Monarch Caterpillar: Anatomy, Life Cycle, and Predators

August 1, 2018 | Caterpillar

Insects are very unique lifeforms because as they grow, their physical bodies change completely. Rather than gradually growing from a child into an adult like mammals do, insects go through a physical transformation process known as metamorphosis. Some insects go through a partial metamorphosis while others, like caterpillars, go through a complete metamorphosis. A monarch caterpillar is a perfect example because it starts out as a mere ground insect but then, it turns into a butterfly that goes airborne.


A monarch caterpillar consists of prolegs, spiracles, true legs, a head, tentacles, thorax, and abdomen. When it becomes a butterfly, its anatomy consists of a forewing, hindwing, antennae, legs, compound eye, proboscis, abdomen, and spiracle.

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Life Stages

There are four stages to the life cycle of a monarch caterpillar. These stages include the egg stage, the larval stage, the pupa stage, and adult stage. The entire metamorphosis process only takes about 4 weeks. The hormones in the caterpillar move quickly throughout the body and they’re responsible for these changes.

Egg Stage

Monarch caterpillars begin at the egg stage. You will normally find these eggs in piles of leaves, especially if you have a garden. They will also inhabit any milkweed pods that you may have too. There are normally lots of eggs bunched together in the same place. If you only see one egg, there is probably several more nearby. Throughout the life of a female monarch caterpillar, she could lay up to 1,200 eggs. Once the eggs are laid, it will take around 4 days for them to hatch. As the hatchlings come out of their eggs, the first things they eat will be their own eggshells because they provide them with vital nutrients.

Larval Stage

The larval stage is when the caterpillars are only hatchlings which have recently come out of their shells. However, the larva caterpillars grow very quickly. It only takes 2 weeks for the larvae to grow about 3,000 times bigger. During this process, the larvae will shed the most from their outer body (also called the exoskeleton).

Pupa Stage

The pupa stage is when the monarch caterpillars are between the larval stage and the adult stage. During the pupa stage, the monarchs will spin a whole bunch of silk and then hang down from it. Actually, they hang upside down and form what appears to be a letter “J.” The monarchs may spend 18 hours like this. Afterward, the exoskeleton of the monarch splits around the head area and the monarch wiggles around to get rid of it. Once the exoskeleton is completely shed, a chrysalis is created. This is what actively begins the metamorphosis of the pupa turning into an adult butterfly.

Adult Stage

It will take up to 10 days for the metamorphosis to finish. Once it does, the monarchs will have colorful wings with interesting patterns on them. They will closely come out of the chrysalis by pushing their way through. At first sight, the monarchs’ wings will look small because they’re folded. Also, their abdomens will seem huge too. But it will only take minutes for the wings to spread out completely. After 60 more minutes, the butterfly will be able to flap their wings and fly away.


Monarch caterpillars are an easy target for a lot of dangerous predators in the environment. These are not large predators either because most of them are insects too. The main predators of monarch caterpillars are spiders, wasps, and ants. When the caterpillars are in their larvae stage, that is when these predators will normally attack. Other predators that monarchs need to concern themselves with are birds, such as grosbeaks and orioles. At the overwintering sites of the monarchs, the biggest threat to them are these birds.

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