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A Look at the
Life of a Monarch Caterpillar

Just moments after a monarch hatches from its egg it devours its own shell. This is a fitting beginning for a creature whose focus is FOOD!

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Eggs are usually laid on the underside of the leaf.
The day before hatching, the top of the egg turns black.

Immediately after emerging, the monarch turns back and eats its own shell.

Let's look at some fast facts about monarch caterpillars:

  • Larva is the scientific word for caterpillar. Larvae is the plural of larva, so "larvae" means "caterpillars." The use of either "caterpillar" or "larva" is correct.
  • Monarchs spend the larval stage of their lives eating--and growing. In fact, the typical monarch increases in mass by 2,000 times while it's a caterpillar. This amazing transformation takes place in only about 9-14 days.
  • The weight a monarch gains as a larva determines the butterfly's size as an adult. Bigger caterpillars become bigger butterflies; smaller caterpillars become smaller adult monarchs.
  • Once a monarch becomes an adult butterfly, it does not grow any more.
  • Larvae go through five growth stages called "instars." This is because, as insects grow, they must shed their exoskeletons as they increase in size. Just as children outgrow their clothes, insects outgrow their skeletons! (Luckily, human skeletons are inside our bodies and grow with us.)
  • In addition to eating and growing, larvae must avoid predators and parasites! Mortality is extremely high. Over 90% of all eggs laid never survive to the chrysalis stage, according to preliminary results of the Monarch Larval Monitoring Project.
  • It's easy to find monarch larvae when you look for leaf damage on milkweed leaves. Predators and parasites may cue-in on leaf damage to find their prey. For this reason, larvae of some butterfly species change their position on the plant often, and move to different plants, as a predator avoidance strategy.
  • When frightened, larvae use a silk lifeline to escape quickly. They can drop to the ground and vanish in the vegetation in an instant. They also often curl up into a ball when touched. Why do you suppose they do this?
  • Only the final monarch generation of summer migrates to Mexico. A butterfly's chance of surviving the winter is greater the more lipids it has stored. This means that the milkweed conditions available to larvae in the north can ultimately affect their chances of surviving the winter!
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Can you find the two larvae pictured here?

Larvae at each of the 5 stages of growth, called "instars."

Notice the typical pattern of "leaf damage" made by 1st instar larvae.

Photo: Elizabeth Howard Photo: Karen Oberhauser Photo: Jim Edson

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