Monarch eggs
Monarch butterflies begin life as eggs.
Female butterflies lay eggs exclusively on milkweed plants because they
are the only plants the monarch larvae, or caterpillars, can eat. There
are many different milkweed species. In North America, monarchs have a choice
of 108 different species on which to lay their eggs.
The female butterfly over a period of a few
weeks may lay up to 500 eggs, but only one at a time. This ensures that
predators can't find them all. Each egg is cone-shaped, ribbed and as tiny
as a pinhead. At average summer temperatures, it takes the caterpillar 4
or 5 days to grow inside the egg. It is ready to hatch when a black dot
appears at the top of the egg. The black dot is the head of the little caterpillar.
Eventually, the caterpillar will chew a hole in its eggshell and crawl out.
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Monarch caterpillars
After hatching, the
tiny caterpillar begins eating its eggshell. Then it starts feeding on the
milkweed leaves.
The caterpillar develops in five stages called instars. At normal summer
temperatures, the first instar larva takes about 3 days to develop to the
second instar. As the caterpillar grows and its skin tightens, if forms
a new, bigger skin inside the old one. Then it breaks the old skin, sheds
it, and becomes the next instar.
During the course of its development, the
caterpillar eats nearly 9000 times its own weight in leaves, increasing
its weight about 2800 times! Milkweed leaves contain bitter substances that
the caterpillar stores. The bitter taste of the caterpillar discourages
birds and mice from eating it. As the caterpillar changes instars, it develops
contrasting black, yellow, and white stripes and movable tentacles that
advertise the fact that it is bitter-tasting.
After about two weeks, the two-inch-long caterpillar
stops eating. It is now ready for the next stage of its development (metamorphosis),
the chrysalis stage. Restless, the caterpillar wanders in search of a spot
that will offer shelter from the scorching sun and the rain. This takes
a few hours.
Once it finds the right spot, the caterpillar
spins a silk button by using a tiny tube on its mouth called a spinneret.
Next, it turns around and attaches the hooks on its hind legs to the silk
button, and then lets go with all its other legs. It now hangs upside down,
forming the shape of the letter J.
A caterpillar
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A monarch chrysalis
The J-shaped
hanging caterpillar transforms into a chrysalis. The skin behind the caterpillar's
neck splits and as the creature wriggles, the skin slides up towards the
hind legs revealing the green chrysalis that has formed inside the larval
skin. Before the old skin can be cast away, the black post (cremaster),
which has many tiny hooks on its tip, has to be twisted into the silk button.
This is a dangerous moment. If the chrysalis is clumsy, it could fall to
the ground and be eaten by ants.
Gradually, the chrysalis hardens into a beautiful
hard, smooth jade-colored case decorated with golden dots. Inside this jewel,
the old caterpillar tissues gradually transform into an adult butterfly.
This process takes about 10 days at normal summer temperatures.
About 12 hours before the butterfly is ready
to emerge, the chrysalis begins to darken and its skin becomes clear enough
to see the orange and black color of the wings inside. The proboscis lies
unfolded along the ventral center; the legs are on either side of it. The
antennae lie alongside the legs. The butterfly's abdomen points backwards
towards the cremaster.
The seam along the back of the chrysalis cracks
and opens just behind the head, and the butterfly emerges.
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Monarch butterflies
After
emerging from the chrysalis, the butterfly usually clings to the empty case
with four of its six legs. At first, the wings are like thick, flat balloons.
Then, blood from the abdomen is pumped into the veins of the wings to inflate
them. The butterfly connects the two halves of its long black proboscis
by coiling and uncoiling it until it works like a drinking straw. After
a few hours the wings harden and the butterfly is ready to fly away.
A butterfly's scientific group name is Lepidoptera,
which in Greek means "scale-winged" (Lepidos = scales; Pteron
= wing). Lepidoptera have two pairs of scale-covered wings and three pairs
of legs. The wingspan of the monarch butterfly (Danaus plexippus) is about
100 millimeters, or 4 inches. Monarch butterflies use only two pairs of
their legs. The front pair is not used and is held folded up against the
body. Females use the taste organs on their feet to find milkweed on which
to lay their eggs.
Butterflies feed and drink nectar of flowers,
sipping the liquid through the proboscis. As the butterfly feeds, it pollinates
the flowers it visits. Pollen sticks to the butterfly's proboscis and is
rubbed off on the next flower it visits, fertilizing the little ovules that
become the seeds of the next generation so new plants can grow from them.
