The following individuals have provided their expertise in identifying adults reared from the field-collected caterpillars: Paul Hammond, Doug Ferguson, Jerry Powell, Don LaFontaine, Paul Opler, Jon Shepard, and Lars Crabo. The following individuals have helped collect and rear caterpillars: Paul Hanson, Ken West, Brian Scaccia, David Carmean, Joshua Miller, Jacob Miller, Kimberly Miller, Jean Miller, Mike LaMana, Carolyn ver Linden, Dana Ross, Norm Anderson, Jack Lattin, Maret Pajute, Gary Parsons, Emma Rossi, Pete Oboyski, and Rich Bowden. Dave McCorkle kindly provided the photograph of Parnassius clodius (79). All other photographs were taken by the author. The preparation of this manuscript was completed with helpful reviews from Paul Hammond, Dana Ross, Mike LaMana, Richard Reardon, and Linda Butler. Roberta Burzynski, U.S. Forest Service, Northeastern Area State and Private Forestry, Radnor, PA, deserves special thanks for her very helpful editing of the final draft. My wife, Jean Miller spent many hours helping me organize and rewrite each draft of the manuscript. Also, this booklet would not have been produced without the encouragement and support of Richard Reardon.

Support for the publication of this booklet came from the USDA Forest Service's National Center of Forest Health Management, Morgantown, West Virginia. In addition the center provided partial support for research on caterpillars of the Cascade Mountains under Cooperative Agreement 42-950010. Partial funding was also provided by the USDA Forest Service Pacific Northwest Forest and Range Experiment Station under Cooperative Agreement PNW 93-0398.

Insects are notably abundant in a wide variety of habitats. The moths and butterflies (Lepidoptera), in particular, are obvious in addition to being well represented among the total number of insects from a given site. The caterpillar is the actively feeding immature stage of moths and butterflies and is perhaps less obvious at first glance but can be abundant on certain plants at certain times of the year. In addition to being abundant, caterpillars are diverse in the number of species present, their appearance, behavior, and life cycle requirements.

Identifying field-collected caterpillars to the species level is essential to performing natural history observations and conducting detailed ecological studies on caterpillars and host plants, parasitoids and host caterpillars, and using caterpillars as indicator species in assessing environmental impacts. Diagnostic keys for identifying species of caterpillars in the Pacific Northwest are not available. In fact, few scientific papers and books can be found that illustrate caterpillars of this region. Stevens et al. (1984) is one of the few sources available for illustrations of caterpillars in the Pacific Northwest. Certain books provide excellent photographs of common caterpillar species of different regions, such as, the Canadian Provinces (Ives and Wong 1988), the Appalachians (McCabe 1991), and eastern deciduous forests (Wagner et al. 1995). Some of the species illustrated in these books also occur in the Pacific Northwest; however, Oregon alone contains over 1600 species of Lepidoptera, and a majority of the Pacific Northwest species do not occur east of the Rocky Mountains.

This booklet is a field guide with keys to the identification of caterpillars commonly found in forests and woodlands of the Pacific Northwest. It contains a brief section on the natural history of caterpillars and describes variations in morphology, color, and pattern that are used to identify caterpillars. It also provides details on how to collect and rear caterpillars, and how to photograph and preserve specimens. Included are a section on nomenclature and a description of the families most commonly found in the Pacific Northwest.

The Pacific Northwest, as considered here, consists of Oregon, Washington, and Idaho. This region contains numerous mountain ranges, part of the Great Basin, the Columbia River Basin, part of the Snake River, the Puget Sound, and the Willamette Valley. The vegetation in this region includes a flora adapted to coastal, desert, and alpine environments, thus it is diverse but the dominant forest tree is Douglas-fir, with ponderosa pine and lodgepole pine also prevalent. Caterpillars that feed on the understory vegetation of these forests are the focus in this booklet.

Only the larval stage of Lepidoptera is called a caterpillar. Caterpillars initially develop in the egg then emerge (eclose) from the egg (unlike eggs which hatch). The caterpillar is called a first instar up to the time of the first molt, which is the shedding of the skin and head capsule. The caterpillar increases in size at the molt and then is called a second instar. Another molt distinguishes another instar and so on. Typically, a caterpillar passes through five instars as it eats and grows. In certain species a caterpillar that will become an adult female may develop through an additional instar and thus grow bigger than the male. Even into the last instar it is usually difficult to distinguish between the sexes.

Caterpillars may be distinguished from other immature insects by a combination of the following features: adfrontal suture on the head capsule; six stemmata (eyespots) on the head capsule; silk gland on the labium (mouthparts); prolegs on abdominal segments A-3, A-4, A-5, A-6, and A-10; or A-5, A-6, and A-10; or A-6 and A-10; and crochets (hooks) on prolegs.

Other caterpillar-like insects that are also terrestrial and feed on foliage are the larvae of sawflies. Sawflies however, usually have only one or a few stemmata, no adfrontal suture, no crochets on the prolegs, and usually prolegs on abdominal segments A-1, A-2, A-7, and A-8, in addition to the prolegs on A-3 to A-6 and A-10.

The variety of form in the body parts serves an important role in the ability to identify the species of the caterpillar at hand. Because the existence of variety in morphology is the result of evolution, the process of a species adapting to its environment, particular life styles can be associated with particular features of morphology as noted in the section on the body regions.

A caterpillar consists of three major body regions: head, thorax, and abdomen.

