Moths, along with butterflies, belong to the order Lepidoptera, the second largest order of insects. They possess two pairs of wings covered with microscopic, overlapping scales having distinctive colors and patterns. The body and legs are covered with scales, or with long, hairlike bristles. The adult lepidopteran lacks mandibles (found in most other insects); it feeds on liquids, mostly nectar, by means of a long proboscis, which is coiled in a spiral under the head when not in use. The proboscis is extended by blood pressure and coiled by muscular action. In some instances, the adult moth does not feed and lacks the proboscis. The head of the adult insect bears a pair of large compound eyes with many facets, and usually also a pair of simple eyes or ocelli. The antennae are long and many-segmented. The lepidopteran life cycle has four stages: the egg; the larva (called caterpillar), which molts several times as it feeds and grows; the pupa, which is a non-feeding, quiescent stage; and the adult (or imago).
The larva is usually the most conspicuous stage in the life cycle. It feeds voraciously, usually on leaves, but in various cases by boring in stems, or fruit, or stored grain, or by chewing on fur, wool, or other material. Unlike the adult, the larva has strong mandibles and other mouth parts for cutting and chewing its food. The larva bears several pairs of simple eyes, but no compound eyes. Each of its three thoracic segments bears a pair of typical many-jointed walking legs, which become the legs of the adult. In addition, several of its ten abdominal segments bear a pair of soft, unsegmented, "prolegs" with hooklike processes, which help in crawling and grasping. The larva of any given species undergoes a specific number of molts before transforming into pupa. The larval skin may be naked, or covered with long hairs, bristles, or spines. Also, many larvae have characteristic colored stripes or other distinctive patterns. The larval stage is frequently highly destructive to crops, fruit and shade trees, stored grain and flour, and other products.
Despite the fact that moths comprise the vast majority of the order Lepidoptera, adult moths are not as commonly seen as butterflies. This is because, while butterflies are active during the day, most moths are nocturnal, and spend the daylight hours resting on tree bark, among leaves, and in secluded areas. Many of them have wing colors and patterns which help to camouflage them in their natural surroundings. Easily recognizable differences between adult butterflies and moths make it easy to distinguish them from one another. For example, butterfly antennae are clubshaped at their tips, while moth antennae taper to a point and frequently are feathery. A moth's body is short and stout and much more hairy than the relatively slender body of the butterfly. A resting butterfly has its wings folded together over its back, while the wings of a moth at rest are spread out. Although some moths are brightly colored, the majority are rather drab. The pupa of a moth lives either in a silk cocoon, or, if naked, hidden under debris or rocks. In most butterflies, the pupa, called chrysalis, is naked and exposed. In flight, both moths and butterflies have the forewing and hind wing of each side coupled by a hooklike device. In size, as measured by the wing span, moths range from less than 0.8 in (25 mm) to over 12 in (30 cm). In a number of moth species, males and females differ in size, color, characteristics of the antennae and of the mouth parts, and other features.
Of the more than 50 families of moths, the following are particularly noteworthy. The family Tineidae includes the clothes moths, whose larvae feed on woolens, furs, and other textiles. Three major agricultural pests in the United States, the "Angoumois grain moth" (which destroys stored grain, especially corn), the potato tuber worm, and the pink bollworm (which damages the blossoms as well as the seed pods of the cotton plant) belong to the family Gelechiidae. Many of the common moths attracted to light at night are members of the family Tortricidae. The spruce budworm, which is a serious pest of pulpwood in North America, also is included in this family. Pyralidae is a large family, with over 1,100 North American species. The European corn borer (which bores into corn stems), and the sod webworm (a pest of turf grasses) are among the most destructive members of this family. Also included here is the wax moth, whose larvae live in beehives and feed on beeswax. In some species in this family, adult moths have a pair of "tympanic membranes" capable of detecting the ultrasonic squeaks of bats. One member of this family, the South American cactus moth Cactoblastis cactorum, has been used successfully in biological control of the prickly pear cactus in Australia. The cactus itself, soon after its introduction into Australia from the Americas, reproduced prolifically and became a major pest on millions of acres of agricultural land. Introduction into Australia of the South American cactus moth, whose larvae feed exclusively on the prickly pear, has almost completely eliminated the host plant. Hawk moths and sphinx moths (family Sphingidae) are large, with a proboscis that is longer than the body when uncoiled. They are strong fliers. The larvae of the sphinx moth, called hornworms, are pests of tomato and tobacco plants. Another large family, with approximately 1,200 North American species, is Geometridae. The larvae of geometrid moths are the familiar inchworms, which move by a "looping" movement, rather than by crawling, since they lack the prolegs on all but two abdominal segments. Inchworms feed on leaves of fruit and shade trees and on vegetable plants, and can be highly destructive. Moths in the family Saturniidae are among the largest and most colorful, with wing spans up to 12 in (30 cm), and large "eyespots" or other markings on the wings. The family Lasciocampidae includes the "tent caterpillar" moths. Their larvae, which hatch from eggs in early spring, construct silken communal "tents" on branches of the crab apple and certain other trees. Their feeding activity almost totally defoliates the host tree. One of the most beneficial and interesting moths is the commercial silkworm moth, Bombyx mori, in the family Bombycidae. This moth, native to China, has been bred in captivity for thousands of years, and is the only example of an insect that has been completely "domesticated," as it no longer exists in the wild. The larvae of the silkworm moth are "fed" mulberry leaves. The long domestication has led to the degeneration of the larva's prolegs, and hence to its inability to climb trees. The source of commercial silk is the cocoon of the pupa. In order to obtain the silk, the pupae have to be destroyed by immersion in hot water, since allowing the adult moth to emerge from the cocoon would break the long, continuous silk thread. Noctuidae is a very large family, with over 3,000 North American species. Larvae of noctuid moths include such serious pests as cutworm and corn earworm. Cutworms are especially destructive of vegetable seedlings which they cut off just above the ground with their powerful mandibles. Tussock moths and gypsy moths belong to the family Lymantriidae. Tussock moth larvae cause extensive defoliation of Douglas fir forests in the Pacific Northwest, while the larvae of gypsy moths play a similar role in the destruction of deciduous trees in the northeastern United States.
Many adult moths are instrumental in pollinating flowers in the course of their feeding. Female moths produce and release into the environment a hormone which attracts males for mating. (Such hormones, which exert their action outside the body of the organism are called "pheromones.") Many pheromones have been chemically analyzed, and a few of them are now produced in the laboratory. Such products are increasingly used as bait which is placed in traps to attract and kill harmful insects, and thus to control their populations. A number of moth species are known to undertake seasonal migrations over long distances.
Borror, Donald J., and Richard E. White. A Field Guide to Insects: America north of Mexico. Peterson Field Guide Series. Boston: Houghton Mifflin, 1987.
Carde, Ring, and Vincent H. Resh, eds. Encyclopedia of Insects. San Diego, CA: Academic Press, 2003.
Covell, C.V. A Field Guide to the Moths of Eastern North America. Peterson Field Guide Series. Boston: Houghton Mifflin, 1984.
Evans, H.E. Insect Biology: A Textbook of Entomology. Reading, MA: Addison-Wesley, 1984.
Holland, W.J. The Moth Book: A Guide to the Moths of North America. New York: Dover, 1968.
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