Speaking of Nature

Speaking of Nature: Royal life of the monarch

It may seem a little early in the year to start thinking about monarch butterflies, but I have spent a great deal of time thinking about insects this summer and I have come to realize (especially with butterflies) that my general understanding of them is somewhat biased. I tend to fixate on the large, flashy adults that flit through the meadows of late summer and add a touch of whimsy to the feeling of a day. This puts me firmly in the majority.

What I tend to forget — and I suppose this is easy enough when you really give it some careful consideration — is the fact that the adult phase of a butterfly is but the end of its life. There are many stages the organism we call a butterfly must pass through before it looks like a butterfly.

With the monarch, we can also add an incredible story of natural history to the already amazing story of the physiological changes that the organism goes through. So why don’t we start in the Northeast, in August, and try to track the various stages of the life of a monarch butterfly. Then, just for fun, we’ll expand the story, which really never ends, to include all of the events that might take place in a full calendar year.

In the beginning of August, all across the Northeast, monarch butterfly eggs started hatching. Each egg was laid on a separate milkweed plant, which would serve as the young monarch’s food supply.

Once free of their eggs (which look like tiny jewels) each of the tiny caterpillars started eating … and growing. It’s important to remember that butterflies, like all insects, have no internal structures that would be analogous to bones. Instead, they have a sort of rigid external shell called an exoskeleton. When humans grow, they add bone to their skeletons and then add muscle and skin to stretch around it. Insects have to do something quite different.

As insects continue to add tissue to the insides of their bodies they stretch their outer “skin” tighter and tighter. Eventually, there is no stretch left and the only solution is to get rid of the old skin and replace it with a larger one.

When a young monarch butterfly (a larva that we call a “caterpillar”) decides to shed its skin, it stops eating and puffs itself up. Muscles flex and strain until the skin bursts open, revealing a fresh new skin decorated with beautiful black, white and yellow stripes. This new skin has plenty of room for growth and the caterpillar gets back to the important business of eating.

These progressively larger and larger versions of the same basic organism remind me of a set of Russian nested dolls. The only difference is that instead of getting progressively smaller, the caterpillars get progressively larger. As with the nested dolls, there are recognizable steps in the progression and entomologists refer to these steps as “instars.”

Adult female monarch butterflies are very careful to lay their eggs on milkweed plants and the caterpillars have to be very careful living on them. Milkweed plants were named for their thick white sap, which is loaded with latex. It’s the latex that gives the sap its white color, but it also makes it a gluey, sticky mess. And, just for fun, the sap is also loaded with toxic chemicals.

Both the latex and the toxins are used as protective measures against hungry insects, and both can cause death. The toxins kill chemically, and the sticky latex can physically kill by suffocating anyone unfortunate or careless enough to get a face full of sap that glues the mouth shut. But monarch caterpillars know how to avoid these problems.

Somewhere in their genes, monarchs carry instructions for the safe consumption of milkweed. The caterpillars normally start their lives on a milkweed plant, so finding food isn’t really a problem. When they are very small they can only attack the edges of the leaves, so they remain relatively safe. It’s only after several instars (a stage in their life between two successive molts), when they start to get large, that their brains instruct them to find the veins that deliver the sap to the leaves and carefully sever them.

Once the flow of sap is reduced, the caterpillars can resume their incessant feeding with far less danger of getting a face full of pressurized, latex-ridden sap. Furthermore, with the flow of sap reduced, they are able to eat the tissue of the leaves without ingesting a fatal does of the toxins.

However, the toxins are still potent enough to make the tissues of the caterpillars themselves toxic. Any bird that tries to eat a monarch caterpillar will regret it. So the caterpillars decorate themselves with bright colors to make themselves easy to see and easy to recognize. A few monarchs die every year at the hands of inexperienced birds, but the message gets across pretty quickly and the remaining monarchs are safe.

After several instars, the caterpillars grow large enough to start the next phase of their lives. A safe place must be located (usually under a large leaf that can provide a little protection) and then a little silken anchor chord glues the caterpillar in place.

Then the caterpillar sheds its skin one last time. What emerges is technically a butterfly (as is the caterpillar) but it no longer looks like an insect at all. Instead of a caterpillar with a recognizable head, or a butterfly with head, legs and wings, what emerges is a beautiful green chrysalis. The organism now looks like a mint-green ice cream cone hanging upside-down in which the tissues of the organism go through astounding two-week rearrangement into a butterfly.

When the chrysalis is finally “ripe,” it cracks open and an adult butterfly crawls out. The butterfly is wet, mushy and extremely vulnerable, but it is also still toxic (a fact advertised with brilliant orange wings). The butterfly pumps fluid into its wings to stretch them out to full size and then they need some time to dry and harden. Once these final measures are seen to, the butterfly can flap its wings and take to the air.

The new monarch butterflies will not find milkweed flowers to feed on, however. Instead, they feed on the goldenrods and asters that paint the fields of our area with yellows, whites and purples in the autumn. They load up on energy-rich nectar and then headed south before a killing frost takes their lives.

The destination of every monarch butterfly is central Mexico, where all of the Monarchs of eastern North America congregate in a fir forest. In all, there are just nine congregation colonies, each containing millions of butterflies that survive by simply hanging in place, month after month, without a single meal. In February, the survivors head north to search for milkweed plants. Males and females mate on the way, and by the time the females reach the first milkweed plants in Texas, their eggs will be ready.

Once a female lays all of her eggs, she will die. Her eggs will hatch, the caterpillars will grow, molt and transform into butterflies. Males and females will mate, and the females will head north again, following a band of blooming milkweed flowers that moves north with the spring.

The female monarchs that finally reach Massachusetts represent the third or fourth generation of monarchs for the year. What is truly amazing is that each of those females carried genes with instructions for the southward migration that they would never make.

Finally, sometime in August, a new generation of monarchs will hatch, grow and follow these precious instructions that call for a turn to the south and a long journey to an unknown, but oddly known, destination.

The caterpillars have hatched and are starting to grow. If you can locate some milkweed plants, see if you can find yourself a beautiful monarch butterfly as it eats and grows.


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