Natural history

Egg

The number of eggs laid varies greatly from fewer than a hundred in some species to more than a thousand in others. The eggs are almost always laid in a specific way, usually on or in a suitable food plant. Eggs of many species are laid singly and widely dispersed; in others they are laid in masses that may be covered with a hardened secretion from the female’s abdominal glands. In a few groups the terminal segments of the female’s abdomen are greatly elongated and bladelike, and the eggs are laid in soft plant tissues or in narrow slits or crevices. In some of the primitive moths (family Prototheoridae) and in some butterflies, the female may merely scatter the eggs in flight in the general vicinity of a suitable food plant. Development of the embryo and emergence of the young larva is often controlled by a mechanism of physiologically enforced inactivity (diapause), which has the effect of timing the emergence of the larva to coincide with suitable conditions of weather and the growth of the food plant. Respiration in the egg is carried on through an aeropyle, a system of air passages in the shell (chorion) that enables oxygen exchange with the environment to occur whether the egg is dry or wet. In a few species of scavenger moths and pierid butterflies (see white and sulfur butterflies), the larvae hatch in the uterus of the female.

Larva, or caterpillar

The larva is the chief, and often the only, feeding stage of the life cycle. Its function is simply to transform very large quantities of plant matter into animal matter and to stay alive during the process. Most larvae molt four or five times as they grow, shedding their exoskeleton in a process called ecdysis. With plant matter being relatively easy to find and eat, it is not surprising that moth and butterfly larvae are fundamentally quite uniform, despite their apparent diversity.

Most larvae continually spin silk threads that adhere to the surfaces on which they live, thus giving them an almost unshakable grip. Silk is also used in the construction of various nests, cases, and shelters that protect the larvae from the elements and from enemies. In moths, silk also serves in the final cocoon, in which the larva will change into a pupa. The larvae of leaf miners are well adapted for life inside a flat leaf, as they are very small, greatly flattened, and more or less legless. The larvae of borers are relatively plain and unornamented, while larvae that live in the open, exposed to the attacks of predators, have evolved a multiplicity of protective adaptations of form, colour, texture, behaviour, and chemical repellents.

The proper timing of larval development and activities is important for species in which a dormant period is necessary. For example, in admiral butterflies (Limenitis) the larvae of the summer generation complete development without pause, but those that hatch in the fall hibernate while still very young. In certain groups larval development is very slow (for reasons not understood), so that there may be only one generation per year. The larvae of the carpenter moths that feed on nutrient-poor wood may take two years to develop. Larvae of many Arctic and northern species regularly have two-year cycles, because of both cooler temperatures, which slow growth rates, and the shorter growing season of their food plants. The usual number of molts (ecdyses) is four or five, but some of the small leaf miners molt only twice. When starved, the larvae of clothes moths (Tineola) have been known to have a dozen molts, sometimes accompanied by a decrease in size.

Larvae seldom travel far from the plants where they begin life. However, in some species there is dispersal of very young larvae, which hang on silk threads and are blown by the wind. Swarms of armyworms (Pseudaletia) may travel long distances along the ground, driven by crowding and lack of food. Just before pupation many larvae stop eating and crawl some distance before settling down to pupate.

Pupa, or chrysalis

The larval stage is followed by the pupa, a resting stage in which the caterpillar undergoes a major rebuilding of body tissues to emerge as a mature adult. For moths, many species pupate in the soil, with little or no cocoon; many others form cocoons in the soil, in leaf litter, or under loose bark. Some cocoons are fastened to twigs or branches or rolled in leaves. The cocoons of leaf miners are usually formed in the “mine” or near it. Cocoons are commonly of silk alone but may also incorporate larval hairs, secretions or waste products, or chewed wood pulp, trash, or leaves. There may be a special seam on the cocoon to facilitate the emergence of the adult moth. Skipper larvae make a flimsy cocoon, generally in a curled leaf where it has been feeding. Of the true butterflies, only the satyr and parnassian butterflies make cocoons; all others pupate naked. In many species the pupa hangs in a head-down position from a silk pad by a stalk (cremaster). The chrysalis of some sulfur butterflies (family Pieridae), swallowtails (family Papilionidae), and gossamer-winged butterflies (family Lycaenidae), is supported in a head-up position by a threadlike silk girdle about the body.

