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CHAPTER 1
WHAT IS A CATERPILLAR?
Whether hairy, spiny, ridged, or smooth, the world's caterpillars all share one common trait, reflected in Eric Carle's children's classic, The Very Hungry Caterpillar. Typically described as "eating machines," they may increase their body mass by up to 1,000 times as they mature. They are the developmental stage of butterflies and moths and have a simple goal — to eat, grow, and become an adult. While a butterfly or moth sometimes survives only long enough to reproduce, the larval period may last days, weeks, months, two to three years, or occasionally even longer in species that are dormant during winter or hot summers.
STRUCTURE
Butterflies and moths have the caterpillar in common. While the adults can often be distinguished from each other by the structure of the antennae and the way the wings are held at rest, there is no simple physical characteristic that distinguishes a butterfly caterpillar from a moth caterpillar. Despite the extreme diversity of color and form in the hundreds of thousands of species, all caterpillars share the same basic features, built on the standard insect plan of head, thorax, and abdomen. The head is large, the thorax (the middle section between the head and the abdomen) is small, and the whole body is long and tubular.
The head
The epicranium, a hard head capsule with a triangular front plate or "frons," has a characteristic inverted, Y-shaped line extending down from the top of the head; this line distinguishes the caterpillar from any other grub. In most caterpillars, the head is conspicuous, although in families such as Lycaenidae it may be retracted into the thorax. There are six simple, lateral eyes (stemmata) to help the caterpillar distinguish between dark and light and give it some spatial awareness. There is a short antenna on each side of the mouth, and the mouthparts consist of a pair of jaws, or mandibles, bounded by an upper flap (labrum) and lower structure (labium). The mandibles swing from side to side, "shearing" through vegetation, and often bear small, sharp, toothlike projections. Located centrally on the lower side of the head is the labial spinneret, the secretory structure through which modified salivary glands discharge silk that is used by the larvae in various ways — sometimes to bind foliage or create a silk web, or during pupation to suspend a chrysalis or construct a cocoon.
Thorax, abdomen, and legs
The thorax is small, muscular, and made up of three segments, each bearing a pair of true, jointed legs. The abdomen, consisting of ten segments, is the largest part of a caterpillar and where food is digested and processed. There are pairs of spiracles (respiratory pores) on all of the abdominal segments except for the last two. The abdominal legs or prolegs are quite different from the true legs, being fleshy and barrel-shaped, and bearing hooks or crochets at the base. Most caterpillars have four pairs of prolegs on the third to sixth abdominal segments and another pair on combined segments nine and ten. Geometridae caterpillars, however, have only two pairs of prolegs, one on the sixth abdominal segment and the other on the tenth, producing a characteristic walking pattern that has given them the nickname of "inchworms" or "loopers." Limacodidae larvae, the so-called "slug" caterpillars, have suckers instead of prolegs and secrete a liquefied silk lubricant to help them glide along.
Setae, spines, and shields
Caterpillars are clothed with hairlike structures called setae, which serve to protect, act as sensors, or secrete substances; for instance, the setae of some species of Pieridae butterfly caterpillars in their early stages produce droplets of fluid, which appear to help deter predators and parasitoids. Further types of ornamentation include fleshy filaments, hardened cones, branching spines, and thoracic shields, all with primarily defensive functions.
DISTINCTIVE LARVAE
Other insects have a similar larval stage, but caterpillars can usually be distinguished from other larvae by their characteristic Y-shaped head marking, more diverse patterning (grubs are frequently quite dull), and by their abdominal prolegs, as most other larvae have stocky true legs but no abdominal legs, or no legs at all. The larvae of sawflies (insects of the order Hymenoptera, which also includes bees and wasps) are very caterpillar-like but have a single lateral eye (not six) and have six to eight (rather than five or fewer) pairs of prolegs.
RANGE AND DIVERSITY
Caterpillars occupy a vast range of habitats, from seed pods to kitchen pantries, and from hot deserts to mountains and even into the Arctic Circle. Adaptation to such different environments has led to extraordinary diversity in appearance and survival strategies. More than half of all species are relatively unstudied "microlepidoptera," the often pale-colored, featureless, and very wormlike larvae of tiny moths, many of which feed concealed within stems, fruits, seeds, and other foodstuffs and materials.
By contrast, the caterpillars of macro-moths and butterflies are often colorful, with showy features such as bristles, spines, and filaments, and make no attempt to conceal themselves. Bright, so-called aposematic coloring is often a "warning" to potential predators that the caterpillars are or might be bad tasting. Very hairy or spiny caterpillars are equally unpalatable to predators such as birds; arming the spines with toxic chemical secretions adds a further layer of defense.
