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Monarchs Begin Life As Very Hungry Caterpillars

Updated: Mar 14, 2023

Monarch butterflies are one of the most recognizable butterflies in Illinois. They are larger butterflies with bright orange and black stripes and white dots on the edges of their wings.

The butterfly is the last stage of this insect’s life cycle.

(Photo via Shutterstock)

Butterflies go through a life cycle called metamorphosis. First it starts as an egg, and the egg hatches into a caterpillar. The caterpillar will grow and then turn into a chrysalis, and then finally the adult butterfly will emerge. The full cycle takes about 30 days to complete. Let’s zoom in on one of the busiest stages of these insects’ lives: the hungry caterpillar.

They start small but grow mighty

When picturing a caterpillar, you might think of something the size of your pinky. However, that size is often about as large as a caterpillar will grow. Caterpillars start tiny and get bigger as they eat.

Can you see four caterpillars? (Photo by Suzy Lyttle)

Monarch caterpillars start by hatching out of their little eggs and then eating the eggs. Then they will start munching on milkweed leaves. When they hatch, monarch caterpillars are about the size of an eyelash. Over their lifetime, they will grow 3,000 times larger than when they hatched, according to the National Wildlife Federation.

To get bigger, the caterpillars will molt their exoskeletons. An exoskeleton is like wearing their bones on the outside, like an armored skin. It would be like us taking off a too-tight sweater to fit into a larger-sized one. Monarch caterpillars molt five times. The stages between each molt are called an instar. The complete caterpillar stage can last 10 to 14 days.

They are picky eaters

Monarch butterflies only lay their eggs on milkweed plants. When the caterpillars hatch, they will only eat the leaves from the milkweed plants. This dependent relationship makes milkweed the host plant of the monarch.

A monarch caterpillar on butterfly weed. (Photo by Suzy Lyttle)

These caterpillars can eat a variety of different species of milkweed. The most iconic type is called common milkweed. It can be spotted in farm fields, roadsides, prairies and hopefully in your neighborhood. Other native milkweeds found in our area include prairie milkweed, purple milkweed, swamp milkweed and butterfly weed.

Their colors are a warning

Eating only milkweed protects the caterpillars from being eaten. Milkweed plants contain different chemicals that make the monarch caterpillars poisonous to potential predators. The toxic plant does not harm the caterpillars, but it could harm predators such as birds, mammals or other insects.

(Photo by Suzy Lyttle)

The bright colors and pattern on the caterpillars let all the other animals know that it is poisonous. If a hungry predator decides to not pay attention to this warning, chances are it will spit out the caterpillar after tasting the toxins.

They have two sets of legs and tentacles

True insects have three body parts (a head, a thorax and an abdomen), six legs and two antennas. Monarch caterpillars are no exception to this rule, but they do have a few extra features. Close to the caterpillar’s head are the six true legs. Further back, they have legs that are a different shape, like rounded suction cups. These back legs are called prolegs. The caterpillars have five sets of prolegs, and each one has tiny hooks helping the caterpillar climb and hold onto leaves and stems.

Monarch caterpillars have poor vision. The antennae on their heads help guide them to food sources and get food to their mouths. What is more visible are the long black tentacles. They have two in front and two in back. They serve as sensory organs, helping the caterpillars navigate their environment. They also trick predators. It makes the caterpillars look like they have two heads and are always watching!

They remember their caterpillar lives

A study conducted by scientists at Georgetown University has changed the way we think about caterpillar metamorphosis. The scientists trained moth caterpillars to dislike a certain smell. Each time the caterpillars smelled the chemical, a little shock would be given. Soon the caterpillars avoided the smell because it reminded them of the shock. This is similar to how dogs get trained with electric fences.

When the caterpillars transformed into adult moths, most of them still avoided the smell because they remembered the shock! There are brain structures in the caterpillar called mushroom bodies that are responsible for learning and tasting. It is now believed that those structures stay intact during the chrysalis phase, allowing the butterflies to remember what foods to eat and what foods to avoid.

Watching a butterfly life cycle play out before your eyes is a magical experience. Invite monarchs and their caterpillars to your yard by planting milkweed plants. If you add in other native plants, you’ll help the monarch butterflies find lots of nectar for their long journey to Mexico, where they overwinter. Visit Plum Creek Nature Center this summer for free milkweed seed packets.


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