Mechanisms of Sperm Transfer and Storage in the Red Flour Beetle (Coleoptera: Tenebrionidae)

This study examines mechanisms of sperm transfer during copulation in Tribolium castaneum (Herbst), red flour beetles, and describes the dynamics of sperm movement within the female reproductive tract. During copulations that range in duration from 30 s to 32 min, T. castaneum males transfer sperm to females in a spermatophore consisting of a membranous sperm sac attached to a gelatinous posterior rod. The spermatophore is deposited in the female bursa copulatrix; shortly afterward, the sperm sac membrane ruptures and sperm are released. Sperm begin to appear in the complex, tubular spermatheca of the female within 10 min after mating. Between 0 and 60 min after mating, the number of sperm stored in the spermatheca exhibits an 8.5-fold increase, and sperm quantity in the bursa declines. Both bursal and spermathecal sperm numbers remain nearly constant between 60 and 120 min after mating. Only ≈4% of the total sperm transferred by mating males are translocated to the female's spermatheca for long-term storage. T. castaneum females continue to use sperm stored in the female spermatheca to fertilize their eggs for up to 140 d following a single mating. Neither male body size nor copulation duration appear to influence the number of sperm transferred during copulation or the number of sperm stored within the female spermatheca.