Monarch butterflies are distasteful to birds
and other predators because of the milkweed toxins in their bodies. These
predators learn to recognize the monarch's striking colors, and as a result,
stay away from them. The Blue Jay is eating a monarch adult that was reared
on a toxic milkweed. The bird then gets sick and vomits. In the future this
bird will avoid eating monarchs.
Blue Jay photos by Lincoln P. Brower; Copyright Lincoln
P. Brower, 2000.
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Monarch butterfly migration
As fall approaches, shorter days and cooler temperatures tell the monarch
butterflies not to breed, and instead, to begin their long migration to
overwintering grounds in Mexico or California. In California they cluster
along the coast mainly on Eucalyptus trees. In Mexico, tens of millions
of monarchs cluster together on Oyamel fir trees and overwinter without
breeding. Fat, stored in their abdomens, fuels their southward flight of
up to three thousand miles. This fat must last until spring, when the butterflies
will begin their flight back north. They are the great-grandchildren of
the butterflies that flew north the previous spring. They have never been
to these overwintering sites, and yet somehow, they find their way - probably
through an inherited behavior pattern.
As the spring equinox approaches on March
21, the butterflies begin mating profusely in the overwintering colonies.
Then they begin flying back in masses to the Gulf Coastal States. Arriving
in late March and early April, they find newly sprouted milkweeds on which
to lay their eggs. Three to four weeks later in early May, this fresh, new
generation of monarchs fly northward to the Great Lakes region, laying eggs
one by one on the milkweeds.
After L. P. Brower, 1995, with permission
of the Journal of the Lepidopterists' Society.
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Monarch butterfly tagging
The fall migration of monarchs is studied
by observation and through recoveries of tagged butterflies. Monarchs are
monitored and tagged as they begin to migrate in late August through early
October. The data obtained in this way help researchers to answer questions
about how the monarchs cross the North American continent, whether they
move along special pathways or in specific directions, and how their migration
is affected by the weather.
One method of tagging the butterfly: The butterfly
is held with one hand. Some of the scales from the upper and lower surfaces
of the front wing are gently removed by a stroking motion of the thumb against
the index finger of the other hand. A label is placed over the cleared area
and attached to the wing by gentle pressure and the butterfly is then released.
The label has a unique number and mailing address to which the finder can
alert the tagger where and when the butterfly was recovered. Tagged butterflies
have been recaptured as far away as 3000 miles from the site where they
were tagged and released.
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Milkweeds
Asclepias curassavica (Tropical Milkweed)
Asclepias eriocarpa (Indian Milkweed)
Milkweeds are herbaceous plants, shrubs or
vines, usually with milky juice (latex) that oozes from broken stems and
leaves. The flowers are arranged in clusters and have a unique structure:
Two seedpods may develop inside each flower,
filled with many seeds, and each seed has a silken parachute attached to
it. The silken parachute helps the seed to drift long distances.
There are about 2000 species in the Milkweed
family. The plants are widely distributed and are most abundant in tropical
and subtropical regions. Some milkweeds, especially those that occur in
North America, contain toxins and are therefore poisonous to cows and other
animals that eat them.
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Insect body structure
All insects have evolved around a basic pattern
of an armored, three-part body. The head has one or more appendages for
eating and drinking, compound eyes, and antennae for smelling. The thorax
has three pairs of legs and can bear one or two pairs of wing. The hind
section of the body, called the abdomen, contains the organs for digestion,
reproduction, excretion, and a large fat reservoir, called the fat body.
Monarch butterflies are also built like this,
with some special adaptations.
