The early stages in ontogenetic development are described in the co-occurring Sphaerophthalmus alatus (Boeck, 1838) and Ctenopyge (Mesoctenopyge) tumida Westergård, 1922, using fragmentary but otherwise well-preserved material from Bornholm, Denmark. The former species is a senior synonym of Ctenopyge (Eoctenopyge) angusta Westergård, 1922, as has been recently proposed, but the early stages of S. alatus from Bornholm are appreciably more spiny than those found of the same species in other places in Scandinavia, and spinosity is retained until later in development. Increasing convexity with development and other morphological changes are noted. Pygidia were previously unknown in C. (M.) tumida. Here, they are represented by early pygidia. These are shield-shaped, with a very spiny margin and a strong axial spine. The distinction between Sphaerophthalmus and Ctenopyge is discussed.
