ABSTRACT From a study of the larvae of some twenty-eight species of Lepidoptera, Dyar(1) concluded that growth, as indicated by the growth of the head in successive instars, follows a regular geometrical progression. Fisher (2), working on Tortrix pronubana Hb., applied the findings of Dyar and worked out the growth ratio, which he found to be 1·37. The following table indicates how the growth ratio is arrived at: Table I.(Fisher, 1924.) In the above table the measurements are given in millimetres, and the modes of the observed widths of the head capsules of the first and second larval instars are taken as the basis of calculation. By dividing the mode for the head widths of the second instar larvae by that for the first instar larvae, a figure representing the ratio of increase is found. By further calculation it is possible to estimate the width of the head capsule for any subsequent instar. The close approximation of calculated and observed measurements furnishes the student of life history of Lepidoptera with a means of checking larval ecdysis and development. Morphologically the larvae of Tenthredinidae bear a close resemblance to those of Lepidoptera, hence it seemed possible that the same general laws with regard to growth might be followed. Material was obtained, therefore, to test the application of Dyar’s law for growth in the larvae of Lepidoptera to growth in the larvae of the Tenthredinidae.