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Glossary items:
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| abdomen: |
the third, hind section
of the body of insects and other arthropods behind the thorax. |
| adaptation: |
a characteristic or trait
of an organism that makes it better able to survive in its environment,
for example, the ability of monarchs to absorb poisons from milkweeds
to protect themselves from their predators. |
| antenna: |
one of a pair of slender,
movable, segmented sensory organs on the head of insects, and other
arthropods. |
| anterior: |
the front end of an organism. |
| appendage: |
any structure attached to
the main body, such as an antenna or a leg. |
| arthropod: |
largest animal group (phylum),
classified (sorted) by the number of body segments and appendages, including
insects, spiders, centipedes, and shrimp. |
| caterpillar: |
the elongated wormlike larva
of a butterfly or moth. |
| chrysalis: |
the pupa of a butterfly
in which the metamorphosis from caterpillar to the adult stage takes
place. |
| compound eyes: |
arthropod eyes made up of
many separate visual units. |
| corolla: |
the collective term for
the petals of a flower. |
| corona: |
a crown-like structure on
the corolla of some flowers, as in milkweeds. |
| cremaster: |
the little, black post which
has many tiny velcro-like hooks on it with which the chrysalis grasps
the silk pad during the larval to chrysalis metamorphosis. |
| dorsal: |
the back side of an animal
opposite its belly side. |
| egg: |
in organisms that reproduce
sexually, the sex cell (gamete) from the female parent with its nourishing
and protective envelopes (shells). |
| exoskeleton: |
an external, supportive
covering on the outside of an animal, as in all arthropods including
insects. |
| fertilization: |
in organisms that reproduce
sexually, the fusion of an egg and a sperm to start the development
of a new individual. |
| insect: |
arthropods with well-defined
head, thorax, and abdomen, three pairs of legs, and sometimes having
one or two pairs of wings. |
| instar: |
one of several immature
stages during the development of an insect - the monarch has five larval
instars. |
| larva: |
the immature feeding stage
of an insect that undergoes a complete metamorphosis from egg to larva
to chrysalis (or pupa) to adult. |
| Lepidoptera: |
an order of insects that
including butterflies and moths characterized by overlapping scales
on their bodies and wings that are often brightly colored. (Lepidos
= scales; Ptera = wings) |
| metamorphosis: |
(a change in form) refers
to the changes in form, shape, and functions during the four stages
of development that insects undergo from egg, to larva, to chrysalis,
to adult. |
| migration: |
the back and forth movements
of animals over long distances between their breeding and wintering
grounds. Monarchs migrate in the fall to Mexico where they overwinter
without breeding, and the same individuals fly back the next spring
to lay their eggs on Gulf Coast milkweeds. |
| milkweed: |
a large family of plants
containing a milky latex that are often poisonous. In North America,
monarch butterflies have a choice of 108 different milkweed species
on which to lay their eggs. |
| molting: |
the periodic shedding and
replacing of the outgrown body covering, such as skin or an exoskelton. |
| organism: |
a living thing that is made
of one or more cells, uses energy, moves, responds to its environment,
adjusts, reproduces, adapts, and has a life span. |
| petal: |
one of the parts of the
corolla of a flower; usually colored; petals maybe joined or seperate. |
| pollen: |
the male reproductive
part of a plant that contains the sperm. |
| pollination: |
the process that transfers
pollen grains from the stamen (male part of a flower) to the stigma
(female part). |
| posterior: |
the back end of an organism. |
| proboscis: |
an extendable and retractable
coiled sucking organ on the ventral part of the head of adult Lepidoptera. |
| prolegs: |
five pairs of fleshy legs
on the abdomen of caterpillars having curved spines; used to hold onto
the food plant while eating and walking. |
| pupa: |
stage in insects in which
the transformation (metamorphosis) from larval to adult features takes
place. In moths, the pupa is usually inside a cocoon, whereas in butterflies,
it is naked and called a chrysalis. |
| reproduction: |
the process by which plants
and animals give rise to offspring. |
| spinneret: |
a specialized nozzle-like
organ connected to an internal silk gland that allows an insect larva
to spin a silk thread. Butterfly caterpillars spin silk threads that
they can follow so as not to get lost, and to suspend their chrysalis
from a leaf or twig. Many moth larvae spin protective silken cocoons
during metamorphosis from larva to pupa. |
| species: |
the fundamental and unique
category in the classification of all living things, for example, the
song sparrow, the bluebird, the monarch butterfly, the azure butterfly,
the lion, the tiger, etc. |
| stamen: |
the male reproductive organ
of a flower made up of an anther (pollen-bearing portion) and a stalk
or filament. |
| thorax: |
the body section between
the head and abdomen in insects that bears the wings and legs. |
| toxin: |
a poisonous substance that
is made by a living organism. |
| ventral: |
the underside, or belly
side of an organism. |
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Press
release
Last Updated 08/06 I.O.
Copyright 2006 Kinder Magic Software
leads to the Video
Screen.
may be used to explore
close-ups of the caterpillar or butterfly 
plays audio explanations
.
cycles through the four
stages of the Monarch 
represents locomotion
opens the Caterpillar or Butterfly
Food Investigation Screen 
rotates the caterpillar
or butterfly 
