Relationship between body size and the morphology of the primordia. (a) The horns of pupae with different body sizes (left: 18 g, right: 14 g). The larger beetle has the longer horn. (b) The primordia of the horn from different body sizes (left: 18 g, right: 14 g). Both of the primordia have similar 2D furrow patterns (concentric furrow patterns and stripe furrows between them), while the overall sizes of primordia are different. The cap (top) regions of the primordia are indicated in red. The images were acquired by µCT scanning. (c) Virtual frontal section images of horn primordia via µCT scanning. The mushroom-like macro structure can be recognized regardless of body size. (d) Relationship between body size and the macro structure size, the density of the furrows, and the depth of the furrows. For macro structure size the correlation coefficient was 0.83 (p < 0.01), for furrow density the correlation coefficient was 0.59 (p < 0.05), and for furrow depth, the correlation coefficient was not significant. In the smallest beetle (white plot: 8.95 g), obvious furrows could not be detected so it was excluded from the analysis (n = 19). Scale bar is 10 mm for (a), 1 mm for (b) and (c).