Errors in Determining Instar Numbers through Head Capsule Measurements of a Lepidopteran—a Laboratory Study and Critique

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Abstract
Larvae of Choristoneura viridis Freeman (Tortricidae) were reared individually on an artificial medium in the laboratory under controlled conditions. The larvae exhibited developmental polymorphism, i.e., some larvae had a total of 6 instars, others 7, and still others 8 instars. Except for instar I, all larval head capsules were recovered and widths measured for each larva in the study population. A frequency distribution curve of capsule widths suggested only 6 instars, when in fact over 57% of individuals showed 7 or more instars. Head capsule width ranges for successive instars, as suggested by the multimodal frequency distribution curve, were not in agreement with known values for the laboratory population, nor were they in agreement when values for the population were segregated by sex irrespective of larval instar group. Peaks in the curve could be interpreted only after the head capsule widths in the population were segregated by instar group. In larval growth regressions for the population as well as for head capsules broken down by sex and/or instar group, standard error of estimate values appeared to be more sensitive than R2 values in reflecting relative precisions of equations. Generated frequency distribution curves of head capsule widths support the argument that frequency distribution of head capsule widths cannot be used to assign instar numbers in lepidopterous species with developmental polymorphism.

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