Caterpillars of the Monarch Butterfly feed on plants in the milkweed family that contain toxins called cardenolides. The caterpillars sequester the toxins from the plant. The toxins are stored in the tissues and protect the adult Monarch Butterflies from predation by birds.
Monarch Larva on Milkweed Leaf
Rafter and colleagues investigated the distribution of cardenolides in the caterpillars and found that the body and gut had similar quantities of cardenolides, but the cardenolides were qualitatively different. Monarch Caterpillars modify the cardenolides from the milkweed plant before they are sequestered. The modified forms of cardenolides are possibly less toxic to the caterpillars or more readily sequestered than the unmodified cardenolides present in the plant tissue. The modified cardenolides are possibly better tolerated by the Chinese Mantid than the toxins in the gut. By avoiding the gut, the Chinese Mantid discards the plant material inside the gut that is more toxic and benefits from the less toxic and nutritious tissues.
*Rafter, Agrawal and Preisser. Ecological Entomology, Volume 38, Issue 1, pages 76–82, February 2013
DOI: 10.1111/j.1365-2311.2012.01408.x
Living With Insects Blog 
Reblogged this on Ann Novek–With the Sky as the Ceiling and the Heart Outdoors.
I always wonder how their skin defenses actually work. I always figured the damage would be done by the time the predator spits them out. It’s insane how other predators figure out the parts to eat and not to eat as well! Trial and error I suppose. Great read, thanks for posting!
The caterpillars have “warning colors” yellow and black and the adults also, orange and black. Toxic insects usually advertise their toxicity. Butterflies concentrate toxins in the wings so if a bird attacks and nips the wing, it is left with a bad taste in its mouth and spits it out. Naive birds will eat them, get sick and then be put off by anything with similar coloration.
SUPER interesting re: how mantids eat them. THAT is the cool fact I learned today. Thanks! =)