Wildlife Watching Wednesday: The Poisonous Monarch Butterfly Caterpillar

By: Tom Berg

Summer is gone, and our friends the monarch butterflies are also gone. They have departed the eastern half of the United States on their annual southern migration, down to their wintering grounds in Mexico. As recently as September they could still be seen visiting the last of our summer flowers. Memories of their striking orange and black wings are still vivid, but even more vivid are the memories of the beautiful monarch caterpillars from earlier in the summer.

Monarch butterfly caterpillars have a distinct yellow-black-white color pattern, and this pattern acts as a warning to predators to leave them alone. They are poisonous! Monarch caterpillars store poisons called cardiac glycosides in their bodies that are ingested when they eat the leaves of milkweed plants. Luckily they are immune to the poison so it does not affect them; it only affects birds and other predators.

Monarch caterpillars hatch from tiny eggs laid on milkweed plants. The caterpillars (or instars, as they are called) grow fast, and as they outgrow their skin they molt into larger caterpillars. They go through five stages of growth, and fifth-stage instars are often 2,000 times larger than newly-hatched first instars! Wow! The fifth stage instars continue eating until it is time to form their chrysalis.

Fully-grown monarch caterpillars attach themselves to a leaf or stem, and then change into a hard chrysalis. Inside the chrysalis the caterpillar goes through the incredible process of metamorphosis, where the pudgy caterpillar transforms into a beautiful and graceful adult monarch butterfly. This metamorphosis usually takes about 10 days.

As the monarch butterfly emerges from the chrysalis, its wings are wet and quite crumpled-up. The orange and black wings look somewhat deformed, but that’s just because they have not been stretched out to their full size yet. The newly emerged butterfly hangs upside down from the empty chrysalis and within an hour its wings have expanded to full size and are fully dry. The orange and black wings warn predators that the butterfly is just as poisonous as the caterpillar!

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