Here you see a monarch butterfly caterpillar on a butterfly weed. The plant (Asclepias tuberosa, L.) is a member of the toxic milkweed family, reportedly the only family of plants on which monarch butterflies (Danaus plexippus) lay their eggs. That’s because their fussy caterpillar larvae will eat nothing else.
Due to their toxic diet, monarch caterpillars become toxic themselves and retain some of that toxicity when they metamorphize into butterflies. The bright colors of both forms of this insect are thought to warn predators that the insects taste terrible and to remind those that have previously eaten a monarch insect how bad they taste. Thus, monarch insects usually don’t worry about hiding.
Nonetheless, they are preyed upon. Among others, robins have been seen eating monarch butterflies and blue birds and grosbeaks have been seen feasting on monarch caterpillars. In addition, monarch eggs and caterpillars are attacked by a number of parasitic insects.
In case this toxicity talk makes you wonder, monarch caterpillars are not harmful to humans who handle them, based on my own experience, but I wouldn’t try to eat one if I were you. (Images taken in Brooklin, Maine, on August 3, 2023.)