Egg | Larva/Caterpillar |
Pupa | Adult
The primary job of the adult stage is to reproduce—to mate
and lay the eggs that will become the next generation. Monarchs do not mate until
they are three to eight days old. When they mate they remain together from one afternoon
until early the next morning—often up to 16 hours! Females begin laying eggs
immediately after their first mating, and both sexes can mate several times during
their lives. Adults in summer generations live from two to five weeks.
Each year, the final generation of monarchs, (which emerges in
late summer and early fall), has an additional job. They migrate to
overwintering grounds (see image below), either in central Mexico for eastern
monarchs or in California for western monarchs. Here they survive the long winter
until conditions which allow them to return to their breeding grounds. These adults
can live up to eight or nine months.
Male and female monarchs can be distinguished easily. Males have
a black spot (indicated by red arrow in image above) on a vein on each hind wing
that is not present on the female. These spots are made of specialized scales which
produce a chemical used during courtship in many species of butterflies and moths,
although such a chemical does not seem to be important in monarch courtship. The
ends of the abdomens are also shaped differently in males and females, and females
often look darker than males and have
wider veins on their wings.
The body of an adult butterfly is divided into the same major parts as the larva:
head, thorax, and abdomen.
There are four main structures on the adult head: eyes,
antennae, palpi, and proboscis. A butterfly’s relatively enormous compound
eyes are made up of thousands of ommatidia (see
image below), each of which senses light and images. The two antennae and
the two palpi, which are densely covered with scales, sense molecules in the air
and gives butterflies a sense of smell. The straw-like proboscis is the butterfly’s
tongue, through which it sucks nectar and water for nourishment. When not in use,
the butterfly curls up its proboscis.
The thorax is made up of three segments, each of which has a pair
of legs attached to it. The second and third segments also have a pair of wings
attached to them. The legs end in tarsi (singular,
tarsus), which grip vegetation and flowers when the
butterfly lands on a plant. Organs on the back of the tarsi "taste" sweet
liquids. Monarchs and other nymphalid butterflies look like they only have four
legs because the two front legs are tiny and curl up next to the thorax.
Monarchs clustered in an Oyamel Fir tree at the
Sierra Chincua overwintering site in central Mexico.