All insects change in form as they grow; this process is called metamorphosis.
There are two kinds of metamorphosis, incomplete (or simple) metamorphosis,
and complete metamorphosis. An example of incomplete metamorphosis is found
in grasshoppers. The young nymphs usually look much like small wingless
adults. The wings develop externally, and there is no prolonged immobile
(pupal) stage. Butterflies and moths undergo complete metamorphosis, in
which there are four distinct stages: egg, larva (caterpillar), pupa, and
adult. Hormones circulating within the body trigger the changes that occur
during metamorphosis.
The development of a butterfly or moth from egg to adult can take anywhere
from three weeks to several years. The shortest life cycles are generally
in areas that have long warm periods. In arctic areas, some moths have
such a short period to grow each year that the caterpillars cannot complete
their development in a single growing season and remain in this stage for
longer than one year. It usually takes these species two years to develop,
but some may take up to ten years. It takes Monarchs about a month to go
through the stages from egg to adult, then the adults live another two
to six weeks in the summer. Monarchs that migrate live all winter, or about
six to nine months.
