The changing natural world at our doorsteps | Illustration and text by Patterson Clark
March 19, 2013
Warmer weather liberates the mourning cloaks
As winter's gloom surrenders to a mild day, a locally scarce and enduringly mysterious butterfly emerges from the bark crevice where it slept for months in the cold.
Flitting and sailing between bleak, bare branches, the mourning cloak drifts by in somber attire, its wings draped in dark, mahogany velvet. But hinting at brighter days to come, the edge of its robe is hemmed with a bold and undulating yellow-white border — the petticoat of spring.
Springtime is fling time for the butterfly. "Males set up a mating area at certain times of day and court females that fly by," says Bob Robbins, the Smithsonian's curator of Lepidoptera. Soon after a successful courtship, a female will seek an elm, willow, hackberry or poplar tree on which to coat a twig with densely packed, barrel-shaped eggs, each starred with eight or nine radiating ribs.
A larva hatches within 10 days to feed on tender tree leaves. As it matures, the caterpillar assumes a distinctive Goth garb: black and spiny, speckled white with a single row of blood-red patches down its back.
You might assume that it would be a loner, but the caterpillar sticks close to its siblings, which feed together in a bristling, tight-knit group.
But by late spring, each caterpillar will splinter off to undergo metamorphosis in a chrysalis strung up beneath a leaf. A new generation of adults takes flight in June, feeding occasionally on flower nectar but most often on oak sap and mammal feces.
What happens next is a mystery. "You will see adults in June or July, but not again until September," Robbins says.
Adults might enter dormancy for the hot months and reemerge in the fall to find a place to spend the winter, or perhaps they produce a second brood that emerges in
September to prepare for hibernation. Whether it's one brood or two "has been unresolved for more than a century," Robbins says. "It remains controversial."
Late-summer caterpillars haven't been found in this area, but if they are present, the leaves they'd be eating would be much tougher than spring leaves and packed with hard-to-digest chemicals.
Fall adults are sexually immature, but "it may be that the mourning cloak is similar to two of its close relatives, the comma and the question mark, two common early-spring butterflies that reach sexual maturity after winter
hibernation," Robbins says.
Another enigma: Like its close relatives, which migrate in the fall, mourning cloaks are sometimes seen flying in a southerly direction in the autumn. "It's possible that mourning cloaks migrate, but whether it's a regular occurrence is questionable," Robbins says. "We don't know where they migrate or why."
The butterfly is much more common in New England than in the Mid-Atlantic. "One would think that if they migrate, we'd see large numbers overwintering here," Robbins says, "but we don't."