Promiscuity and elongated sperm storage organs work cooperatively as a cryptic female choice mechanism in an earwig

► I examined the function of exaggerated sperm storage in promiscuous female earwigs. ► Males can displace only 20% of sperm stored in an elongated spermatheca per mating. ► Larger males had higher competitiveness and repeated matings with individual females. ► Larger males also sired more sons, which inherit their larger body size. ► A simulation study showed 20% displacement of sperm is optimal for gaining larger sons.