The heads of caterpillars also show incredible diversity in coloration, patterning, and shape, again as a defense, some resembling "faces," with horns, false eyes, nose, and mouth. Others protect their head by having "head-like" posteriors, presumably a bid to fool predators (at least half of the time) into attacking the less vulnerable end. Many species, however, are cryptically colored to blend with their environment. Some even change hue according to the part of the host plant they are feeding on, such as certain lycaenid butterfly caterpillars, which are green when consuming leaves but become red, yellow, or orange if they eat flower buds and petals.
CHAPTER 2
FROM EGGS TO PUPATION
Like all eggs, butterfly and moth eggs are fragile and attractive to predators. The relatively slow-moving larvae that hatch from them are also vulnerable and, in order to survive their complex phases of development and reach pupation, the final stage before adulthood, they must deploy a remarkable range of strategies that have evolved to meet the challenges of their habitats.
EGGS — LAYING AND HATCHING
Using visual and olfactory stimuli, female butterflies and moths often carefully select a spot on or close to a specific host plant, where their miniscule eggs can hatch in safety, although some moths distribute eggs randomly, conferring the benefits of a broad host plant range. Differing in size and shape according to species, eggs may be laid singly or in glued-together masses of up to 1,000, on upper or lower surfaces of leaves, on buds or flowers, encircled around twigs, on the ground, on rocks, or on other non-plant substrates. Being so small and often cryptically colored, perhaps resembling plant parts, fungi, detritus, or even bird droppings, caterpillar eggs are rarely found by casual observers.
The eggs usually develop rapidly, hatching within two to ten days, depending on temperature. Sometimes, though, they are programmed to delay hatching, spending adverse weather conditions — extreme cold or heat — in a state of developmental arrest, known as diapause. They then hatch only when the host plants they feed on reappear.
LARVAL STAGES
The caterpillar hatches by cutting a hole in the shell with its mandibles and, according to species, may consume the entire eggshell on the way out or leave the empty eggshell with a telltale exit hole. The new larva immediately sets about feeding and protecting itself. It may move to a safer location on the plant, cover itself with a silk-tied leaf shelter, or, in the case of gregarious larvae, join with its siblings in creating an extensive silk-web nest. Caterpillars at the newly hatched stage, known as the first instar, usually feed rapidly, often doubling in size within a few days. Once the larval "skin," or integument, tightens and appears stretched, with a swelling at the head caused by the larger, inelastic head capsule of the next instar, the larva is nearing its first molt, or ecdysis. Before molting, larvae find a site hidden from predators, spin a small pad of silk to which they attach their claspers, and remain motionless for 12 to 48 hours.
Molting, which takes only minutes, begins at the head end, with the integument splitting and slipping backward along the body as the larva moves slightly forward. In most species, the next instar consumes the old integument and soon resumes feeding. Newly molted larvae often show temporary paler coloration, which disappears within 2 to 12 hours. Some species have four or six instars, but most have five, molting four times. However, where larval development is interrupted multiple times by diapause, seven to nine instars can occur.
FEEDING AND GROWING
Caterpillars are programmed to eat as much as possible in order to grow and mature. The period from egg hatch to pupation may be as little as ten days, although in species with multiple dormancies caterpillars can live for two to three years, with one Arctic species taking up to seven years to complete development. An approximate doubling of length occurs in each successive instar. Between 60 and 80 percent of the total plant mass eaten by a developing caterpillar is consumed in the final instar. Size is relative, however, as the largest saturniid silkmoth and hawkmoth caterpillars grow up to 6 in (150 mm) in length, while the final instars of "micromoths" may reach only 3/16 in (5 mm).
Species also grow at different rates and in different seasons, depending on their preferred food. Some feed only on leaf buds, others on young leaves, mature leaves, flower buds, flowers, seeds, or even stems. Buds, flowers, and seeds are more nutritious (generally with more nitrogen) than leaves or stems, promoting faster growth but within a shorter growth period. Food sources such as grasses and evergreen needles are low in nutrition but hugely abundant over vast areas, so caterpillars exploiting these resources grow slowly but with little competition.
DEALING WITH ENVIRONMENTAL EXTREMES
Caterpillars, like all insects, are cold blooded and depend on environmental conditions to achieve the optimum body temperatures for development. For the majority of species, the range of body temperatures favoring development is 59–86°F (15–30°C). When temperatures remain below 41°F (5°C), with periods below 32°F (0°C) and limited, low-angle sunshine, many caterpillars are unable to develop. To survive long, hard winters, they have to change their physiology and enter a dormant state of suspended animation, or diapause. In late summer or fall, some caterpillars prepare for overwintering by seeking refuges, such as curled leaves, seed pods, under rocks, or other sheltered locations, where they will be buffered against the elements. Here, a lowered metabolic rate and radical biochemical changes, including synthesis of a kind of "antifreeze," protect them against extreme cold. Species living in hot, dry Mediterranean or desert climates, where temperatures frequently reach 100–115°F (38–45°C) and plant life is often sparse, face a similar challenge, entering summer dormancy, or estivation, and delaying pupation and adult emergence until fall, when conditions are better for survival and reproduction.