The duration of the pupal stage differs greatly within various groups and sometimes even in different generations of a single species. In small, rapidly developing species, it may last only a week or 10 days during the summer or for many months in cases of hibernation or estivation. Pupae have been known to remain alive for three years in abnormal conditions and still produce adults. The danger of desiccation is greatest in small and exarate pupae (those in which the appendages are not fixed to the body by a skin or sheath) and least in large or compact ones.

The adult

Although fully formed, the adult may remain quiescent within the pupal case for a long time until conditions are right for its emergence. To escape from the cocoon, the pupae of some groups have cocoon-cutting structures such as the movable mandibles of certain primitive moths, the saw-toothed structures on the head of blotch leaf miners, and the bladelike structures on the wing bases of giant silkworm moths. In puss moths (Cerura) and some others, the cocoon is partially dissolved by alkaline secretions.

Once out of the cocoon, the adult crawls upward to where it can hang with the head up and the back down. The insect then forces its body fluids into the thorax by contracting its abdomen and pumping blood into the unexpanded wing pads, causing them to take on the size and shape of the adult wings. The adult may be able to fly in a few minutes or may have to hang for several hours until its wings have stiffened sufficiently for flight.

Whereas the larva is the nutritive stage of the life cycle, the adult is the reproductive stage. Its mobility is necessary to bring males and females together for copulation and to disperse the species into new areas. Nutrition is also an essential adult function in many primitive groups, such as the mandibulate moths (family Micropterigidae). In most of the highly mobile species, much of the energy necessary for flight is obtained from nectar or other liquid foods taken in by the adult. In a few groups the adult mouthparts have become so reduced that they do not function. In these species, all nutrition is obtained during the larval stage.

Food selection by the adult

Adults locate their food sources by both sight and scent, the former being especially important in diurnal species and the latter in many diurnal and in most nocturnal species. The chief source is floral nectar, but sap (especially if fermenting), overripe fruits, homopteran honeydew (sugar-containing secretions from homopteran insects), fecal matter, and carrion are sometimes used. There are mutualistic relationships of a broad sort between species of lepidopterans with flower-visiting adults and the plants whose flowers they visit and pollinate. However, these relationships are seldom specific or obligate, since only rarely are the plant and the lepidopteran mutually dependent. Exceptions exist among some orchids and members of the morning glory family, both of which have very deep tubular flowers. These appear to be pollinated only by certain hawk moths (family Sphingidae) with very long “tongues” (proboscises). The mutualism of the yucca moths and yucca plants is obligate in that the moth larvae feed only in yucca fruits and the latter can develop only from moth-pollinated flowers. The female yucca moth has special tentacles on the mouthparts, with which it gathers and carries balls of pollen (see community ecology: The coevolutionary process).

Courtship and mating

The sequence of mating activities is usually initiated by the female, which gives off specific odorous substances (pheromones) that attract males, sometimes even before she emerges from the pupa. These are detected by structures (scent sensilla) on the male’s antennae. Males with very large, feathery antennae, such as those of the giant silkworm moths, can locate females from 5 to 6 km (3 to 4 miles) away and may form courting swarms about them. A species may have a particular time of day or night for this release of pheromones. Males of some species, such as the European ghost moth (Hepialus humuli), at dusk may form dancing swarms into which the females fly.

Once the male has located the female, a more or less elaborate courtship may ensue. The sight of characteristic colours and patterns may be a requisite for this among the brightly coloured butterflies. Male pheromones also may play an essential part. Distributed from special scent scales (androconia) on the wings, body, or legs, the pheromones ensure the receptivity of the female. Finally, the accessory genitalic structures must fit together, not only mechanically but also in such a way that stimulates sensory nerve organs of the female. Some butterfly courtships are very active. In the sulphurs, for example, the male and female fly high in the air, with elaborate behavioral sequences in which each act serves as a stimulus for the next act. Such highly specific sequences are important barriers to hybridization between otherwise similar species.

