Adult Monarch
The monarchs are flying! By the tens of thousands, these carefree, luminous little creatures have been floating around our town for several weeks now, riding the updrafts and sailing by my office window, probing flower beds with their busy probosces, and congregating soundlessly in pine trees along the golf courses, doubtless discussing plans for the next stage of their journey. For aimless as they may seem, they abide with us for only a little while, a lunch stop along the path of a carefully conceived, almost unimaginable migration. Aha! —-but therein lies a tale.
Monarch Larva
Butterflies have held a treasured place in human history for thousands of years, and their images can be found in petroglyphs, revealing the ancient view that they were sent by gods as messengers. Chaucer knew them as boterflyes, the Dutch as botervliege—and the source of the term is obscure. From James A. Scott’s handsome text we learn of the extensive studies of butterfly biology, perhaps the most remarkable feature of which is metamorphosis. The process begins with the fertilized egg, about the size of a pinhead, placed with surgical precision on the underside of a milkweed leaf by the adult female. In 3-12 days a gaily striped caterpillar emerges and instantly begins feeding on the milkweed plant. Within two more weeks the larva multiplies its original weight 2,700 fold, an elephantine growth which, if applied to a six-pound human baby, would mean an adult weight of 8 tons. After shedding its skin five times, the larva attaches to a perch, weaves a dense mat of silk, and sheds again to become a blue-green pupa (a chrysalis larva, from the Greek khrusos, gold). This becomes transparent in about two weeks and cracks; the adult gingerly appears and begins pumping fluid into its fleshy wings. The adult Danaus plexippus then flies off to seek its fortune. Thus the caterpillar leads a ponderous, gluttonous existence and moves only a few feet; the adult enjoys a far-ranging existence but eats little.
A few functions of the adult are worth scrutiny. Unlike birds, the flight machinery of butterflies involves very tiny muscles, only 2-3 mm in length, attached to long tendons; and these do not connect the skeleton to the wings, but instead connect segments of the thorax. The function of these parts can be represented by the two halves of the thorax, which is a rather rigid, chitinous structure, and the lower half resembles a rowboat. The wings resemble the oars of the rowboat. The upper half of the thorax is hinged in the middle, and when the longitudinally aligned flight muscles contract, the upper half bends upward, forcing the wings down; the opposite happens when the muscles relax.
Spring mating in monarchs is carried on with enviable gusto, lasting 2-3 hours for each mating. Monarchs lack pheromones, the attractant chemicals by which most animals find and identify each other, and rely instead on visual and tactile signals to determine readiness for mating. The male twists his abdomen sideways, his valvae grasp her abdomen, and he turns to face away from her and injects a spermatophore into her mating tube. Heliconiini, the passion-vine butterfly, by contrast, not only produces pheromones for mate attraction, but also for mate repulsion at the end of mating, employing a structure with the indelicate name of a “stink club”.
One of the most enduring mysteries of the monarchs had been the location of their hibernation grounds. Thanks to the patient toil of Fred Urquhart, PhD, a Canadian zoologist, this mystery was solved and beautifully described in National Geographic, August 1976. He located a 20-acre site high in the Sierra Madre of central Mexico, far from the smog and turmoil of city life. At 9,000 feet elevation, winter temperatures hover from just below freezing to just above—a perfect arrangement for these cold-blooded insects to live in semi-hibernation, burning little of their fat until they begin their flight north in spring. Following reports of observers in Mexico, and confirming the observations with tagging experiments, Urquhart came upon the site and found it populated by oyamel trees. Myriads of monarchs were roosting in the trees and carpeting the ground, bending the tree limbs with their weight. The life cycle therefore begins typically with birth in the upper Midwest and New England. These gallant little aviators begin their migration to Mexico in the summer and fall, and spend the winter in Mexico. They then migrate all the way back to Canada and New England the following spring, a round trip of 6-8,000 miles, with no instructions from outside sources. Death occurs at 8-12 months age after reproduction.
I pondered these points the last time I drove north, and saw a monarch at least every 200 yards, almost all of them flying lazily across the road at a perilous altitude of 5-15 feet—and almost all were heading due west, re-enacting once again an annual migration that must surely have begun long before man came to North America. The purpose of a this, if any, is quite beyond me; I can’t take it all in. Yet it seems grander than the squabble of nations over oil fields, or the weekly displays on our football fields and baseball diamonds. It is a pageant for the ages.
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