Sperm competition and penis structure in some libellulid dragonflies (Anisoptera)

Within and between species the copulation of libellulid dragonflies varies in duration (seconds to >1 h), situation (perched or in flight) and in the amount of copulatory movement that occurs (from no visible activity to persistent rocking movements of the base of the male’s abdomen). Three ways in which sperm may be removed from the spermathecae of females are considered. In the first, in species which make long copulations accompanied by bouts of very small but fast rocking movements, sperm may be gradually filtered from the ejaculate by rows of flagellar spines as the flagellum is oscillated back and forth within the duct. In the second, strong but slower copulatory movements occur which may drive the spiny cornua in and out of the spermathecal ducts, filtering sperm as they do so. In the third, in which there are also marked copulatory movements, the grooved cornua lack spines and the male’s own ejaculate, or a diluted fraction of it. may be used to wash out rival sperm. Finally, in a fourth group of species in which copulation is brief and aerial, and the flagella lack spines but possess large terminal hooks, there is probably less sperm removal from the spermathecae.