At least 30 specimens of each form of Macrosiphum rubiellum, Theobald, and Aphis idaei, van der Goot, were collected, mounted and their principal organs measured.
Correlations were calculated between different parts on the right side and the conclusion reached that the ratios of lengths of organs were unstable characteristics of the species examined. High correlations were obtained between the same part on different sides of the body.
The mean values and standard errors are tabulated and a method given for assigning limits of variation within a species. The standard errors obtained were of the order of 7 per cent. of the mean.
The asymmetry, kurtosis and skewness of the distributions were calculated to test the effect of possible non-normality on the foregoing work.