MONARCH CATERPILLAR IN CLASSIC J SHAPE
When a caterpillar hangs in a classic J shape, it is ready to change into the pupa (PYOO-puh) stage. I actually saw this poise. One day I was visiting a pond in the village and saw beautiful swamp milkweed (Asclepias incarnata) growing near a pond’s edge. This was the only time I noticed monarch caterpillars eating milkweed flowers; they had already eaten all the leaves on the milkweed plants. I visited the site for a few days. It wasn’t long before the caterpillars were no longer seen on milkweed plants. They may have found shelter on another plant or twig to begin the next stage of their life cycle.
Watching caterpillars in the wild is interesting. One day after a summer rain, I witnessed a caterpillar hiding under a milkweed leaf. The caterpillar was upside down with the abdomen prolegs clinging to a milkweed leaf. The caterpillar was peacefully fast asleep. Milkweed leaves can be large and this may help a caterpillar stay dry or hide from predators at night. Don’t be surprised if caterpillars go missing in late afternoon. They usually return to the top of milkweed leaves in the daytime once the sun warms them up by mid morning.
In the fifth and last instar stage when a caterpillar is ready to shed its final skin it will no longer eat between the larva and adult butterfly stage. The caterpillar has an average of two or three weeks to complete its journey as a larva.
It is amazing to watch a caterpillar shed its last skin and change into a pupa or sac. The whole transformation takes place within a matter of minutes. Once the pupa is formed it is soft and needs to harden in order to protect what is within.
CATERPILLAR (LARVA) – GUSANO
The caterpillar (larva) emerges from an egg in approximately four days. After a tiny caterpillar emerges from an egg, it is hungry and needs to eat the egg shell because it is rich in protein. The larva eats the egg in a circular pattern. Then the caterpillar climbs to the top of a milkweed leaf and becomes a munching machine. If there are other holes it may be remains of other eggs. It could be that a spider, mite or even a monarch caterpillar ate the eggs.
Caterpillars have lively colors with bands of yellow, black and off-white striped skin. A young larva may roll up in a tight ball if handled. Perhaps the caterpillar is trying to protect itself from being attacked. A caterpillar will grow beyond its skin five times and each time the caterpillar needs to shed its skin. This is known as molting. Stages between molts are called Instars. A caterpillar remains soft and vulnerable until a new skin has a chance to harden and it is best not to handle caterpillars because of molting.
There are several threats to the caterpillar. Tachnid flies look for a host for its own larvae and may lay eggs on milkweed or on a caterpillar. The parasitic wasp is also a threat. Additionally, there are various bacteria and viruses that could be harmful. Notice how big the caterpillar is in the illustration? The larva appears to be in its fourth Instar.
A caterpillar has three body parts: Head, thorax and abdomen. There are two sets of tubercles, one at the head and another at the end of the abdomen. The antennae like tubercles may confuse predators because the larva mimics two heads. The front tubercles aid in sensing and smell.
The thorax segment has three pairs of true legs. All insects have six true legs. There are four sets of prolegs or false legs in the abdominal segment. Usually these are visible as in the illustration. They look like pads and have little hooks that help the caterpillar attach itself to a leaf.
There are prolegs at the end of the abdomen in the illustration; the anal prolegs are used by the caterpillar to spin a white silk button. When a caterpillar splits its exoskeleton for the last time, the larva hangs upside down and starts to molt first starting with the head. When the old skin is near the rear end of the abdomen, a cremaster is exposed. This is a strong dark appendage that is uses to attach to a silk button that a caterpillar made before it started to pupate. The newly formed chrysalis will harden and hang upside down on the underside of a leaf or twig until a monarch butterfly is born.
THE EGG – HUEVECILLO
The male and female monarch butterfly will fly from Mexico to Texas. There they will mate and shortly thereafter die. The mother butterfly urgently needs to find milkweed to deposit her eggs on. This last act will insure the next generation of monarch butterflies. There are four generations that wind their way towards Canada. The fifth generation is the one that lives the longest and returns to Mexico in early fall. How does the butterfly know where milkweed grows?
The butterfly flits over a field or garden looking for milkweed. Watch the flight pattern and you may see a mother butterfly looking for a host plant. Plants have ultraviolet patterns on their leaves and flower petals making them visible to pollinators such as the monarch butterfly. Humans can’t see this but a butterfly can. A monarch uses a combination of visual and chemical cues to find milkweed.
Once a mother butterfly lands on milkweed, she uses sensory organs in her feet and head to make sure it is milkweed. A monarch may even drum on a milkweed leaf with her forefeet to be certain. The forefeet are located close to the head.
Notice that there are three eggs on the milkweed plant in the illustration. The mother butterfly carries approximately 200 eggs in her stomach. She touches the abdomen to the underside of a leaf and deposits an egg. The mother usually deposits only one egg per plant. There is a reason for this. It is probable that the first caterpillar to emerge from an egg will scout for other eggs. A caterpillar is able to defend its own food territory when there is no competition. Eggs have protein. Sometimes a mother butterfly is in a hurry and may deposit more than one egg on the same plant. This is known as egg dumping.
By depositing eggs on different milkweed plants, eggs have a better chance of survival if something goes wrong. There are many risks to milkweed including: Some plants may not be as hardy as others; a storm could destroy a plant; milkweed might be removed because a gardener thinks it is a weed; a land developer could clear cut a large track of land removing all native plants; farmers may plant monoculture crops, genetically engineered crops and may spray crops with pesticides.
There is a loss of approximately 3,000 acres of farmland each year to development. Roadside crews may use herbicides (ER-beh-syds) which poison milkweed, a plant that grows along roadways, one of the major corridors of butterflies. If there is no milkweed along the migration trail, there will be no monarch butterfly migration. This is why it is so important to let milkweed grow.