Animated coloured scanning electron micrograph (SEM) of Monarch caterpillar, young larval stage (Danaus plexippus). Monarch butterfly caterpillars (Danaus plexippus) hatch from eggs laid one at a time and cemented to the undersides of young leaves. Newly hatched caterpillars (larvae) feed exclusively on milkweed leaves. Milkweed latex contains alkaloids that are harmless to caterpillars, but that sicken their predators. Birds learn to avoid the bright warning colours of monarch caterpillars and adults. A caterpillar eventually attaches itself to a twig, sheds, then forms a chrysalis from which an adult butterfly emerges. An adult lives six weeks in summer but those that migrate can live six to nine months over a mild winter.