Minnesota Night Life
A glimpse of the fleeting existence of moths.
Photography by James Sogaard. Text by Susan Weller.
On a warm evening, beneath a moonless or cloudy sky, from early spring to the killing frosts of autumn, both dazzling and demure creatures step out for their brief adult existence. They are Minnesota’s moths. Many adult moths live a week or less—just long enough to find mates and lay eggs. Females sit quietly on a plant and emit an airborne chemical mixture (pheromone) unique to their species. The pheromone becomes a plume of scent borne on the breeze. Up to five miles away, a single molecule landing on a male’s antenna is enough to cause the male to search for the female, and he travels upwind following her scent.
Even during their short lives, adult moths do need to drink. Some species have rudimentary mouthparts capable only of sponging water from small puddles; others have a well-developed proboscis for sipping nectar from flowers.
Nectar-feeding moths are often strong fliers. Sphinx or hawk moths, familiar to gardeners, hover like hummingbirds at large, trumpet-shaped flowers. The nectar fuels their arduous flight and search for mates and egg-laying sites.
Most moths feed at night at sap flows—places where sap leaks through the bark of a tree—especially in early spring before flowers bloom. Go out with a flashlight at night, and you will see them gathering like pigs at a trough, sucking up sap.
Caterpillars actually do most of the serious feeding (see sidebar on metamorphosis), but only a few species are economic pests. Caterpillars of most species are not harmful, and they are an important link in the ecosystem. Many birds and small mammals eat caterpillars and pupae. Furthermore, caterpillars are great recyclers, returning 2 to 5 percent of the forest biomass to the ground in the form of castings rich in nitrogen, potassium, carbon, and other nutrients.
To escape predators, many caterpillars are camouflaged by color and shape and only move to feed at night. Caterpillars resemble their plant hosts, or they have disruptive coloration. Colors and structures that break up the caterpillar’s outline or match the background render them practically invisible.
Some species are protected by spines and toxins. They advertise their toxicity to potential predators with bright warning colors. Other species feed only on plants with particular toxic chemicals, and then use these plant toxins for their own defense. Like monarch butterfly caterpillars, milkweed tussock caterpillars gain protection by eating milkweeds, which contain compounds toxic to birds and mammals.
Moths are members of the insect order Lepidoptera, meaning "scaly wings." They occur on all continents except Antarctica. There are more than 200,000 species of Lepidoptera. Even though butterflies are the most obvious and best-known lepidopterans, they make up just 10 percent of the species in the order and are really just specialized moths. Just as hummingbirds are highly modified songbirds, butterflies are highly modified, day-flying moths.
How many moth species live in Minnesota? We don’t know, because so many of the smaller ones (micro-Lepidoptera) haven’t been collected or described. Illinois, with a less diverse flora than Minnesota, has recorded more than 1,100 species of Lepidoptera, but records are incomplete for some of the largest moth families.
Insect habitat needs are complex and poorly understood. Many of our moths need overwintering sites as well as plant hosts for caterpillars and nectar sources for adults. The more we learn about moths, the better able we will be to conserve them and ensure that future generations will enjoy this beautiful Minnesota night life.
James Sogaard, Long Lake, is a free-lance photographer and amateur entomologist. Susan Weller is curator of lepidopterans at the Bell Museum of Natural History, Minneapolis. Her childhood fascination with caterpillars led to a career studying moths.
Metamorphosis: Adaptation to Two Niches
The vast majority of insects—including beetles, flies, ants, wasps, bees, butterflies, and moths—undergo complete metamorphosis. This radical transformation requires the death-defying disintegration of the insect in its juvenile form. After a drastic reorganization of its tissues, the insect emerges as a sophisticated adult to begin a new and altogether different life.
As a winged adult, the insect is able to travel farther and faster than it could as an earthbound larva. It can seek out different foods, so it doesn’t compete with its young. Its circle of available mates expands greatly.
Metamorphosis is a successful survival strategy. Nearly 90 percent of all insect species—more than half the known living species of animals in the world—undergo complete metamorphosis. Moths are an attractive and easily observed example of such insects.
Moth larvae live in terrestrial (rarely aquatic) environments. They feed almost exclusively on plants. Every one of their juvenile physiological features is geared to survival as plant-eaters. Moth larvae have simple eyes, in most cases needed only to perceive the edges of leaves. They have legs and fleshy abdominal prolegs for clinging and grasping, musculature to creep about, and powerful mandibles for cutting and chewing plant material.
Juvenile hormones suppress adult development, allowing the larva to remain a well-adapted eating machine. As it eats, the larva outgrows its "skin," which is actually a flexible external skeleton, or cuticle. Complex hormone cycles allow the larva to grow a new, larger cuticle and shed the old, smaller one. Some species molt up to seven times, outgrowing their skin quickly as they increase in weight several thousand times from egg to final molt. The phases between molts are called instars.
At the onset of the pupal stage, a larva searches for shelter. It will soon become immobile and helpless for two weeks to more than a year, depending on the species. Larvae of many species have well-developed silk glands and spin cocoons around themselves, often with a leaf. A few species burrow naked into the soil.
With the last molt, a tough, thick pupal skin encases the entire insect, which appears at rest. Inside, however, a tumult of cellular activity ensues. Pulsing steroids direct digestive juices to destroy larval tissue. Spared groups of formative cells, awash in the pulpy, creamy debris of disintegrated tissue, begin to build the body anew, generating a sexually mature adult.
Emerging from cocoons fully developed for flight, adult male moths soar off to find mates. Males bear two pairs of wings, wing musculature, and new legs for perching.
Not all adult female moths are so mobile. Some lack wings or the muscles for flight. Instead, their bodies are devoted to egg laying. They disperse volatile pheromones to attract mates.
Adult moths’ lives, devoted largely to reproducing, are usually brief. Because they are not growing or developing, adults don’t need nearly as much food as larvae do. As a result, their mouthparts have changed completely. Gone are the powerful cutting mandibles. Maxillae become the long, tubular proboscis, which locates and sips fluids to sustain flight, mating, and egg production. Other anatomical refinements of both males and females that aid them in their newfound mobility include compound eyes, antennae with keen scent receptors, and ultrasonic hearing in some species.
Metamorphosis has allowed insects unprecedented staying power in the evolutionary scheme of history. Almost all insect orders present in the Triassic period, some 225 million years ago, still exist today. No higher order of animal can boast such success. Through sheer numbers, insects have become the true inheritors of the earth.
—Susan Kaneko Binkley