Transformation: A Swallow-tale

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“Transformation…Transformation is a marvelous thing…I am thinking especially of the transformation of butterflies. Though wonderful to watch, transformation from larva to pupa or from pupa to butterfly is not a particularly pleasant process for the subject involved. There comes for every caterpillar a difficult moment when he begins to feel pervaded by an odd sense of discomfort. It is a tight feeling — here about the neck and elsewhere, and then an unbearable itch. Of course he has moulted a few times before, but that is nothing in comparison to the tickle and urge that he feels now. He must shed that tight dry skin, or die.” - Vladimir Nabokov, On Transformation (1951)

Emerging in the spring with warming weather, the adult Western Tiger Swallowtail (Papilio rutulus) hangs from its chrysalis, allowing the blood-like hemolymph circulating through its body to fill its four wet, wrinkled wings. Soon it takes flight, a flashing yellow-and-black blur four inches across that seldom stops to rest—such is its exigency. In a month’s time it must find a partner and mate, and the female must lay her hundred or so eggs on the leaves of their preferred host species: cottonwoods, willows, aspens. During this period the adults eat little, and with the summer ending and the eggs laid, they die.

The tiny green eggs hatch within four or five days, and hence begins the bulk—almost seventy percent—of the swallowtail’s life. This is the larval stage, and mostly it revolves around eating. The caterpillar is a defoliating machine, equipped with mandibles for mowing down leaves and a long digestive tube to deal with the copious roughage. Unsurprisingly, all this eating leads to prodigious growth in a short period of time. Western swallowtails undergo a molt, or ecdysis, five times throughout their larval lives, each shedding triggered by a cascade of hormones as the larva matures. Between molts, the larva is known as an instar, and in swallowtails the instars follow an interesting morphology. Early instars look like bird droppings, the better to avoid predation by cater-pillaging passerines. Poop, as it turns out, seldom invites gustatory interest. Later instars take on a snake-like appearance, turning bright green and developing distinctive eyespots—a rather unconvincing ruse to us, but one apparently worth adopting.

After six to seven weeks of eating, ecdysis, and eluding predators, the caterpillar undergoes a final transformation, into a pupa. It attaches itself bottom-up to a suitable surface with a pad of silk and sheds the last bit of its larval skin. Underneath is a chitinous pupal casing called the chrysalis—another hormonally-induced integument—and it is within this casing that the real magic of metamorphosis occurs. All through the caterpillar’s life, its body contained specialized groups of cells called imaginal discs that lay quiescent during its development. In pupation the discs spring to life, giving rise to the appendages and accessories of the imago, or adult: wings, legs, thorax, abdomen, antennae, compound eyes, proboscis, scales. All of the larval cells are broken down and re-appropriated as fuel for the imaginal discs, a genetically-guided restructuring from within. This is known as complete metamorphosis, and there is nothing quite like it in the world.

Pupation lasts anywhere from a mere fifteen days in the summer to several months, when cooler weather forces end-of-season pupae to go dormant until the spring. These overwintering chrysalises turn a drab brown to match their surroundings—a stark contrast to the pale green chrysalis of summer. With the warming sun the pupae are bestirred, and the swallowtail cycle starts anew.