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Female genitalia of insects are formed by several structures whose functions are poorly understood. The signa are sclerotized structures located on the inner wall of the corpus bursa of females of many Lepidoptera species. In this paper, we first describe seven hypotheses concerning the function of signa and derive several predictions from them. Then, we test several of these hypotheses with studies of four butterfly species (Callophrys xami Reakirt, Eueides isabella Cramer, E. lineata Salvin & Godman, and Heliconius ismenius Doubleday) and conclude that in these species signa are mainly used for breaking the envelope of spermatophores. These results are in agreement with the idea that signa are a product of sexual coevolution.