The head consists of a well sclerotized head capsule. In most species the head capsule is marked with an adfrontal suture and typically contains six stemmata or eyespots. Also on the head capsule is one pair of small, three-segmented antennae located close to the base of the mandibles, which are the chewing organs.

The mouthparts have many components and occur on the head. The components are: a labrum, mandibles, maxillae, and a labium. The labrum serves as an upper lip and may be notched to function as a leaf guide assisting in orienting food between the mandibles. The mandibles are located below the labrum and are paired, opposable, and hardened tooth-like structures used to bite and crush food. The maxillae are located behind the mandibles and contain sensory organs for detecting whether foliage is acceptable food or not. The labium is located behind the maxillae and houses the silk gland, which emits a strand of silk used for producing pads, life lines, and cocoons. The overall shape of the head capsule, color patterns, the location of hairs on the head, and the morphology of the mouthparts are helpful in identifying species of caterpillars. These features require the aid of a microscope and will not be emphasized here.

The thorax has three-segments, each with a pair of segmented legs. The three thoracic segments are the: prothorax, nearest the head (T-1); mesothorax, in the middle (T-2); and metathorax, connecting to the abdomen (T-3). The thoracic legs assist in locomotion and clinging to substrates. Some caterpillars, in particular certain leaf mining species, have no segmented legs on the thorax. Each side of the prothorax has a spiracle, which is an opening of the respiratory system. The identification of caterpillars is further aided by the presence or absence and shape of sclerotized plates, the location of primary setae (and setal clusters), the location, color and shape of the prothoracic spiracle, and morphology of the legs. Refer to Peterson (1962) and Stehr (1987) for further details on the use of these features in identifying caterpillars to families and species.

The typical abdomen has ten segments, A-1 to A-10. Segments A-1 to A-8 have spiracles and an anal plate may occur on A-10. Depending on the family group, certain abdominal segments have fleshy prolegs bearing crochets (hooks): segments A-3 to A-6, the midabdominal prolegs, and A-10, the anal prolegs. The typical pattern for prolegs is one pair per segment on A-3 to A-6, and A-10. However, some caterpillars have prolegs only on A-5, A-6 and A-10 which is characteristic of the Plusiinae of the Noctuidae; or A-6 and A-10 in the Geometridae. Some leaf mining caterpillars have reduced prolegs merely indicated by the presence of crochets on the abdominal wall, while in others even the crochets have disappeared. If prolegs occur on segments A-1, A-2 or A-7 to A-9, the specimen is most likely a sawfly.

The prolegs on the abdomen are not true legs because they are fleshy extensions of the body wall and not segmented appendages. The crochets at the end of the prolegs occur in a variety of configurations that are characteristic of family groups. For instance, the crochets may occur in the form of a closed or open circle, an ellipse, paired longitudinal lines, or a transverse line. Also, the crochets may be inserted into the flesh of the proleg in single rows (uniserial), double rows (biserial), or triple rows (triserial). Similarly, the tips of the crochets may form a single row (uniordinal), a double row (biordinal), or a triple row (triordinal). All of these features are useful in the identification of caterpillars but are best seen with magnification and not used here for field identification.

An array of projecting features may occur on various caterpillars. The location and number of many of the following traits may allow for a quick and accurate identification of a particular caterpillar. The projections may be attached to the body wall such as soft and flexible hairs, or modified hairs that are sclerotized and hard or stiffened into spines. Also, projections may be extensions of the body wall in the form of warts, tubercles, or horns.

The type and arrangement of hairs are used to place a caterpillar into the groups devised for this booklet. Hairs may be short or long, single or in clusters or tufts, tapering to a point or with a clubbed end, and in many colors. A few single, short hairs may be seen in a majority of species, such as most Noctuidae, Geometridae, Tortricidae, and many more families. Such hairs are called primary setae and typically occur in particular positions on the body segments. Hairs that are densely scattered over the body are called secondary setae. Examples occur in the Noctuidae, most notably in species of Acronicta and the Lycaenidae. When long hairs are present the caterpillar usually possesses a more dense array of hairs, long or short or both, overall. For instance, most Arctiidae (Hyphantria cunea and Spilosoma virginica have long, densely packed hairs. Also, long hairs may occur in clusters (hair pencils) or in densely packed tufts (tussocks) in the middorsal area. Tufts usually occur in association with glands where the hairs serve as a wick for the gland exudate. Examples of a combination of hair pencils and tussocks can be seen in many Lymantriidae, the tussock caterpillars, such as in species of Dasychira and Orgyia.

Spines may have a single point (chalaza) or multiple points (scolus). Spines typically occur at defined positions along a certain region of the body (e.g. dorsal, subdorsal) at the locations of the primary setae. Numerous species, namely Saturniidae and Nymphalidae, have spines of various kinds and in various colors. For examples see Nymphalis antiopa, Polygonia faunus, Hemileuca eglanterina, and Hyalophora euryalus.

These are small bumps or short finger-like projections extending from the body wall.

Extensions of the body wall into fleshy short bump-like or long filamentous projections are tubercles, often occurring in pairs or in a series encircling one or more segments. An example of short tubercles can be seen in Aethaloida packardaria and Sicya crocearia, while longer filamentous tubercles are illustrated by Danaus plexippus.

Extensions of the body wall drawn into relatively short, pointed fleshy projections are horns. As found in sphingid caterpillars, commonly called hornworms, the horn occurs singly, typically in the middorsal area, and posteriorly on A-8. For examples see Pachysphinx occidentalis, Paonias excaecatus, and Sphinx sequioae. Some late instar hornworms may have a reduced horn, even to the degree that only a "button" or circle remains.

