Do the butterflies that eat poisonous stuff have poison in them?
Butterflies that eat toxic (poisonous) plants while they are caterpillars have many different adaptations for dealing with plant toxins.
Some butterflies keep the toxins in their bodies to protect themselves against predators while they are still caterpillars and retain these toxins when they metamorphose into adults. They sometimes store the plant toxins in specific parts of the body. Monarchs have toxins that they get from eating milkweed in their wings. A bird that takes a bite out of the wing learns that the monarch is not good to eat, and the monarch survives to fly another day, although perhaps not as effortlessly as it would have with undamaged wings.
Other caterpillars that eat toxic plants do not store the toxins in their bodies. They have enzymes in their guts that detoxify the toxins just as we do (our enzymes are mainly in the liver). If they have just eaten, however, their foregut (the first gut segment) might be full of nasty plant juices that they can regurgitate onto predators that try to eat them.
One of my favourite examples of dealing with plant toxins comes from the tobacco hornworm, which is the caterpillar of a large moth. Nicotine in tobacco is highly toxic to most organisms (including humans). Tobacco hornworms “breathe” nicotine out of their spiracles, which are tiny holes on the sides of their bodies that they use to take in oxygen and expel carbon dioxide, like our nostrils. Having stinky “nicotine breath” protects the caterpillars from predators such as spiders. – Dr. Naomi Cappuccino