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Male structures for clasping females during precopula interactions and mating (“claspers”) have evolved in many groups of arthropods. Several hypotheses regarding the function of claspers have been proposed. We describe how males of the true bug Stenomacra marginella (Heteroptera: Largidae) move their genital claspers during sexual interactions, and present the results of experiments in which we tested whether claspers are necessary to achieve intromission. When one and both claspers were partially amputated, the probability of successful intromission decreased from 62 to 0% and from 57 to 3%, respectively. Behavioral observations indicate that the claspers open the valves that cover the female genital opening. We consider the possibility that claspers in S. marginella may have multiple functions.