The typical caterpillar body is cylindrical. Variations of the cylindrical shape include bodies that are flattened, humped, otherwise swollen, or constricted. The flattened shape is indicative of a leaf-mining habit while the cylindrical shape is characteristic of borers, tunnelers, and external leaf-feeders. The humps, swellings, and constrictions may help a caterpillar to blend into its surroundings. The location and size of humps and constrictions can be useful for identifying certain species.

Obvious bulges in the body profile can be found in many species. A large, dorsal, pyramid-like, posterior swelling occurs in Amphipyra pyramidoides and Feralia februalis. Thoracic and midabdominal swellings, either dorsally or ringing a segment, occur in species of Catocala, Schizura, and Zale lunata.

A distinctive narrowing of the body is also a feature of special note in certain caterpillars. Among the Hesperiidae (see Epargyreus clarus) the neck region is noticeably constricted.

Caterpillars display a wide range of color patterns. Typical hues are brown, tan, cream, white, silver, grey, black, red, pink, orange, yellow, green, blue, and purple. These colors are displayed in a wide assortment of patterns that can be categorized as bands, lines, rings, streaks, dashes, circles, dots, saddles, and patches. The location of the color and its pattern can be helpful in caterpillar identification; however, the patterns may differ subtlely or markedly from one instar to another as in Lophocampa maculata and Nadata gibbosa. The most common locations of definitive patterns are middorsal, subdorsal, lateral, sublateral, and ventral. The following patterns are found in various species.

Wide lines extending from head to tail along the middle of the back, as in Lithophane georgii.

Wide lines extending from head to tail along the sides where the spiracles occur. In some species the top edge of the band barely touches the spiracles and may appear to be a subspiracular band, for example, Anomogyna mustelina and Acronicta perdita.

Narrow lines extending from head to tail along the middle of the back, as in Cosmia calami and Acronicta marmorata.

Narrow lines extending from head to tail more or less halfway between the middle of the back and the spiracular area, as in Operophtera bruceata and Achytonix epipaschia.

Narrow lines extending from head to tail along the sides where the spiracles occur. In some species the top edge of the line barely touches the spiracles and may appear to be a subspiracular line, for example, Drepanulatrix foeminaria, Aseptis fumosa and Hemileuca eglanterina.

Bands of color going around the body segment, often in two or three alternating colors as in Danaus plexippus and Tyria jacobaeae. Also, the intersegmental area may be colored in a manner that shows a faint ring pattern, as in Eulithis xylina.

Narrow lines of color longer than half the width of a body segment. See Epargyreus clarus, Mitoura grynea barryi, and Pseudorthodes irrorata.

Narrow lines of color shorter than half the width of a body segment and located along the middle of the back, for example, Phlogophora periculosa and Malacosoma disstria.

Narrow lines of color shorter than half the width of a body segment and located along the subdorsal area of the body, for example, Malacosoma californicum.

Relatively large spots of a solid color (or middle of spot variably colored) located along the middle of the back, as in Zotheca tranquilla, Acronicta funeralis, and Leucoma salicis.

Specks, usually white or black, randomly and usually densely scattered over the body. Some species may show black specks at the base of primary hairs, which are not scattered. See Ypsolopha cervella, Paonias excaecatus and Nymphalis antiopa for white speckles and Catocala aholibah for black speckles.

Irregularly shaped patches of color extending across multiple segments along the middorsal area, as in Furcula cinerea, Schizura unicornis, and Polygonia faunus.

Bands or lines of color that extend from side to side across the back but not all the way around the body, as in Phryganidia californica.

Bands or lines of color that extend from side to side across the back of a posterior abdominal segment, as in Orthosia pacifica and Nadata gibbosa.

Lines, usually white, yellow, or black, that extend from anterior lateral areas to posterior subdorsal or dorsal areas, for example, Pachysphinx occidentalis and Paonias excaecatus.

With rare exception caterpillars are herbivorous, that is, they feed on plants. Most typically caterpillars feed on foliage, but also on roots, within branches and woody stems, in seeds, and on flowers. Few species of caterpillars are predaceous, feeding on animals.

The caterpillars of many moth and butterfly species may have restricted ranges of suitable host plants upon which they can feed. Such species are termed monophagous or host plant specialists. In such cases the caterpillar may feed only on one species, on only a few species, or on all species within one genus of plant. For instance, any of the species of Drepanulatrix only feed on certain species of Ceanothus.

On the other hand, many caterpillars are generalists and are termed polyphagous. That is, the caterpillar can feed upon any plant among a wide range of plant species and still develop into an adult in the usual period of time. Generalist feeders often are able to live on plant species belonging to a wide array of families. For instance, the caterpillars of Neoalcis californiaria, Hesperumia sulphuraria, and Aseptis binotata can develop on 15 to 23 plant species belonging to 10-12 plant families. Although the caterpillars may be polyphagous they may exhibit preferences for certain plant species. Few caterpillars such as Neoalcis californiaria and Anomogyna mustelina, are capable of feeding on both coniferous foliage and leaves of flowering plants.

Within a given environment caterpillars can be found in a variety of habitats and microhabitats. In general, they may be aquatic or terrestrial. Caterpillars can be found in fruits, roots and stems as borers or miners, in leaves as miners, on the surface of leaves as skeletonizers or chewers, in galls, or in the nests of other insects, such as ants and bees.

