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Insects and other arthropods

Monarch caterpillar

Caterpillar chewing a milkweed leaf
Caterpillar chewing a milkweed leaf
  • Caterpillar chewing a milkweed leaf
  • 2nd instar caterpillar
  • 4th instar caterpillar
  • 5th instar caterpillar
  • Fully grown caterpillar

A monarch caterpillar needs to eat as soon as it hatches; in fact, its first meal is its own eggshell!


Right from the start, a tiny monarch caterpillar sports its characteristic stripes. On its head, it has a pair of short antennae, two compound eyes and six pairs of simple eyes, called ocelli. Its eyesight is not very good, and it mainly uses its antennae to find its way around.

On its body, the caterpillar has two pairs of black filaments: one near its head (2nd segment) and another shorter one near its hind end (11th segment). These filaments are sensory organs.

To move around, a monarch caterpillar uses three sets of prolegs, in addition to the six real legs that it has in common with all insects.

Feeling too tight!

Like all arthropods, the caterpillar has to shed its “skin” in order to grow because its exoskeleton is too rigid to allow it to expand. A larva moults several times as it grows, passing through several larval stages (instars) – this happens 5 times for monarchs.

  • 1st stage : about 2 mm
  • 2nd stage : about 6 mm
  • 3rd stage : about 15 mm
  • 4th stage : about 35 mm
  • 5th stage : about 50 mm

To store up as much energy as possible, the caterpillar eats milkweed, its host plant, practically non-stop.

It does take a break just before it moults, though. That’s a good sign to watch for if you want to see it moulting!

For a monarch, the 5 larval stages occur over the space of anywhere from 9 to 15 days.

A monarch caterpillar grows to 2,700 times its starting weight! If humans grew that fast, we’d be as tall as the Statue of Liberty in just 2 weeks.

A colourful warning

The bigger a caterpillar gets, the brighter its stripes become. These colours do an important job for the insect, by warning predators that it’s poisonous (this is called aposematic colouring). Predators quickly learn to recognize a monarch and to avoid it. It’s a good thing for the monarchs, because only a few have to pay with their lives for predators to get the message. But in the end, all monarchs benefit from this strategy!

Poison on the menu

Monarchs acquire their chemical protection by eating milkweed – all parts of this plant contain toxic latex. The cardenolides in the latex are stored in the caterpillar’s tissues, without harming it.

A relative advantage

Despite this means of defence, 90% of monarchs in the wild don’t make it past the egg and caterpillar stages. Part of the reason for this is that the cardenolides accumulate in an insect’s body over time. This means that young caterpillars are more edible than ones in later larval stages. As well, the cardenolides are more harmful to mammals and birds. Other predators and parasites like reptiles, amphibians, arthropods, etc. aren’t particularly affected by the toxin.

In the end, fewer than 10% of monarchs survive until they turn into chrysalises. While that might seem like a huge loss, remember that it’s part of the balance of nature and allows other animals to thrive, too!

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