The males of many species show definite territorial behaviour, defending a particular perch and area against the intrusion of other males but darting out at passing females. Among butterflies, certain admirals (Limenitis), coppers (Lycaena), and hackberry butterflies (Asterocampa) are noted for this behaviour. Congregations of males of many butterflies about hilltops may have a certain territorial and premating function. So may the large aggregations of various butterflies living in wet places.

Migration

Many Lepidoptera are famous migrators. The American monarch butterfly (Danaus plexippus) is the only species known to perform an annual long-distance two-way migration; i.e., the same individuals fly southward in the autumn and northward in the spring. Monarchs have also crossed the Pacific Ocean, colonizing Hawaii and Australia; occasionally they reach Africa and Europe. The cosmopolitan painted lady (Vanessa cardui) stages mass flights nearly everywhere it is found. However, these are one-way flights and are therefore emigrations rather than true migrations. Many other species in Europe and North America fly northward in large numbers, often reaching regions in which they cannot survive the winters. In Europe the painted lady and many moths reach Britain and Scandinavia from central Europe. In North America the painted lady, cloudless sulphur (Phoebis sennae), and many owlet moths often reach Canada. Many spectacular emigrations occur in the tropics, where swarms numbering in the millions may fly out to sea and become lost. The best-known group having these mass movements are pierid butterflies, but mass flights of certain large day-flying uraniid moths (Urania leila and U. fulgens) have also been recorded. The usual explanation of such mass population movements, that they serve to extend the range of the species, is far from adequate in many instances.

Studies employing radar technologies revealed that migratory lepidopterans can travel at speeds of 100 km (62 miles) per hour by selecting fast-blowing wind currents a few hundred metres above ground. They also are able to identify currents that will carry them in the direction of their destination and can fine-tune their direction in response to crosswinds, enabling them to maintain their course. The deliberate selection of favourable winds combined with corrections for wind drift have been estimated to extend by 40 percent the distance that the insects are able to migrate. These behaviours, which are fundamental to the success of long-distance lepidopteran migrations, parallel those observed in migratory birds.

Environmental hazards

As primary consumers of green plants, lepidopterans are enormously important in food chains, not only because of the very large number of species in the order and the diversity of their food habits but also because of their abundance. Lepidopterans, in turn, are eaten by a host of predators, parasites, and scavengers. All stages in their life cycles are under continual attack.

The major invertebrate predators on lepidopterans include centipedes, spiders, mantids, bugs (homopterans), ground beetles, ants, and both social and solitary wasps. Important predators among vertebrates include toads and tree frogs, lizards, birds, rodents, bats, and monkeys. The invertebrates generally locate their prey by scent or sight, whereas most of the vertebrates hunt by sight. The exception are the bats, which hunt by acoustic echolocation (the so-called bat “sonar”).

The chief groups of parasites that attack lepidopterans are tachinid flies and many wasps, chiefly the ichneumon, chalcid, and cynipid wasps. More precisely called parasitoids, these insects probably have a greater impact on caterpillar populations than do the direct predators. Female parasitoids locate suitable hosts, chiefly by scent, and lay their eggs in, on, or near them. The parasitoid larvae live inside their hosts, gradually feeding on their tissues and almost invariably consuming them almost completely. Unless some of the caterpillars’ toxic or repellent secretions serve to discourage them, lepidopterans seem to have evolved few defenses against parasitoids. The high reproductive rate of lepidopterans is important in countering losses to parasitoids as well as other adversities.

Small red chigger mites often ride about on adult lepidopterans but probably do them no harm. However, a few other mites live and breed in the tympanic cavities of owlet moths, destroying their auditory structures. Curiously, these mites regularly settle in only one of a moth’s two tympanic cavities and thus only half-deafen it. It is thought that by leaving the moth with one good “ear,” the mite reduces the likelihood of the moth, and hence of the mite itself, being captured and eaten by a bat. Lepidopterans are also subject to attack by a considerable number of protozoa, roundworms, bacteria, viruses, and fungi that affect the larvae chiefly during peaks of abundance and crowding. Some of these organisms have been used by humans as a means of controlling injurious species.

