In queenless ants, gamergates (mated egg-laying workers) fulfil the reproductive task normally reserved for the queen. Every worker is a potential gamergate, thus we expect pronounced conflicts over sexual reproduction within their colonies. In the queenless ant genus Diacamma, gamergates inhibit nest mates from mating by aggressively removing ('mutilating') a pair of small appendages on the thorax, termed gemmae, shortly after eclosion. Dissection and serial sectioning of the reproductive tracts of both mutilated and unmutilated individuals of Diacamma sp. from Japan at different ages revealed that mutilation inhibits the development of the bursa copulatrix and the spermatheca, two structures fundamental for sexual reproduction. The precursor of the bursa copulatrix develops into a fully functional structure in unmutilated individuals, whereas it degenerates irreversibly in mutilated callows. Experimental manipulations showed that the removal of the gemmae is not the sole factor regulating this development. The spermathecal epithelium and accessory spermathecal gland of unmutilated individuals are thicker than that of mutilated individuals, indicating a higher degree of activity in the former. Mutilated females are therefore left incapable of copulating and less competent for long-time sperm storage.