During its lifetime, an individual Monarch butterfly will have eight completely different outer surfaces, each in a different size, shape, and color combination. Four generations will typically complete the life cycle in a year. The generations overlap to some extent, with most butterflies flying for up to two months while the final generation (the individuals who hibernate) have a life span of up to eight months. One adult female will typically produce 300 to 1100 eggs...which tells us something about the survival prospects of a butterfly. The one, two, or ten eggs that become adult butterflies have literally been naturally selected as the fittest to survive out of a thousand.
The life cycle of a butterfly includes four distinct phases: egg, larva (growing through five changes of skin, or instars), pupa, and adult.
Monarch Butterfly Eggs
If possible, the female Monarch will place just one egg on each of about a thousand milkweed plants. She may or may not ever see the baby butterflies that hatch from those eggs. She has no noticeable instinct to watch her babies grow up, or interact with them in any way while they are growing. Her only maternal act is to try to give each potential caterpillar enough food for it to eat without destroying its food plant.
Monarch butterfly eggs are whitish, egg-shaped, typically a little more than a millimeter long and a little less than a millimeter wide. Bigger butterflies tend to lay bigger eggs, and individuals tend to lay their biggest eggs first. You can see the egg on the underside of a milkweed leaf if one is present. Under a magnifying glass you might even see the little ridges on the sides of the egg.
Monarch Butterfly Larvae
Caterpillars usually hatch in three to eight days. They are already “very hungry caterpillars." The first thing they eat is their own empty eggshells. After this, the only animal protein meal they will ever need, they become vegetarians, nibbling little holes in milkweed leaves. Milkweed is their only food. Monarch hatchlings are nondescript pale greenish animals. Barely two millimeters long when they stretch themselves out of the eggshells, their translucent bodies grow up to six millimeters long during the few days they spend outgrowing their first, drab larval skins.
Second instar Monarch caterpillars have tiny bristly hairs on the skin that begins to show bands of yellow, black, and white. This skin grows up to a full centimeter long. Each of these changes of skin normally lasts less than a week.
The third larval skin is striped and begins to develop the distinctive tentacles a Monarch caterpillar uses to flail about and distract predators. (Some predators will bite off part of a tentacle, giving the caterpillar a chance to escape.) During this stage the caterpillar grows another half centimeter in length, and begins to nibble around the edges of milkweed leaves as well as making holes in the middle.
Adult Monarchs have six legs but usually stand and walk on only four, holding their front legs up around their heads. By the third instar Monarch caterpillars are starting to develop this feature, with front legs noticeably shorter than the two other pairs of legs on the thorax. (Like most caterpillars they also have four pairs of “prolegs" on the abdomen and a pair of “claspers" at the back end, and also use their jaws to hold on to things while walking. The tentacles are short and may look like even more legs.)
In the fourth instar the caterpillars finally grow a full inch long. White spots are visible on their prolegs, and the tentacles, which remain solid black, grow long enough to be whipped about.
In the final stage, as “mature caterpillars," Monarch larvae can be 5 cm long—almost three inches—and weigh as much as 1.5 grams. The tentacles are now quite long and can be lashed dramatically. Tentacle lashing is basically a stress reaction, so the fact that caterpillars reared in captivity do it more often than wild caterpillars do probably indicates that they feel stressed, rather than that they “wave hello." Wild Monarch caterpillars are likely to climb up onto your hand if you hold out a finger in front of them, while cage-reared ones may go into fits of tentacle-whipping that probably indicate that they’d prefer to be left alone.
Caterpillars whose vision has been studied seem to be so nearsighted that they probably are not able to see all of a human at one time. Monarch caterpillars don’t seem to explore much of the world around them, and may spend the whole caterpillar stage of their lives on one milkweed plant. What causes cage-reared caterpillars to seem more stressed than wild ones may be that they sense the presence of other caterpillars in the same cage in addition to the proximity of humans.
Monarch Caterpillar Becoming a Chrysalis
Monarch Caterpillar going into cocoon
The Monarch Butterfly Chrysalis
All butterflies undergo an extreme makeover in the pupal stage. When the caterpillars have eaten as much as they can, they stop eating and look for a safe place to rest. Some species spin cocoons of silk around themselves, burrow into the ground, or hide among dead leaves. Monarch caterpillars do none of these things, but attach a little pad of silk to the underside of a sturdy surface, lock their claspers, hang upside down, curl their heads up toward their midsections, and wait. After about a day in this position the final larval skin splits and drops off, revealing a smooth, rounded green chrysalis. The chrysalis is a case with no eyes, legs, or other movable parts. It shows no signs of life for about two weeks, during which the caterpillar inside is reshaping itself into a butterfly.
A few predators even smaller than the pupal butterfly can attack Monarch chrysalides, which is another good reason why, if you raise butterflies in captivity, environmental scientists recommend not trying to rear a lot of them at one time. Tiny wasps can bite through the chrysalis and kill the butterfly inside.
Monarch Emerging from a Chrysalis
Monarch Caterpillar emerging from cocoon
The adult butterfly is sometimes called the imago, the “image" by which the species is best known. It crawls slowly out of the back of its chrysalis, looking dull, wet, and crumpled. The experiences of struggling out through the chrysalis, then resting on the empty shell for a few hours, are essential in order for the butterfly to have a healthy adult life. Watching Monarch butterflies’ wings develop, as the colors brighten and the butterfly stretches, flexes, but remains on its perch, is often described as a lesson in patience. These butterflies have bigger, stronger wings than other species and take more time before they are ready to fly.
Adult Monarchs need a lot of space. They don’t form pair bonds; individuals usually mate more than once, and flutter about looking for different partners as well as different milkweed plants. Again, ecologists warn that this behavior pattern is typical of species that need biodiversity to survive, and should be allowed to roam naturally rather than “farmed" or bred in captivity.
While most big moths live entirely on fat stored up from the food they ate as caterpillars, adult butterflies require some food. They eat liquids slurped up through the proboscis, which looks like a coarse, curly hair and functions like a drinking straw. (They also touch food sources with their feet, which probably have some sense of taste or at least of succulence.) Monarchs get most of their nourishment from flower nectar. They can take nectar from a variety of sources, but their favorite flowers grow on big, tall plants and bear drops of nectar in clumps of narrow tube-shaped petals.
Like many butterflies, Monarchs seem to prefer muddy, polluted water. They need bio chemicals from milkweed, which are toxic to most animals, as part of their diet and probably get some benefits from the minerals in mud.
It’s interesting to watch Monarchs flutter and jostle among other butterflies, and other insects, at flowers and puddles. These other insects are neither predators nor competitors, so the Monarchs seem to tolerate their company more than they tolerate other Monarchs’ company. Monarchs like to crowd together only during hibernation. When two Monarchs do fly together, they are usually play-fighting. Males are usually a little bigger than females and sometimes manage to knock them to the ground, which is rougher play than is usually observed in butterflies. (Nevertheless, even Monarchs don’t hurt each other. One observer reported that about one out of three collision flights ended with the participants mating.)
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