet in search of a good pupation site. It is looking for a firm, sheltered surface where it can hang in a J-shape before shedding its skin one final time to reveal the chrysalis beneath. This wandering behavior is completely normal and not a sign that something is wrong.

How Long Does the Caterpillar Stage Last?

From hatching to chrysalis formation, the caterpillar stage takes approximately 10 to 14 days under normal conditions. Warmer temperatures speed up development, while cooler temperatures slow it down. A caterpillar raised at 80 degrees Fahrenheit may complete all five instars in 10 days, while one raised at 65 degrees might take closer to 18 days.

In the wild, weather variation means the timeline is not always predictable. Rain, cool nights, and reduced food availability can all extend the time spent in each instar. Indoors under controlled conditions, development tends to follow the faster end of the timeline.

How to Tell the Instars Apart

The most reliable way to identify the instar is by size. A caterpillar under 6mm is almost certainly a first instar, while one over 40mm is in the fifth instar. Between those extremes, the sharpness of the stripes and the visibility of the tentacle filaments help narrow it down.

The head capsule is another useful clue. In the first and second instars, the head is very large relative to the body and appears mostly black. In later instars the head is still dark but the white and yellow patterns become more visible on it. By the fifth instar, the head has distinct banding that mirrors the body.

If you are raising monarchs at home and want to track each stage, photographing your caterpillars daily is an easy way to document their progress. The differences between consecutive instars can seem subtle when you are watching them every day, but comparing photos from day one to day twelve makes the transformation obvious.

What Happens After the Fifth Instar?

Once the fifth instar caterpillar finds a pupation site, it spins a small silk pad and attaches its hind end to it. It hangs in a J-shape for roughly 12 to 16 hours, and then the skin splits one final time to reveal the bright green chrysalis inside. The chrysalis is studded with gold dots near the top that catch the light.

The chrysalis stage lasts about 10 to 14 days, after which a fully formed adult monarch butterfly emerges. The entire life cycle from egg to butterfly takes roughly 4 weeks, though the caterpillar stage is only about two of those weeks. Most of that two-week window is spent eating.

Choosing the right milkweed species matters throughout every instar. Our host plant guide for gardeners covers which milkweed varieties monarchs prefer and how to keep a steady supply through the season.

Key Takeaways

• Monarch caterpillars grow through 5 instars over 10 to 14 days, starting at 2mm and reaching about 2 inches by the fifth instar.
• Each molt reveals a larger caterpillar with more pronounced yellow, white, and black stripes and more visible tentacle filaments.
• The fifth instar has the longest duration (5 to 7 days) and is marked by heavy feeding followed by a gut purge and wandering behavior before pupation.
• Temperature directly affects development speed, with warmer conditions producing faster progression through each instar.

Frequently Asked Questions

How do I know which instar my monarch caterpillar is in?

Size is the most reliable indicator. First instars are under 6mm, second instars are 6 to 9mm, third instars are 10 to 14mm, fourth instars are 25 to 35mm, and fifth instars are 45 to 55mm. The boldness of the stripe pattern and the length of the tentacle filaments also increase with each successive instar.

How long does each monarch caterpillar instar last?

The first through fourth instars each last roughly 2 to 3 days. The fifth instar lasts 5 to 7 days. Total time in the caterpillar stage is about 10 to 14 days, depending on temperature and food availability.

Do monarch caterpillars eat anything besides milkweed?

Monarch caterpillars are specialists and require milkweed to survive. They will not eat other plants. The cardenolides in milkweed are what make monarchs toxic to predators, so milkweed is not just food but also a source of chemical defense at every instar stage.

Why did my fifth instar caterpillar stop eating and start wandering?

This is completely normal. When a fifth instar caterpillar stops eating, purges its gut, and begins wandering, it is preparing to form a chrysalis. It is searching for a safe pupation site. Do not try to stop this behavior or move it back to the milkweed plant. Just let it find a spot and hang in its J-shape.

Can I raise monarch caterpillars indoors through all five instars?

Yes, and many butterfly enthusiasts do. You will need a clean enclosure, regular fresh milkweed, and daily cleaning to remove frass. The main challenges are keeping the milkweed supply consistent and making sure the fifth instar caterpillar has somewhere to hang when it is ready to pupate. A stick or mesh ceiling in the enclosure works well for this.