Caterpillars may reenter diapause multiple times if environmental stimuli signal the onset of unfavorable conditions. Post-diapause, checkerspot (Euphydryas) butterfly caterpillars recommence feeding in late winter or early spring on fresh host plant growth, but if a lack of moisture affects that growth, the larvae become dormant, potentially living for two to three years with only short periods of development annually. Caterpillars living at high elevations, such as those of the Arctic Fritillary (Boloria chariclea), depend on timely snowmelt to enable them to feed and complete development in time for the normal midsummer flight period. In late spring, after diapause, these caterpillars appear to measure day length to determine if they can complete development in time. If not, they overwinter twice, as an early instar then a late instar. Climate, elevation, and food plant also affect the number of broods developed during a year.
PREPARING TO PUPATE
When nearing maturity and pupation, full-fed larvae often change color, most "shrink" to a certain degree, and some enter a wandering phase, the "wanderers" seeking sites away from the host plant. Some go underground, some hide, and others build a protective cocoon or blend in with the background, either through coloration or by creating a broken outline. This high degree of crypsis, and the talent of wandering prepupal caterpillars for finding secluded pupation sites, means the particularly vulnerable pupal stage of butterfly and moth metamorphosis is the least likely to be seen.
CHAPTER 3
THE MIRACLE OF METAMORPHOSIS
Perhaps the most celebrated trait of Lepidoptera is their capacity to metamorphose — changing their body structure and appearance so completely that larvae and adults look as if they are two quite separate species. While most insects metamorphose, some practice "incomplete metamorphosis," with no pupal stage; larvae hatch from eggs and are usually a miniature version of the adult. Insects undergoing complete metamorphosis, which also include beetles, flies, and wasps, are considered more highly evolved. Fossil records suggest that metamorphosis began to occur up to 300 million years ago and conferred an evolutionary advantage on metamorphosing species, because their different forms and habitats ensured that adults and larvae did not compete for the same resources.
MAKING THE CHANGE
Pupation describes the transition of a species from active eating machine (caterpillar) to the immobile, non-feeding preparatory stage (pupa), which will ultimately yield the adult butterfly or moth. The term "chrysalis" is generally used for hard-cased butterfly pupae or the casing itself, while many moths spin a protective outer silk "cocoon" around themselves. Pupae are formed in one of five basic modes: loose on the ground, within a silken cocoon or leaf shelter, underground in an earthen cell, hanging by the terminal end (cremaster) attached to a silk pad, or attached upright by the cremaster with a supporting silk girdle. Loose pupae are common in moths but rare in butterflies. Skippers — butterflies from the Hesperiidae family — commonly form pupae within tied leaf or grass shelters, while hanging pupae are characteristic of Nymphalidae butterflies, and girdled pupae are found in species from the butterfly families Papilionidae, Pieridae, and Lycaenidae. While some moth larvae spin cocoons on leaves, twigs, or branches, many burrow in leaf litter or to varying depths in the ground. Several species incorporate protective materials with their silk into the cocoon to strengthen it, such as chewed bark and their own stinging setae. Others add twigs or bits of vegetation to help disguise the cocoon.
When a pupation site is selected, and silk pads, shelters, or cocoons are complete, the prepupal larva shrinks a little and waits motionless for the final molt to occur. The outer skin then softens, splits, and falls away, leaving the pupal case that has formed beneath. In most species, final coloration of the pupa matches its immediate environment. Even the Monarch butterfly (Danaus plexippus), which is brightly colored at all larval stages to alert predators to the toxins it contains, does not advertise this fact at pupation; its green pupa blends with the foliage around it. While pupae may occasionally wriggle if disturbed, they are generally unobtrusive, remaining hidden or camouflaged to avoid predation at this crucial stage.
Larvae that form hanging pupae adopt a characteristic J shape. After 12 to 48 hours, the larval skin splits behind the head, revealing not another caterpillar integument, or skin, but a fleshy, soft integument, usually green, yellow, or orange. With much wriggling, the larval skin moves down the body, revealing increasingly more of the soft, new pupa. Once the shed skin reaches the terminal segment, the pupal cremaster probes and seeks the silk pad spun earlier by the prepupal larva. With hanging pupae this is a critical phase; if the cremaster fails to make contact with the silk pad to which it attaches with tiny hooks, the soft pupa will fall, and likely perish. After attachment, more wriggling usually results in the shed skin dropping away, and eventually the pupa stops moving, hardens, and assumes the coloration that allows it to blend in with its environment.
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Excerpted from "The Book of Caterpillars"
by .
Copyright © 2017 Quarto Publishing plc.
Excerpted by permission of The University of Chicago Press.
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