'Maier, C.T.; Lemmon, C.R.; Fengler, J.M.; Schweitzer, D.F.; Reardon, R.C.; Caterpillars on the Foliage of Conifers in the Northeastern United States. Morgantown, WV. USDA Forest Service. Forest Health Technology Enterprise Team Connecticut Agricultural Experiment Station. FHTET-2004-01. March 2004. 151 p.
We shall use a few specialized terms to describe the caterpillars of moths and butterflies. In this section, terms that refer to structures or areas in the illustrations are printed in boldface type at first mention. These and other specialized terms are defined in the Glossary of Specialized Terms.
The caterpillar (or larva) has a more or less cylindrical body composed of three main body parts: the head, the thorax, and the abdomen (Figures 1 and 2). The head (Figure 3) appears to be a single unit, but actually it is comprised of six fused segments that create a tough, hardened (sclerotized) head capsule. The head bears the feeding and sensory appendages. On each side of the head is a typically rounded area called a lobe that is above the stemmata, six simple light-sensitive eyes arranged in a curved row. In the middle of the front of the head is a conspicuous plate known as the frons or frontal triangle. Below the frons is the clypeus, a narrow plate that runs across the front of the lower head. Beneath it is the labrum or upper lip, a simple sclerotized plate that may or may not be notched. A notched labrum is used to guide food to the mandibles, a sturdy pair of opposable-toothed jaws that are located behind the labrum and usually extend below it. On each side of the head is a short antenna, which is a sensory structure that lies between the stemmata and the clypeus or labrum. The head has other structures, but most of them are visible only from the rear or are internal. Within the head are the modified salivary glands that produce silk, a proteinaceous substance used for aerial dispersal, construction of shelters (tents, feeding webs, and cocoons), escape from predators, and other things.
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| Figure 1. Side view of a caterpillar called a looper (Family Geometridae). |
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| Figure 2. Side view of a caterpillar called a cutworm (Family Noctuidae). |
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| Figure 3. Front of a generalized head of a caterpillar |
Each of the three thoracic segments (T1 to T3) has a pair of "true" legs with claws (Figures 1 and 2). The first thoracic segment (T1) is covered dorsally by a prothoracic shield or plate. The color pattern of this shield is valuable for identifying species of leafrollers (Family Tortricidae) and other caterpillars.
The abdomen consists of 10 visible segments (A1 to A10). Most caterpillars have a single pair of prolegs or fleshy false legs near the bottom of segments A3 to A6 and A10 (Figures 1 and 2). In a few groups, the prolegs are reduced to two or three pairs (especially loopers and inchworms) or in size (many needleminers). The prolegs on A10 usually are well developed. The ends of the prolegs have a circular or other arrangement of crochets or tiny hooks that are used to cling to foliage, bark, silk, or other substances. Experts sometimes use the arrangement and the length of the crochets to identify caterpillars to the family level. Although crochets are briefly mentioned in some descriptions, they usually will not be used to identify caterpillars in this manual. Abdominal segments A1 to A8, as well as thoracic segment T1, have a spiracle on each side of the body. Spiracles are usually circular or oval openings to the respiratory system of insects. In some caterpillars, there is a distinct anal plate or sclerotized shield on the top of A10.
The body of a caterpillar may be adorned with hairs, spines, horns, warts, swellings, tubercles, and other structures that are helpful in distinguishing species. Chaetotaxy, the arrangement and the nomenclature of body hairs, is useful for separating the families of Lepidoptera, but its use here would unduly burden the reader.
The body may have distinctive colors or patterns of color. Many caterpillars have longitudinal (lengthwise) lines or stripes on their thorax and abdomen (sometimes extending to the head). These longitudinal markings frequently allow them to blend into their background. These marks facilitate identification even though they may vary in width or color, even within a species. No standard terminology exists for the various colored streaks and patterns; however, some jargon is necessary to refer to the location where a colored mark or a structure is located. Our descriptive terms are shown in Figures 4 and 5. The areas from top to bottom are: middorsal, subdorsal, supraspiracular, spiracular, subspiracular, and subventral. These and additional specialized terms for markings are defined in the glossary.
Larvae of other insect groups can resemble caterpillars. For example, beetle larvae can be elongate, but they do not have abdominal prolegs. Sawflies, which also may eat the needles of conifers, have more than five pairs of abdominal prolegs, and these lack crochets.
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| Figure 4. Top view of a generalized abdominal segment |
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| Figure 5. Side view of a generalized abdominal segment |
