On “Dyar'S Rule” and Its Application to Sawfly Larvae.*

Abstract

Forty years ago Dyar (1890) sought to establish a criterion by which it would be possible to determine, by simple mathematics, the correctness or incorrectness of the number of instars observed. This was based upon the apparent close agreement between calculated and actual widths of the heads of all larval instars of 28 species of caterpillars, which had from four to ten stadia. In most cases, the calculated and observed widths were either identical or within 0.1 mm., though, apparently, a discrepancy of 0.2 mm. was not considered significant. In six cases of greater deviation in various instars among these species, the explanation offered was “abnormal development”—either dwarfing, because the larva was “unhealthy” and failed to cast its skin, etc., or unusual growth. In five other cases, probable incorrect measurements were admitted. From these data, Dyar found that the size of the larval head is (relatively) static during a stadium and that “… the widths of the head of a larva in its successive stages follow a regular geometrical progression …” (It was suggested that the width of the larval head is the most convenient measurement to take.)