Most caterpillars feed and develop as solitary individuals; however, the caterpillars of a few species aggregate, some of which construct nests. For instance, the caterpillars of Lophocampa argentata aggregate on branches of Douglas-fir but do not construct nests. The caterpillars of Hyphantria cunea and Malacosoma disstria occur in large colonies living in silk nests spun across twigs and branches of trees.

The growth rate of a caterpillar depends upon at least three factors: genetics, temperature, and nutritional quality of the food. The genetic basis for growth can be observed in the developmental time of caterpillars from different females. The average time for development among the caterpillars of a certain female can differ from that of another female of the same species. This trait can be inherited.

Caterpillar growth rates are dependent upon temperature. Growth rates are slow at cold temperatures and up to a certain point faster at warm temperatures. Caterpillars of species that occur in very cold climates may take more than one or two years to complete development because the short warm season limits feeding and growth.

Dependence of caterpillar development upon the nutritional quality of vegetation is strongly influenced by the amount of protein (nitrogen), water content, and allelochemicals. Most plants contain between 1% and 7% nitrogen by weight. Experimental manipulation of nitrogen in the diet of a caterpillar usually shows faster growth when nitrogen is provided at the higher end of the normal range. Likewise, caterpillar growth is enhanced when water content of their food is at the higher end of the normal range. Allelochemicals are plant-derived chemicals that may stimulate or deter feeding by caterpillars. Some of the better known allelochemicals are terpenes, alkaloids, and various proteins. These chemicals may also act as poisons to the caterpillar or in certain instances the caterpillar may store poisons and in turn become toxic to potential predators. Many of the poisonous caterpillars are aposematic (brightly colored). For instance, the brightly colored caterpillar of the monarch butterfly, Danaus plexippus, is poisonous to most prospective predators due to the storage of plant derived cardiac glycosides.

Caterpillars may switch between active and inactive periods based on the time of day. Diurnal caterpillars are active during daylight and nocturnal caterpillars are active during the night. The caterpillars of some species move among leaves and among plants in a daily pattern, often using branches, stems, and trunks as their pathways. Daily patterns in the behavior of caterpillars influence methods and opportunity for observation. While nocturnal habits of moths are well studied, few studies exist regarding observations of caterpillars throughout the night.

Species of butterflies and moths that have one generation per year (univoltine) will be in the caterpillar stage at a particular time, such as spring, summer, or fall. Typically, a univoltine species may be found in the caterpillar stage for a period of 8-10 weeks. Caterpillars of species that exhibit multiple generations per year (multivoltine) may be found for an extended period of time throughout spring, summer, and fall. Caterpillars from various generations in multivoltine species may be found over a period of 20-24 weeks.

A caterpillar may persist for eight to ten days or for eight or more months. The short-lived caterpillars belong to small species that mature after a week or two of feeding. Many such species exhibit multiple generations per year. The long-lived caterpillars belong to species that eclose from eggs in fall, spend winter (overwinter) in a state of suspended development due to the cold, and then complete development during the spring of the next year. These species have only one generation per year. Only a few of the common species in the Pacific Northwest overwinter as a caterpillar. Some of these are the arctiids, Gnophaela latipennis, Lophocampa argentata, and Pyrrharctia isabella; the geometrid, Neoalcis californiaria; and the dioptid, Phryganidia californica. A majority of the Lepidoptera in the Pacific Northwest overwinter as pupae.

Metamorphosis, the process of changing from a caterpillar into an adult occurs within the stage of life called a pupa. In butterflies the pupa is called a chrysalis. In moths the pupa may be covered in silk, which is called a cocoon, or the pupa is naked but perhaps encased in rolled leaves or in the soil. When a caterpillar has attained a critical size it will change its behavior from feeding to searching for or creating a site to pupate. The pupal stage may last for nine to twelve days, as in Ypsolopha cervella, or for more than one year, as in Coloradia pandora. In most species the pupa overwinters. Typically, overwintering pupae are in diapause, a state of arrested development. The adult will not mature and emerge from the pupa at the appropriate time unless the pupa is first exposed to a period of cold.

Caterpillars have many natural enemies. Predators devour caterpillars, often in great quantities. Some of the most important predators are rodents; reptiles; bats; birds; spiders; nematodes; and other insects like beetles, true bugs, and fly and wasp parasitoids. Also, many pathogens cuase fatal diseases in caterpillars. Some of the most important pathogens are viruses, bacteria, protozoans, microsporidians, and fungi.

Caterpillars are not without defense mechanisms against such natural enemies. Physical and physiological protective features include stinging hairs (Hemileuca eglanterina), camouflage (Semiothisa new species), hiding in rolled leaves, storage of allelochemicals (bad tasting and poisonous), glands that emit repellant chemicals, and an ability to encapsulate foreign bodies. Behavioral protective features include flashing bright colors to startle predators, spitting, and feigning death.

In addition to natural enemies extreme mortality may be caused by chemical pesticides. Here also the caterpillar has certain defense mechanisms, namely, detoxication enzymes which breakdown pesticides (and plant allelochemicals) to nontoxic elements.

Numerous techniques may be used for collecting caterpillars. One of the basic approaches is to visually search plants, in particular where feeding damage and perhaps feces (frass) can be seen. Another method of search involves clipping foliage into a bag and then inspecting the foliage indoors where a microscope may prove helpful. Tools and techniques used for the collection of caterpillars include a sweep net, beating sheet, burlap skirts, funnel traps, and sifting soil. The sweep net is similar to an aerial net but used to brush over vegetation in a manner that dislodges caterpillars from their host plant. The beating sheet is held under a plant while the foliage is shaken by beating branches with a stout pole. Caterpillars fall off the foliage and onto the sheet. Burlap skirts can be tied around tree trunks to trap caterpillars while they are enroute between feeding sites and resting or hiding places. Funnel traps can be set under plants to collect caterpillars as they drop from the foliage. Sifting soil will result in the capture of root feeders and caterpillars in the soil preparing to pupate.

