Abstract: Head width is an important individual size parameter of insects，but the measurement of head width has some limitations，such as effects of human error，high time cost，and the use of a single parameter．With the development of computer technology，some size parameters of insects，including aquatic insects，which were previously difficult to ascertain，can now be obtained through image-based analysis．To promote the application of these size parameters in studies on phenology and age structure of insects，especially aquatic insects，it is important to clarify the correlations between size parameters and head width．Two species of aquatic insects，Ephemera wuchowensis Hsu，1937 and Caenis lubrica Tong and Dudgeon，2002，were selected for this study．These species have large differences in individual size throughout the larval period，and were selected to study the correlations between head width and individual size parameters obtained based on digital images，including area，perimeter，width and height of bounding rectangle，major and minor of fit ellipse，and Feret’s diameter．Digital images of the insects were obtained by scanning using a modified scanner，and the image analyses were carried out using ImageJ software．Correlation analysis of parameters based on binary linear regression and multivariate linear regression．The results show that all the individual size parameters of the two species of aquatic insects obtained based on ImageJ analysis had very high and significantly positive correlations with head width，and the correlation between area and head width was the most prominent．And the correlation between area and other size parameters is also very significant，which is a good representation of individual size．The results provide technical support and a theoretical basis for promoting the application of digital image analysis technology in biological and ecological studies on aquatic insects and other insects．