(A) Main steps of our program for unsupervised segmentation of the cuticle and body volume. (a) Sagittal cut of a Myrmoteras ant worker with cuticle segmented by our unsupervised program. Starting from the original slice (b), image colors are adjusted and the vial is detected and cropped. We then apply a pixel intensity threshold (c) and filter contours inside another to extract cuticle volume (d). We lastly fill cuticle contours to extract body volume (e). These steps were applied to all image slices composing the scans to obtain the global cuticle and body volume. (B) Allometric relationship between cuticle volume and body volume in ant workers, queens, and males compared to the outgroup. Each data point represents the average cuticle volume and body volume per species per caste. The table summarizes the regression parameters obtained from SMA regression. CI, confidence interval. (C) Ancestral state reconstruction of the size-standardized worker cuticle investment for 434 species (174 genera). Colored branches indicate predicted values of traits, and the density plot reflects the density of tip values. Time is expressed in Ma, and major subfamilies are labeled as follows: Ambly, Amblyoponinae; Dolicho, Dolichoderinae; Ectato, Ectatominae; Pr, Proceratiinae; Pseudo, Pseudomyrmecinae. Warmer (redder) colors represent decreasing cuticle investment, whereas cooler colors (darker blues) represent higher cuticle investments. Key genera are provided as landmarks, and a similar tree with full details of species as well as ancestral state reconstructions for queens and males is provided in fig. S2.