Male genitalia of 25 spp. are studied using scanning electron microscopy, and the structure of the fourth penile segment is described. Remarkable diversity exists among spp., particularly in the size and shape of the lateral lobes, and the morphology of the cornua. There are also differences in the number of cornua among the taxa surveyed. The surface of the lobes of many spp. is covered with spines which anchor the penis during copulation, and may trap and remove sperm when the penis is collapsed and withdrawn following copulation. Spp. are categorized according to the morphology of the penis and inferred patterns of sperm removal. Type 1 taxa possess relatively large, broad, flat lateral lobes, and lack cornua, or possess cornua that are greatly reduced in size. These spp. are believed to displace sperm in the bursa copulatrix before depositing their own sperm, thereby gaining positional priority during oviposition. Type 2 spp. possess elongated lateral lobes and/or cornua. These taxa are believed to engage in a mixed strategy of sperm displacement and sperm removal. Optimization of these characters on a phylogeny of the 3 genera indicates that the Type 1 sperm displacement strategy is ancestral, and that the Type 2 strategy was subsequently derived within the majority of the Libellula s.s. taxa.