Monarch Butterflies:
Migration
and Life Cycle
(Danaus
Plexippus)
Where are Monarch Butterflies 
found?
Monarchs can be found all around the world in subtropical
to tropical areas. They tend to be found
in open habitats including meadows, fields, marshes, and cleared roadsides.
Migration of a Monarch Butterfly
Butterflies live a rather
complicated life cycle. Depending on the
time of year that a monarch emerges, they will react in different ways.
Fall Migration
Late summer early fall monarchs tend to be physically and
behaviorally different from those that come out in spring or early summer. Usually these butterflies go into a
reproductive diapause. Diapause is a
states of stopped development of the reproductive organs.
Reproductive diapause is manipulated by the nervous system
and other hormones. Unfavorable
environmental conditions also brings the onset of diapause in monarchs. The factors that truly affect it are day length,
temperature, and host-plant quality.
Day Length
Decreasing day length is one of the main triggers for the
suspended development. In a series of
experiments, Liz Goehring of University of Minnesota found that monarchs that
were grown in conditions where there was constant short and long day lengths
were mostly reproductive. In the other
case, monarchs nurtured under decreasing day light were most likely to be in
diapause.
Temperature
Increasing or
decreasing temperatures have affects on all animals. As the seasons turn to fall, temperatures
begin to vary more virulently. Fall
temperatures tend to be warm during the days, and much cooler at night. These fluctuations contribute to the onset of
diapause in monarchs.
Host Plant Quality
Again as the seasons change, natural plant flora changes
also. Plants toward the end of the
summer and the beginning of fall begin to senesce,
or grow old. Monarchs can sense this
change, especially when leaves begin to fall.
Monarchs reared on these older plants tend to be in diapose.
Overall, a combination of
all of these induces the diapause in monarchs.
Though only one of these cues can bring the onset of diapause.
Spring Migration
Most monarchs from North America tend to migrate in the winter
to trees at sites in Mexico and California. The monarchs cluster together,
covering entire trees, including the whole trunks of trees.
Mating period for the butterflies is a 3-5 week period,
which occurs just as winter ends and the days grow longer. About the middle of March, monarchs in Mexico
leave to find milkweed plants in the north and east to lay their eggs on.
Coming out of diapause for monarchs depends on the
individual. Many of the monarchs came
from a wide range of environments and are of different ages, thus, the way each
reacts in coming out of diapause is different.
Overall, males tend to come out of diapause earlier than females. Several factors affect the longevity of the
diapause.
Availability of Milkweed
Monarchs stay in
the winter tends to be sparse with milkweed plants. Female monarchs can only lay their eggs on
milkweed plants. Thus, if there are no
milkweed plants, the females cannot reproduce.
Milkweed is not necessary for completion of diapause.
Mating
Females must mate before fertile eggs can be laid. Though females have been found to more likely
become reproductively mature after they have mated. Post diapause females tended to produce
mature eggs more rapidly if mated before.
Though not a requirement, females can complete diapause and become
mature without mating.
Day Length
Just as it is important in the fall time for going into
diapause, it is expected that it will signal changes to come out of
diapause. However, scientist have no
evidence to support this accusation.
Migration Patterns
North American Monarchs are
thought to migrate to California. There
are at least 25 predictable overwintering sites in California, in addition to
numerous other temporary sites. About 5%
of the overall monarch population are California Monarchs.
The migration patterns of
monarch butterflies are depicted in the following diagrams.
Figure 1- This displays the Fall migration of North American
Monarchs.
Figure 2- This displays the Spring Migration of Monarch
Butterflies.
Various Structures of the Monarch
Butterfly
http://www.monarchlab.umn.edu/SO/so.html
Egg
The
butterfly egg is surrounded by a hard outer layer called chorion. This a hard protective coating to protect the
developing larva. Wax lines the 
inner
egg to keep the egg from drying out. Each egg has one
to many tiny funnel-shaped openings at one end, called micropyles. These holes are what allow the sperm to
fertilize an egg, thus, the holes runs all the way through to the inside of the
egg. The ridges along the outside of the
egg are formed in the female butterfly before she lays the egg. The egg is only 1/8th of an inch
long. Each egg is laid singly by the
female monarch underneath a milkweed leaf.
The larva will emerge in 3-5 days.
Larva/Caterpillar
Just
like all insects the caterpillar is composed of three sections: The head, the
body with a thorax, and the abdomen. The
structure is what is shown below:
The
head of the caterpillar contains a very short antennae, mouthparts (upper lip,
mandible, and lower lip), six pairs of simple eyes, called ocelli. A caterpillar’s eyesight is very poor despite
how many. It uses its antennae as a
guide.
Each
thoracic part of the caterpillar has a pair of jointed or true legs. In the abdominal area, there are prolegs or
false legs, which contain little hooks that the caterpillar uses to hook on to
leaves. The tentacles that are found on
the caterpillar are used for sensing.
The caterpillar will molt four times while it is
growing. Each time it molt it will eat
the molted old skin.
Pupa (Chrysalis)
At
some point in the caterpillar’s life cycle, it stops eating and begins to form
a chrysalis. The caterpillar spins
itself a silk mat from which to hang from.
This is made by the spinneret on the bottom of the head. As the caterpillar sheds its skin, it stabs a
stem into the silk mat for it to hang, which is called the cremaster.
A
misconception is that the metamorphism occurs all at this time. But right up until the point the caterpillar
formed the pupa, many physiological changes took place to prepare for the
transformation into a butterfly. The
whole process takes 10-14 days during regular summer conditions.
Adult
The primary job of the
adult monarch is to mate and reproduce.
One mating session for a monarch butterfly usually lasts from one
afternoon to the next morning, which at times can be as long as 16 hours. Although, monarchs cannot mate until they are
three to eight days old. Upon mating, the
females almost 
immediately
start laying the eggs. Unlike most
insect species, both sexes can mate several times in their life time.
The usual life span for a
summer adult is 3-5 weeks. Although a
migrating monarch, from the latest generation in the summer, can be as old as
8-9 months. These monarchs have to make
it through all the winter months to reproduce in the summer.
Monarchs do not grow at all
in their adult stage. They must maintain
nourishment and health for flight.
Providing most of their energy, they drink the nectar from flowers,
which contains 20% sugars.
The diagram above is of the
basic structure of the monarch butterfly.
Pictures and Info collected
from;
http://www.monarchlab.umn.edu/SO/so.html
For more information about
monarch butterflies, feel free to explore these websites.