Abstract: 【Aim】 Most beetles fly with their elytra open, but there are a few species that fly with their elytra almost closed (e.g. most cetoniines and some species of dung beetles). Closed elytra during flight can significantly reduce air resistance and improve flight speed, but limit the motion amplitude and stress state of the hindwing base articulation sclerites (axillaries). And the axillaries play a role of power transmission hub. Previous studies have found that axillaries have complex internal and external morphological structures, but it is not clear what factors affect such complex morphological structures, and the relationship between the opening and closing behaviors of the elytra and the axillaries during flight is still unknown. 【Methods】 A total of 27 species from 10 families of Scarabaeoidea and 6 subfamilies of Scarabaeidae were scanned using Micro CT. The threedimensional morphologies of axillary sclerites ［1st axillary (1Ax), 2nd axillary (2Ax) and 3rd axillary (3Ax)］ were reconstructed and analyzed by computer aided three-dimensional reconstruction. Among them, 17 species fly with the elytra open, and 10 species fly with the elytra closed, covering 83% of the family level of the Scarabaeoidea. 【Results】 The morphological characters of the 1Ax, 2Ax and 3Ax of the scarabs with elytra open during flight are different from those of the scarabs with elytra closed. The front end of the head of 1Ax is narrow, and the neck is broad and convex. The ridge of 2Ax is wide, and the positions of proximal lobe and distal lobe are different. The head of 3Ax is broad and the inner apex angle is acute, and the tail of 3Ax is relatively short. Axillaries of the same group have similar morphologies. The morphologies of the axillaries of cetoniines and its sister group are different from those of the tribe of Gymnopleurini. 【Conclusion】 The opening and closing statuses of elytra during flight have an influence on the morphology of the axillary sclerites, and the morphology of axillary sclerites is not only affected by the pressure of flight selection, but also affected by phylogeny.

Key words: Scarabaeoidea, hindwing axillaries, threedimensional reconstruction, flight modes