The Monarch butterfly, Danaus plexippus (L.) can be found at Fermilab beginning in June and sometimes in great numbers in late summer, early fall. It is one of the few butterflies that migrates. The butterflies that travel south in the fall overwinter in the south. They may reproduce in the southern wintering grounds or enroute to the North in the spring. The butterflies you see in the summer at Fermilab are not the same individuals that left there the preceeding fall, but their offspring. Two or more summer generations may be produced in the North and then the fall generation returns to the same wintering grounds even though it is three or more generations removed from that of the previous winter. The principle wintering grounds are in Mexico, but some monarchs overwinter in Florida, Cuba or southern California.

A female monarch lays its eggs (1) on a sprouted milkweed plant. The eggs hatch in four to five days producing tiny yellow, black and white banded larvae (caterpillars). These caterpillars (2) will feed solely on milkweed and eat enormous quantities because they are growing fast. They will grow to 2,700 times their original size in only two weeks, molting five times in the process. At three weeks old the caterpillar will enter the pupa stage (3) and gradually change into an emerald green case ringed with golden dots called a chrysalis (4). Inside the chrysalis, the caterpillar rebuilds into an adult butterfly (5) which, at five weeks old, climbs out of the chrysalis (6) (7) head first. Its bright orange and black wings (8) (9) signal to predators, "Beware!" Why? Monarchs are what they eat! The milkweed's "milk" or white latex is both acidic and somewhat poisonous to many animals. Since a monarch cater-pillar feeds solely on milkweed, it absorbs these substances into its body and stores them throughout its life. Therefore, the monarch tastes awful to many of its predators.