An excellent means of acquiring caterpillars is to capture live adult females and rear the caterpillars from her eggs. In some species the female must be coaxed into depositing eggs by providing proper conditions of light, temperature, humidity, flying space, and a substrate for oviposition. The caterpillar of Egira februalis was obtained from eggs deposited from a live trapped female.

Careful collecting can conserve habitat where caterpillars and other organisms live. Avoid trampling plants and disturbing unstable soils. Try to grow plants for the food your caterpillars require or at least prune wild plants with care and an understanding that you may need more foliage later. Collect only as many caterpillars as you can raise, and only raise as many as can be supported without harm to local host plants. Once your observations are completed release specimens back into the environment from which they were acquired. Do not introduce exotic species and be aware of legal obligations regarding collecting on private and public lands. In particular, heed the importance of protecting rare and endangered species.

The rearing of caterpillars is helpful in: associating field-collected larvae with the adult, testing food plants for suitability, or associating parasitoids and diseases with the caterpillar stage of respective species. Caterpillars can be observed in the field, or reared in cages in the field or indoors.

Rearing indoors has numerous advantages and disadvantages. One advantage is that the specimen is less likely to be lost. Another advantage is that the caterpillar will likely grow faster indoors because of warmer temperatures. Faster growth allows the observer to see size and color patterns for each instar ahead of the presence of the same instar in the field. However, indoor rearing requires that food must be provided by potted plants, clipped foliage from the field, or artificial diets. Unsuitable rearing conditions will result in high mortality. Temperature control, dehydration, fungal growth, starvation, cannibalism, and overcrowding are common problems. Additionally, the use of closed containers may cause problems due to excessive condensation and poor sanitation. The severity of the impact of cannibalism and disease may be reduced by raising the caterpillars individually. The presence of slightly moistened peat moss is helpful at the time of pupation. The peat moss provides a medium within which the caterpillar can bury itself, and the moisture helps to prevent dessication which can be a major obstacle when rearing in dry indoor conditions.

A color slide or print will provide a record of the caterpillar at various times of development. An excellent photograph can be acquired by using all of the following: (1) A 35 mm, single lens reflex camera with exchangable lenses. Instamatic type cameras will not allow the photographer to get close to the subject or to fill the frame with the subject. The photographs presented here were taken with a 50 mm macro lens mounted on a 25 mm extension tube. (2) Film with a low ASA rating. The photographs in this handbook were taken with color slide film ASA 25. This film speed provides superior quality in grain but requires more light than faster films. (3) A flash system, either a bracket or a ring flash. I use a bracket system which consists of two flash units that are mounted on opposite sides of the camera. The lens, film, and flash units allow shooting pictures at f/16 and f/22 at a distance of about 20 mm from camera lens to caterpillar. Photographs can be taken in the field but shadows, wind, cluttered backgrounds, and other unwanted features (like other insects) may interfere with obtaining the best picture. Most of the caterpillars illustrated in this booklet were field-collected but photographed in a staged indoor setting.

Caterpillars should be preserved as voucher specimens for eventual study of traits that photographs do not reveal. Improperly preserved caterpillars will rot and turn black. The simplest method for preservation is to heat water to about 180oC. If you don't have a thermometer you can obtain an appropriate temperature by bringing the water to a boil and then let it sit off the burner for a couple of minutes before putting the caterpillar in the water. Extremely hot water may cause the caterpillar to burst. After it has been in the hot water for three or four minutes, transfer the caterpillar to 70% ethyl alcohol (isopropyly alcohol is less desirable) for permanent storage. Although this technique will provide a properly inflated specimen, an unfortunate side effect is that the caterpillar will lose most or all of its color. See Peterson (1962) and Stehr (1987) for additional ideas on preserving caterpillars.

The common names of caterpillars often describe the appearance of the caterpillar or where it lives. The names then are clues to what the caterpillar looks like or where it might be collected. Names such as the linden looper (17), the alfalfa semilooper (38), the western spruce budworm (75), the green oak caterpillar (81, 82), and the fall webworm (102) are typical examples. Unlike the unique scientific name, one particular species may be honored with multiple common names and some common names may be used for more than one species. The common names used here were found in Essig (1929), Ives and Wong (1988), Hinchliff (1994), and Wagner et al. (1995), and from the list of common names that has been officially adopted by the Entomological Society of America (Stoetzel 1989).

The scientific name of all organisms is based in Latin or Greek and consists of at least two parts and often a third. Also, the last name of the author who described the species is sometimes included (not so in this booklet). The first name indicates the genus and is always capitalized. The second name, which is not capitalized, along with the first name represents the species. Some species have a third name that indicates a subspecies. Subspecies status is applied to distinct populations that are geographically separated. Individuals of different subspecies within a species could interbreed and produce fertile offspring. Individuals of different species do not naturally interbreed or would not produce fertile offspring.

No two animals are allowed to have the same scientific name. A species will possess a list of invalid scientific names due to a history of taxonomic revisions and a variety of expert opinions as to the identity of the species across its range. Often species with distinct color forms will have many invalid names because the various forms were initially thought to be distinct species.

