As caterpillars, monarch butterflies need to eat milkweed plants to thrive. Milkweed plants are quite toxic to most insects and monarch butterflies have evolved the ability to sequester the cardiac glycoside toxin within milkweed and ways to circumvent the gooey latex that comes out of the leaf. The plant uses latex to glue an unlucky insects mouth shut. Here are a few pictures from the past week of some large monarch caterpillars.
Evidence that a monarch caterpillar sniped the milkweed vein to prevent latex from flowing
Gooey Latex from milkweed
More monarch caterpillars