Here you see several Monarch larvae, or caterpillars, eating milkweed. In the wild you won't find more than 1 or 2 on a plant, but in captivity the population density can become very high. They get along together quite well. Milkweed is the Monarch larva's only food. The plant's thick, glue-like sap is poisonous to most creatures. The Monarch assimilates the poisons into its own tissues, making it an unpleasant mouthful to birds and many other insect eaters.
The caterpillars of many butterflies and moths are commonly known as "worms", even though they really aren't worms at all. For example, the "cabbage worm" is actually the larva of the common white butterflies you see everywhere. The "tomato worm" and "tobacco worm" are the larvae of hawk moths, also known as sphinx moths, which are about the size of hummingbirds and hover similarly when feeding on nectar. "Apple worms" grow up to become small moths. "Corn earworms" become medium-sized moths.
Most varieties of larvae are difficult to raise in captivity. They wander away from their food plants and become lost, or they have difficulty getting along with one another. The Monarch doesn't do those things, so it is one of the easiest caterpillars to raise. The ones you see above aren't in any kind of cage or box; they're on plants that sit openly in a vase on a table.
One female butterfly may lay as many as 500 tiny eggs, one at a time, on the leaves of various milkweed plants. In the wild, the number that survive to become butterflies can be counted on your fingers. Spiders and other predators consume the eggs and young larvae. Parasitic flies lay eggs on larger larvae; their maggots consume the larva from the inside out, usually waiting until after it has become a chrysalis before they bore their way out. People who have the larvae's favorite food growing nearby may find chrysalises on their eaves, gutters, fences, and garden plants; not realizing what these are, they destroy them.
By bringing them indoors, we shield them from their predators, increasing the survival rate to over 90%. This is achieved by collecting them as eggs and raising them indoors throughout their larval and pupal stages. It takes an experienced eye to readily discern a creamy yellowish-white Monarch egg on the underside of a milkweed leaf. Other objects of similar color and size may be found there, but upon close examination with a magnifying glass the Monarch egg is found to be exquisitely beautiful. It is not perfectly round, but is drawn up to a narrow tip on the end away from the leaf's surface. It is not smooth, but is faceted with several rows of indentations, radiating out from the tip and down the sides.
About three days after it is laid, the egg darkens, becoming gray on the sides and black at the tip. The caterpillar's skin has formed and is showing through the translucent shell. Soon it chews its way out, turns around to devour the rest of the shell, and then wanders a short distance away to begin munching milkweed leaves.
The newly-hatched larva is only a little more than a millimeter in length and has a gray body with a shiny black head. After a couple of days it has filled out its skin, which has stretched to about 3 mm long and now shows distinct banding. It becomes dormant for several hours as a new, looser skin forms within. The old skin splits and out crawls a much more comfortable caterpillar that exhibits the characteristic yellow, white, and black striping of the Monarch larva.
Molting is a major ordeal for the caterpillar. It typically seeks a place of solitude, where it hopes to remain undisturbed while waiting for the new skin to finish forming within. It spins a thin mat of silk into which it digs its rear prolegs so that the skin will remain attached to the surface as the caterpillar wriggles free.
Immediately after molting, the caterpillar's head and legs are pale in color, and the old head covering remains attached to the caterpillar's face, as seen in the photo below. The caterpillar thrashes its head from side to side, rubbing the old head covering against the surface until it loosens and falls away.
It will molt in this manner three more times before reaching its full length of about 6 cm. After each molt it rests awhile as the new, larger mouth parts harden. Then it turns around to eat its old skin before going back to its business of consuming milkweed to fill out its new skin. The larvae shown above and below have just molted into their final growth stage.
Rarely, a molt may go wrong. Part of the skin may remain attached to the caterpillar's posterior end, or the old head covering may remain attached to its face. In either case, the caterpillar can no longer eat; it has no choice but to slowly starve to death.
A larva in its final growth stage will consume several milkweed leaves over a period of about three days. The total time from hatching to pupation is almost two weeks. When it is nearly ready to pupate, the larva becomes restless and may wander away from the plant in search of a sturdy, sheltering support from which to suspend itself by its rear legs.
From this position it will pupate.
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