In general, caterpillars representing about 21 families of Lepidoptera are commonly collected in the Pacific Northwest. These families are briefly described in alphabetical order. The number of species given in parentheses for each family is only an estimate due to the dynamics of name changes and recognized species status as well as incomplete records for the Pacific Northwest.

the woollybear caterpillars. (30 species) External leaf feeders occurring on conifers and broadleaf plants. Many species covered by densely packed, wispy hairs that are longer than the width of the body. Other species covered by densely packed hairs that are shorter than the width of the body. Nearly all of the species overwinter in the caterpillar stage. A full grown caterpillar may be 60 mm in length. For most of the species the adult moths are night-flying.

the oak worm caterpillars. (1 species) Phryganidia californica, which occurs on oak and chinquapin. External leaf feeders. The adults are night-flying moths.

the caterpillars of hook-tip moths. (2 species) Drepana arcuata which is common, and D. bilineata, which is not common. External leaf feeders occurring primarily on broadleaf plants, in particular on alder species. Two features are characteristic: the rearward projecting knob on A-10, and the prolegs on A-10 which project backwards and lack crochets. Full grown caterpillars may be 30 mm in length. The adults are night-flying moths.

the caterpillars of gelechiid moths. (200 species, but poorly documented) Habits and habitats varied: external leaf feeders, borers, tunnelers, in flowers, in seeds, and leaf tiers; occurring on conifers and broadleaf plants. The caterpillars are fairly nondescript making field identification difficult. Identification is best achieved using keys which rely on the arrangement of hairs. Most full grown caterpillars are less than 15 mm in length. The adult moths are night-flying.

the inchworms or loopers. (400 species) This family is second to the noctuids in containing the most species of Lepidoptera in the Pacific Northwest. Either one pair of midabdominal prolegs which occur on segment A-6, or two pairs, which occur on segments A-5 and A-6 (Campaea perlata). Most species feed either on conifers or broadleaf plants, few on herbs and grasses. Full grown caterpillars in small species may be only 20 mm while large species may be 70-80 mm long. For most of the species the adult moths are night-flying.

the caterpillars of skippers. (40 species) External leaf feeders but may be found within tied leaves, occurring primarily on broadleaf plants, including many species associated with grasses. The most diagnostic trait for identification is that the first segment of the thorax is constricted. See Epargyreus clarus. A full grown caterpillar may be 50 mm in length. The adults are day-flying butterflies.

the tent caterpillars and caterpillars of lappet moths. (6 species) External leaf feeders, occurring primarily on broadleaf plants. Medium to long hairs, which may be densely packed on the body. The hairs are often more densely arranged sublaterally as in Phyllodesma americana. Among the common hairy caterpillars of the Pacific Northwest, the biordinal crochets of lasiocampids are unique. A full grown caterpillar may be 100 mm in length. The adults are night-flying moths.

the caterpillars of hairstreaks, elfins, blues and coppers. (60 species) External leaf feeders, occurring primarily on broadleaf plants. Covered with many short hairs, giving the appearance of velvet. The head is usually concealed from above by the prothorax. The dorsal aspect of the body is humped, a feature best seen in lateral view. A full grown caterpillar may be 30 mm in length. The adults are day-flying butterflies.

the tussock caterpillars. (8 species) External leaf feeders, occurring on conifers and broadleaf plants. Typically, tufts of dense hair occur middorsally on segments A-1 to A-4; a brightly colored (red or orange) exposed gland middorsally on segments A-6 and A-7, as in Orgyia antiqua. An introduced species, Leucoma salicis, does not fit the description of a typical lymantriid caterpillar in the Pacific Northwest. A full grown caterpillar may be 60-70 mm in length. The adults are night-flying moths.

the caterpillars of cutworms, armyworms, semi-loopers, and underwings. (850 species) This family contains the highest number of species among all families of Lepidoptera in the Pacific Northwest. Habits and habitats varied, some species occur in the soil, others bore in stems, and many are external leaf feeders. Host plants include conifers, broadleaf trees and shrubs, herbs, and grasses. Caterpillars may be hairy as in some species of Acronicta nearly naked as in the species of Lithophane, brightly colored as in Zotheca tranquilla, or cryptic as in the species of Catocala. The Plusiinae are recognized by the presence of only two pairs of midabdominal prolegs (on A-5 and A-6), all other noctuids have four pairs of midabdominal prolegs. Full grown caterpillars of the smaller species may be 15-20 mm, while large species may be 70 mm in length. The adults are moths and nearly all species are night-flying.

the caterpillars of prominents. (20 species) External leaf feeders, occurring primarily on broadleaf plants. Prolegs of A-10 may be extremely short or extremely long relative to the midabdominal prolegs and are often elevated above the plant surface as in Furcula cinerea. Caterpillars may be 60 mm when full grown. The adults are night-flying moths.

the caterpillars of brush-footed butterflies; fritillaries, commas, admirals, crescents, checkerspots, and tortoiseshells. (50 species) External leaf feeders, occurring primarily on broadleaf plants. Many species have spines middorsally on A-7 but not middorsally on A-9. The head of nymphalid caterpillars may possess spines as in the species of Nymphalis. These spines are not of the stinging type as in some saturniids. A full grown caterpillar may be 75 mm in length. The adults are day-flying butterflies.

the caterpillars of swallowtails and parnassians. (10 species) External leaf feeders. Body color in swallowtails is a mixture of green, yellow, and black. Many of the swallowtail caterpillars feed on species of Apiaceae (umbelliferous plants). The parnassians are black with yellow spots and feed on Dicentra or Sedum. Caterpillars of Papilionidae possess an osmeterium, which is an eversible forked pouch, on the prothorax. A full grown caterpillar may be 70 mm in length. The adults are day-flying butterflies.

