Keystone WILD! Notes

WINTER 2007-2008

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Creature Feature
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Monarch Life Cycle -- Visit the Full Size version to see details
All insects change in form as they grow, a process called metamorphosis. Butterflies and moths undergo complete metamorphosis, in which there are four distinct stages: egg, larva, pupa, and adult. Monarchs take about 6 weeks to go from egg to adult.

Egg (fig. 1)
The female Monarch usually lays a single egg on a plant, often on the bottom of a leaf near the top of the plant. The eggs, which are only about the size of a pencil point, hatch about four days after they are laid.

Larva (caterpillar)
This is only stage in which a Monarch grows, and the larvae eat only one type of leaf: milkweed. The larva spends 21-27 days devouring the leaves of the milkweed plant on which its egg was laid (fig. 2). When the caterpillar outgrows its skin, it sheds, or molts. The intervals between molts are called instars; a Monarch larva goes through five instars during its 3-4 weeks of feeding on milkweed, growing larger with each molt (fig. 3).

Pupa (chrysalis)
When the larva is ready to pupate, or transform into a butterfly, it attaches itself to a branch stem with silken thread which it forms into a “button,” and sheds its skin for the last time. This final molt reveals a shiny, green, gold-speckled case, or chrysalis, that protects the pupa (fig. 4). The adult emerges in 10-14 days. Shortly before it is ready to emerge, the pupa turns very dark and the orange and black wings of the adult become visible through the waxy-looking pupa case (fig. 5).

Adult (butterfly) 
To escape from its chrysalis, the adult starts swallowing air; its body expands and the chrysalis splits down the back. The adult crawls out and hangs from its pupa case (fig. 6), gently forcing blood into the veins of its soft, wet wings. After 3-4 hours, the wings have fully extended and dried. If the Monarch does not fully extend its wings, they will dry in their crumpled shape and the butterfly will not be able to fly or feed. For this reason, you should never disturb a butterfly while it is drying its wings.

The primary job of the adult Monarch is to mate and lay eggs. Over the summer there are three or four generations of Monarchs, and these summer adults live from two to five weeks (fig. 7). However, the final generation of Monarchs, which emerges in early fall, has an additional job: migrating to central Mexico or California to survive the long winter. This final generation can live for 8 to 9 months.

Male or Female?
Male and female Monarchs are easy to tell apart. Males have a black spot on a vein on each hind wing that is not present on the female. Also, females often look darker than males.

In all the world, no butterflies migrate like the Monarchs of North America. They travel much farther than all other tropical butterflies - up to three thousand miles. They are the only butterflies to make such a long, two-way migration every year. As with salmon who return to their spawning grounds, no one is really sure how the Monarchs know the way to Mexico. To complicate matters, the Monarchs who fly south are not the same ones that come back to Pennsylvania. On their way back north from Mexico, the butterflies lay eggs through the southern states and then die. These offspring lay eggs along the way north to Tennessee, and then they die. Therefore, in our area, it is often the granddaughters of the original migrating Monarchs that actually complete the return trip!

What do they eat?
Larvae (caterpillars)
Monarch larvae only eat one thing: milkweed leaves. A female will usually lay only one egg per milkweed plant to make sure there is enough food for each larva. Most species of milkweed are toxic to plant eaters. When Monarch larvae ingest milkweed, the plants' toxins accumulate in their bodies and wings. Predators learn to avoid Monarchs because the larvae and adults taste bad and may even make them sick.

Adult butterflies
Nectar from flowers, which is about 20% sugar, provides most of an adult Monarch’s food. Monarchs use their eyes to find flowers, but once they land on a potential food source, they use taste receptors on their feet to find the actual nectar. To attract Monarchs in western Pennsylvania, you can plant such flowers as Butterfly Bush, Marigold, Black-eyed Susan, and, of course, Milkweed.

Monarch Watch
The University of Kansas has a long-running program called Monarch Watch, in which butterfly lovers of all ages attach special tags to the wings of migrating monarchs (fig. 8). Volunteers along the migratory path have a capture-and-release system for documenting the tagged butterflies, and the information is entered into a database. The University of Kansas recently made the database accessible online, and you can learn more at

Butterfly trivia:

  • The Aztec believed Monarchs to be the incarnation of fallen warriors, wearing the colors of battle.
  • In German folklore, butterflies were thought to be witches trying to steal cream from the milk pails.
  • Many cultures relate butterflies to spirits or the human soul. In several languages, the same word is used for both “butterfly” and “soul.

Artwork: Mark A. Klingler Text: Cathy Klingler

Sources: and Dr. John Rawlins, Carnegie Museum of Natural History


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