What is fascinating about the monarch butterfly and some of the other milkweed specialists is that
they do not just feed on the plants, digest the substances, and then build up their own body
substances. Rather, they store some of the components of the milkweedsap in their body. When
a milkweed stem or leaf is damaged, it exudes a white sap. All you have to do is to scratch the
stem with your finger nail and the white sap oozes out and streams down the stem until it
gradually hardens. When, for example, a monarch larva bites into a leaf vein or stalk, the sticky
(latex-containing) milky sap seeps out and the larva ingests it. It draws out of the sap a particular
group of substances known as cardiac glycosides(cardenolides), and instead of breaking them
down or excreting them, it stores them in its tissues. The concentration of cardiac glycosides in
the tissues of a monarch is substantially higher than it is in the tissues of common milkweed.
Interestingly, it is not only the larva that sequesters these substances; they are also retained in the
adult, which has gone through the complete metamorphosis from caterpillar to butterfly. So part
of the milkweed becomes an essential part of its insect predators.