the caterpillars of whites and sulphurs. (20 species) External leaf feeders, occurring primarily on broadleaf plants. Covered with many very short hairs, giving the appearance of velvet. A full grown caterpillar may be 50 mm in length. The adults are day-flying butterflies.

the caterpillars of plutellids or diamondback moths. (25 species, but poorly documented) Usually external leaf feeders that may loosely tie leaves together, occurring primarily on broadleaf plants. Body color is often a shade of yellow to green. Generally small, rarely exceeding a full grown length of 15 mm. See Ypsolopha cervella. The adults are night-flying moths.

the caterpillars of snout moths. (200 species, but poorly documented) Typically borers in plant stems and fruits, occurring on conifers, broadleaf plants, and in nests of other insects. Identification is best achieved using keys that rely on the arrangement of hairs. Usually less than 30 mm in length when full grown. The adults are night-flying moths.

the caterpillars of silk moths. (12 species) External leaf feeders, occurring primarily on conifers and broadleaf plants. Many species possess scoli which will be present middorsally on A-8 and A-9 but not A-7. The spines of some species are capable of inflicting a painful sting as in Hemileuca eglanterina. The head of silkworm caterpillars lacks spines, whereas the head of nymphalid caterpillars may have spines. A full grown caterpillar may exceed 100 mm in length. The adults are moths and most species are night-flying.

the caterpillars of satyrs. (12 species) External leaf feeders, occurring on grasses. Covered with many very short hairs, giving the appearance of velvet as in Cercyonis pegala. A full grown caterpillar may be 40 mm in length. The adults are day-flying butterflies.

the hornworms. (25 species) External leaf feeders, occurring primarily on broadleaf plants. A single middorsal horn usually occurs on A-8. Caterpillars often rest in a prayer-like pose, with the head and true legs raised above the plant surface. A full grown caterpillar may exceed 100 mm in length. The adults are moths and nearly all species are night-flying.

the caterpillars of thyatirids. (10 species) External leaf feeders but are found in loosely tied leaves, occurring primarily on broadleaf plants. The tail end (prolegs are reduced) is often raised above the plant surface. Caterpillars may be 40 mm in length when full grown. The adults are night-flying moths.

the leaf-tier caterpillars. (300 species, but poorly documented) Typically external leaf feeders but often in a rolled leaf. Many species are leaf miners as early instars, occurring on conifers and broadleaf plants. Identification is best achieved using keys that rely on the arrangement of hairs; however, some species are distinctively marked allowing field identification. A full grown caterpillar of a large tortricid may reach 20-25 mm as in Choristoneura rosaceana. The caterpillars move quickly in reverse when disturbed. The adults are night-flying moths.

adfrontal suture

a seam-like line on the frontal area of the caterpillar head capsule.

allelochemical

a chemical derived from plants that may stimulate or deter feeding by caterpillars.

anal proleg

a proleg on the last segment of the abdomen.

anterior

to be in front; the area that is the forward-most section of a body part.

aposomatic

a brightly colored organism that is typically poisonous.

biordinal

the tips of the crochets occur in a pattern of two rows.

biserial

the base of the crochets occur in a pattern of two rows.

bivoltine

two generations per year.

cardiac glycoside

an allelochemical found in certain plants such as milkweed and poisonous to most animals.

chalaza (pl. chalazae)

a single hair arising from an elevated base.

chrysalis

the pupa of a butterfly.

cleft

nothched.

cocoon

the pupa of moths wrapped in silk.

crochets

hooks present on the ends of the prolegs.

cryptic

an organism that blends into its surroundings.

diapause

a condition of arrested development.

diurnal

active during the day.

dorsal

the top or back area of the body.

dorsum

the upper side of the body.

eclose

to emerge from.

frass

insect feces.

gregarious

living in a group.

head capsule

the sclerotized (hardened) part of the head.

herbivorous

feeding on foliage.

holometabolous

an insect whose development involves a pupal stage.

inchworm

the caterpillar of geometrids which has only one pair of midabdominal prolegs.

instar

an immature insect between molts.

labium

the bottom-most part of the mouthparts upon which the caterpillar silk gland is located.

labrum

a flap-like part of the mouthparts just below the face used as a feeding guide.

larva

one of the immature stages in the life cycle of a holometabolous insect.

lateral

an area along the side of a body part.

mandible

a component of the mouthparts used for cutting and chewing food.

mesothorax

the second (middle) segment of the thorax from the head, abbreviated as T2.

metathorax

the third (last) segment of the thorax from the head, abbreviated as T3.

midabdominal proleg

prolegs on the middle abdominal segments.

middorsal

an area in the middle of the top region of a body part.

monophagous

feeding on only one kind of plant.

multiserial

the base of the crochets occur in a pattern of multiple rows.

multivoltine

more than two generations per year.

nocturnal

active during the night.

occiput

the top portion of the head capsule.

omnivorous

feeding on both plants and animals.

osmeterium

an eversible forked pouch in Papilionidae located middorsally on the prothorax.

oviposition

the laying of an egg.

parasitoid

a specialized insect predator whose young develop entirely on a single host and kill the host.

plusiine noctuids

a group of moths whose caterpillar typically has two pair of midabdominal prolegs.

polyphagous

feeding on many plant species.

posterior

to be behind; the area that is the back end of a body part.