Protection against danger

It is chiefly against the visually hunting predators that lepidopterans have evolved a multiplicity of defense mechanisms. The adults of many groups, such as skippers, many butterflies, hawk moths, and many underwing moths, have fast and erratic flight. When escaping, they dart or fall to cover and often remain immobile for some time. They have a good chance of survival, especially if their colour matches their surroundings. Larvae, especially when small, drop suddenly when disturbed, either dangling from a silk thread or falling to concealment on the ground. The larvae of some owlet moths can jump several inches. Dense, loose hairs and scales make many moths slippery and may facilitate their escape from sticky spiderwebs.

Certain owlet moths and possibly some measuring worm and snout moths that are subject to predation by bats are able to receive and identify the bats’ navigational sound pulses. Upon hearing bat pulses, these moths perform radical evasive maneuvers when the bat sound is loud (hence close) or dive to the ground when the bat pulse is weaker (indicating that the danger is farther away). The greater wax moth (Galleria mellonella) can hear ultrasonic frequencies close to 300,000 hertz, which is in excess of the highest echolocation frequency known to be used by bats (212,000 hertz).

Targets such as prominent coloured eyespots or tails on the hind wings attract attention and focus the attacks of predators onto parts of the body that are less vulnerable to injury. Such spots are likely to be seized and torn off, but this does the moth or butterfly no real harm and gives it time to escape without vital injury.

Many species manage to hide very effectively from predators. Cutworms and other caterpillars hide in litter by day and feed only at night. Many moths hide in crevices or under loose bark. Some of these seem to have especially flat bodies for this purpose. Hibernating butterflies spend the winter in hollow trees or among dead leaves, where they hang immobile. The larvae of a great many moths, most skippers, and many butterflies live in individual nests of rolled, folded, or webbed leaves or grass. Sod webworms live in silk-lined tunnels in turf. Wood borers, especially those in rootstocks and deep tunnels, are relatively secure. Many larvae aggregate in communal nests—e.g., tent caterpillar moths (Malacosoma), the ermine moth (Yponomeuta), and the Mexican social white butterfly (Eucheira socialis). The larvae of the bagworm moths (family Psychidae) and casebearer moths (family Coleophoridae) live and pupate in individual portable cases that are often masked with bits of leaf or twig. Some larvae, such as those of the green measuring worm moths (family Geometridae), attach bits of leaves or flower petals to themselves. Cocoons are frequently camouflaged with leaves or debris. The chrysalis of puss moths (Cerura) and some dagger moths (Acronicta) is hard, woody, and inconspicuous.

Camouflage is a form of concealing coloration. Such a cryptic, or hidden, appearance occurs when the natural coloration or pattern of an organism allows it to blend in with its background. Concealing coloration works only in appropriate surroundings and only when accompanied by proper behaviour, which is usually immobility. Great numbers of larval, pupal, and adult Lepidoptera are thus protected in their usual locations on plants. Leaf-eating larvae usually blend into leafy environments. Many caterpillars have stripes that simulate leaf veins. The double-toothed prominent caterpillar (Nerice) has a jagged outline resembling the edge of an elm leaf. A great many measuring worms, or inchworms, are notably twiglike, with long, slender, stiffly held bodies. Many other caterpillars, especially those of prominent moths (family Notodontidae), have irregular shapes that resemble twisted dead leaves. Likewise, many adult moths that rest during the day among leaves or on bark are cryptically coloured and patterned. Their behavioral mechanisms, such as aligning stripes on their wings with patterns of the tree bark, help them blend in with the backgrounds on which they rest.

Many larvae and adults are disruptively marked with bold contrasting patches or bands of colour that break up their outlines into two or more seemingly unrelated masses. Many adult moths and butterflies have coloration that serves to startle and thus momentarily delay an attacker. Moths with cryptic forewings and butterflies with cryptic wing undersides show only these surfaces when they are at rest. When moths such as the underwing moths (Catocala) are disturbed, they move the cryptic forewings to expose bright patches of colour on the upper surface of the hind wings. When butterflies such as the morphos, hairstreaks, and anglewings are disturbed, they take flight, exposing brightly coloured upper wing surfaces. Regardless of the method, the effect is startling to predators. When the animal lands, the bright surfaces are suddenly hidden, causing it to disappear into the background. A similar “startle effect” protects larvae that have prominent spots that look like large eyes.

