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 13 monarch caterpillar DHJanzen100176.jpg high resolution
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| A last instar (full sized, just before stopping feeding to molt into the caterpillar) monarch butterfly caterpillar, perched ostentatiously on the top of a milkweed leaf. This aposymatic caterpillar does not have (known) mimics in northern forests, but there are many tropical brightly ringed caterpillars (e.g., in Costa Rica) that may well be part of Mullerian and Batesian mimicry systems in which monarch caterpillars are embedded. The defenses of this caterpillar are the cardiac glycosides derived acquired directly from the milkweed foliage that it is feeding on (and is its only food plant - various species of Asclepias). Whether it modifies them slightly before sequestering them, and where they are sequestered in the body is not known to me. The caterpillar itself is quite tough (in the rubbery sense) just as is the adult monarch butterfly, presumably selected for through being handled roughly by a predator and then rejected once tasted (the caterpillar is not tough enough to survive being swallowed and then regurgitated). Lincoln Brower and his students showed that if the caterpillar is forced to feed on cabbage (no cardiac glycosides, though cabbage does have secondary compound defenses which are apparently ignored by the monarch caterpillar physiology), it is edible to birds and so are the resultant monarch butterflies. | ||
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