24 ILLINOIS AUDUBON BULLETIN 
In the southern areas not affected by polar air masses the monarchs 
spend the winter as free-fiying, non-roosting individuals. At tempera- 
tures of 60° to 70°F. they actively feed on nectar from nearby flowers. 
Along the northern limits of the winter range, where the temperature 
occasionally drops below 55°F., the butterflies exist,in roosting colonies 
during cool periods and at night. 
Generally, the spring migration begins near the end of February. 
In contrast to the leisurely flight south, the northbound migrants fly 
faster and do not linger during feeding. They may reach the central 
and southern portions of the United States in May and June to lay 
their eggs on the new growth of various species of milkweed (Asclepias 
spp.). While only one or two generations are produced in the north, 
three or four generations may complete development in the southern 
portions of the range. 
The period of development from egg to adult is approximately 33 
days. This time may be shortened as much as ten days at warmer 
temperatures, or it may be extended as much as fifteen days at cooler 
fall temperatures or in southern Canada. The egg, the size of a 
pinhead, is laid on the underside of an apical leaf and hatches in three 
to six days. The larva, with alternate bands of black, cream color, and 
yellow, has a pair of long black filaments on the second thoracic 
segment and on the eighth abdominal segment. During the next 10 to 
17 days the larva moults five times. The mature larva then leaves the 
plant in search of a place to pupate. The chrysalis is hung head down 
from tree limbs, the underside of leaves of a variety of plants, or 
almost any place which will provide support and protection. 
Some newly emerged adults may stay in the vicinity of their 
breeding grounds, but most move northward to mate and lay eggs. 
Therefore, a given field in August may have tattered and faded spring 
migrants, freshly emerged adults, and a mixture of all immature 
stages. Those butterflies found in the extreme northern fringes may 
actually be those which were reared on more southern breeding 
grounds. Late in the season spring migrants die; and their progeny, 
which number in the thousands, begin a leisurely trip to the southern 
winter range. 
(Reprinted by permission of the author and the Illinois State Museum) 
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