Migration and Life Cycle
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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