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Caterpillars of Pacific Northwest
Forests and Woodlands

Caterpillar Morphology

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.

Fig. 1. Schematic drawing of a caterpillar (redrawn from Wagner et al. 1995).

Caterpillars may be distinguished from other immature insects by a combination of the following features (Fig. 1):

adfrontal suture on the head capsule;
six stemmata (eyespots) on the head capsule;
silk gland on the labium (mouthparts);
prolegs on abdominal segments A3, A4, A5, A6, and Al0; or A5, A6, and Al0; or A6 and A10
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 A1, A2, A7, and A8, in addition to the prolegs on A3-A6 and A10.

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 variety in morphology is the result 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.

Body Regions

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

Head. 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. 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.

Thorax. The thorax has three segments, each with a pair of segmented legs. The three thoracic segments are the: prothorax, nearest the head (T1); mesothorax, in the middle (T2); and metathorax, connecting to the abdomen (T3). 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.

Abdomen. The typical abdomen has ten segments, A1-A10. Segments A1-A8 have spiracles, and an anal plate may occur on A10. Depending on the family group, certain abdominal segments have fleshy prolegs bearing crochets (hooks): segments A3-A6, the midabdominal prolegs, and A10, the anal prolegs. The typical pattern for prolegs is one pair per segment on A3-A6 and Al0. However, some caterpillars have prolegs only on A5, A6 and Al0, which is characteristic of the Plusiinae of the Noctuidae; or on A6 and A10 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 A1, A2 or A7-A9, 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 are 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 be 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.

Hairs. The type and arrangement of hairs are used to place a caterpillar into the groups devised here. 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. Hair 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. 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.

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

Tubercles. 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.

Horns. 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 A8. 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.

Body Shape

The typical caterpillar body is cylindrical. Variations of the cylindrical shape include bodies that are flattened, humped or 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.

Humps. 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 in Zale lunata.

Constrictions. 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.

Colors and Patterns

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 subtly 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.

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

Spiracular longitudinal bands. 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.

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

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

Spiracular longitudinal lines. 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.

Rings. 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.

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

Middorsal dashes. 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.

Subdorsal dashes. 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.

Middorsal line of circular or elliptical spots. 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.

Scattered speckles. 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.

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

Dorsal transverse bands or lines. 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.

Anal transverse bands or lines. 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.

Oblique lines on midabdominal segments. 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.

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