Final answer:
Monarch caterpillars acquire cardiac glycosides from milkweed plants, making them distasteful or poisonous to predators. These chemicals are an evolutionary defense mechanism, passed from caterpillars to adult butterflies.
Step-by-step explanation:
Monarch butterfly caterpillars are not good to eat because they contain cardiac glycosides (terpenoids), which are chemicals that make them distasteful or potentially poisonous to predators. These cardiac glycosides are acquired by caterpillars as they feed on milkweed plants. The milkweed is the caterpillar's primary food source, and it is where they ingest the chemicals that protect them from being eaten by predators. This adaptation not only helps in the survival of the caterpillars but also allows the toxins to be passed onto the adult monarch butterflies, which continue to display warning coloration to deter predators. Some other insects and herbivores have developed the means to detoxify milkweed's poisons and also use it as a food source, but for most predators, these cardiac glycosides act as a strong deterrent.