Many species produce startling sounds. Hawkmoth caterpillars and the pupae of many gossamer-winged butterflies make squeaking or grating sounds when disturbed. The adult African death’s head moth (Acherontia atropos) makes a loud chirping sound. Ageronia butterflies, when startled into flight, make a loud clicking sound by means of a structure on the wings. These sounds may have a startling and therefore delaying effect on a predator.

Certain butterflies and moths possess repellent or toxic substances that provide protection against predators. Sometimes these are secured directly from the plant on which the larva feeds, such as the toxic glycosides present in high concentrations within plants eaten by milkweed butterflies (family Nymphalidae) such as the monarch. More often, the toxin is produced by the insect itself and stored in the body, so that the predator must taste the insect to know it is toxic. The toxin often occurs in the blood—e.g., hydrogen cyanide in burnet moths. Other toxins may be in the gut or may be the product of special glands, which release the toxin at the time of an attack. Tiger moths (family Arctiidae) give off drops of repellent from glands on the prothorax. Many groups show reflex bleeding (autohemorrhization) from leg and body joints when disturbed. The larvae of swallowtail butterflies (Papilio) and tussock moths (family Lymantriidae) give off strong-smelling, volatile substances from extrusible scent organs (osmeteria). The caterpillars of many prominent moths spray formic acid from ventral prothoracic glands. Many larvae and some adults possess hollow barbed hairs that introduce toxins into potential predators, causing pain and swelling. Caterpillars of the slug moths (family Limacodidae), flannel moths (family Megalopygidae), Io moths, and some tussock moths are noted for this. A few adult moths (e.g., the garden, or great, tiger moth [Arctia caja]) inject toxins through sharp spines on their hind legs. The great majority of protected forms are aposematic; that is, they have markings, shapes, or behaviour that draw attention. They are thus easily recognized and remembered by predators, which, after trying to feed on only one or two individuals, will leave other similarly patterned individuals alone.

Not all warning mechanisms are visual. Inedible tiger moths make high-pitched grating sounds by means of timbal (drumlike) organs. These sounds are inaudible to humans but can be heard by bats, and they may function to warn the bat of the moth’s inedibility. This allows the moth to avoid being captured and tasted. Some authorities think that these sounds also may interfere with the bats’ acoustic orientation system, preventing them from detecting the moth.

The protective advantage gained by a distasteful or dangerous insect that advertises that defense, either by aposematic coloration or acoustic warning, may also be utilized by similar harmless and edible insects. The evolution of such resemblances results in a phenomenon known as mimicry. The distasteful insect, called the model, may even belong to an insect order completely different from that of the mimic. For example, members of various lepidopteran families mimic wasps, bees, or beetles. The clearwing moths are particularly effective mimics of certain stinging wasps, the resemblance being carried to details of the shape and coloration of the wings, abdomen, and legs.

The occurrence in a population of two or more distinct hereditary variants, or morphs, is known in many lepidopterans. Each morph may have a different adaptive value, linked with such physiological features as resistance to cold or to toxins in the environment. Striking variation in appearance may have great adaptive value by confusing predators, making it more difficult for them to learn the appearance of the prey. Mixed populations of both light and dark (melanic) individuals may survive better in habitats containing both light and dark backgrounds on which they rest during the day. This may also allow the entire population to survive if the environment changes either through normal succession of forest growth or because of human-made phenomena such as industrial pollution. In England, investigations of the peppered moth (Biston betularia) have abundantly documented the evolution of “industrial” and “natural” melanism and have shown that major genetic population changes can take place very rapidly.

Striking polymorphisms occur in some mimetic species, notably the African swallowtail (Papilio dardanus). The occurrence of different species of inedible butterfly models in various geographic regions has been accompanied by the evolution of correspondingly different mimetic females of this single species of swallowtail. In North America the tiger swallowtail (P. glaucus) has mostly black females wherever it coexists with the distasteful pipevine swallowtail (Battus philenor), which is also black. However, where B. philenor does not occur, P. glaucus females tend to be all nonmimetic yellow forms like the males because, without the black models, black has no protective significance. Some very striking mimetic polymorphisms occur among Neotropical Heliconius butterflies and their various models and mimics.