predaceous

feeding on animals.

proleg

fleshy protuberance of the body wall creating a nonjointed leg.

prothoracic shield

a sclerotized plate on the first thoracic segment.

prothorax

the first segment of the thorax behind the head, abbreviated as T1.

pupa

the life stage of a holometabolous insect that follows the larval stage and within which metamorphosis occurs.

sclerotized

a hardened area of the body skin.

scolus (pl. scoli)

a spine with multiple points.

semilooper

the caterpillar of plusiine noctuids.

setae

hairs.

silk gland

a silk-excreting gland in caterpillars on the underside of the labium.

spiracle

the opening of the respiratory system located along the lateral side of the body.

stemma (pl. stemmata)

the eyes on the head capsule, also called eyespots.

subdorsal

the body area part way between dorsal and lateral.

sublateral

the area below the lateral region of a body part.

subspiracular

the body area immediately below the spiracles.

triordinal

the tips of the crochets occur in a pattern of three rows.

triserial

the base of the crochets occur in a pattern of three rows.

uniordinal

the tips of the crochets occur in one row.

uniserial

the base of the crochets occur in one row.

univoltine

one generation per year.

urticating

stinging (hairs).

venter

the underside of the body.

ventral

the area on the underside of a body part.

warts

small bumps or short finger-like projections extending from the body wall.

Covell, C.V., Jr. 1984. A field guide to the moths of eastern North America. Boston: Houghton Mifflin Co.; 496 p.

Dornfeld, E.J. 1980. The butterflies of Oregon. Forest Grove, OR: Timber Press; 276 p.

Essig, E.O. 1929. Insects of western North America. New York, NY: The MacMillan Co.; 1035 p.

Furniss, R.L.; Carolin, V.M. 1977. Western forest insects. Miscellaneous Publication 1339. Washington, DC: U.S. Department of Agriculture, Forest Service; 654 p.

Hinchliff, J. 1994. An atlas of Oregon butterflies. Corvallis, OR: Oregon State University Bookstore; 176 p.

Ives, W.G.H.; Wong, H.R. 1988. Tree and shrub insects of the Prairie Provinces. Information Report NOR-X-292. Edmonton, AB: Canadian Forestry Service, Northern Forest Centre; 327 p.

Johnson, W.T.: Lyon, W.H. 1991. Insects that feed on trees and shrubs. 2d ed. Ithaca, NY: Cornell University Press; 560 p.

McCabe, T. 1991. Atlas of Adirondack caterpillars. Museum Bulletin 470. Albany, NY: State Education Department, New York State Museum; 114 p.

McGugan, B.M. (compiler). 1958. Forest Lepidoptera of Canada. Vol. 1: Papilionidae to Arctiidae. Publication 1034. Ottawa, ON: Canada Department of Agriculture, Forest Biology Division; p. 1-76.

Miller, J.C. 1995. Caterpillars of Pacific Northwest Forests and Woodlands. USDA, US For. Serv. FHM-NC-06-95; 80p.

Peterson, A. 1962. Larvae of insects. Part I: Lepidoptera and Hymenoptera. Ann Arbor, MI: Printed for the author by Edwards Bros.; 315 p.

Prentice, R.M. 1962. Forest Lepidoptera of Canada. Part II: Nycteolidae, Notodontidae, Noctuidae, Liparidae. Bulletin 128. Ottawa, ON: Canada Department of Forestry; p. 77-281.

Prentice, R.M. 1963. Forest Lepidoptera of Canada. Part III: Lasiocampidae, Drepanidae, Thyatiridae, Geometridae. Publication 1013. Ottawa, ON: Canada Department of Forestry; p. 282-543.

Prentice, R.M. 1965. Forest Lepidoptera of Canada. Part IV: Microlepidoptera. Publication 1142. Ottawa, ON: Canada Department of Forestry; p. 544-840.

Scoble, M.J. 1995. The Lepidoptera: form, function and diversity. Oxford, UK: The Oxford University Press; 404 p.

Stamp, N.E.; Casey, T.M. (eds.). 1993. Caterpillars: ecological and evolutionary constraints on foraging. New York: Chapman and Hall; 587 p.

Stehr, F.W. (ed.). 1987. Immature insects. Vol. 1. Dubuque, IA: Kendall Hunt Publishing Co.; 754 p.

Stevens, R.E.; Carolin, V.M.; Markin, P. 1984. Lepidoptera associated with western spruce budworm. Agric. Handbk. 622. Washington, DC: U.S. Department of Agriculture, Forest Service; 63 p.

Stoetzel, M.B. (compiler). 1989. Common names of insects and related organisms. Lanham, MD: Entomological Society of America; 199 p.

Tietz, H.M. 1972. An index to the described life histories, early ages and hosts of the macrolepidoptera of the Continental United States and Canada. Sarasota, FL: Allyn Museum of Entomology; 1041 p.

Wagner, D.L.; Henry, J.J; Peacock, J.W.; McManus, M.L.; Reardon, R.C. 1995. Common caterpillars of eastern deciduous forests. FHM-NC-04-95. Morgantown, WV: U.S. Department of Agriculture, Forest Service, National Center of Forest Health Management; 31 p.

Wagner, D.L, V. Giles, R.C. Reardon, and M. McManus. 1997. Caterpillars of Eastern Forests. USDA, US For. Serv., FHTET-96-34; 113p.

Wright, A.B. 1993. Peterson’s first guide to caterpillars. Boston: Houghton Mifflin Co.; 128 p.