MALE DISTURBANCE, REPEATED INSEMINATION AND SPERM COMPETITION IN THE DAMSELFLY COENAGRION SCITULUM (ZYGOPTERA, COENAGRIONIDAE)

Before mating, all male odonates translocate sperm from the testes (IX segment) to the penis (II segment), but in Coenagrion scitulum this behaviour is repeated up to six times during copulation. The aim of this study was to find an explanation for this unusual behaviour. Copulation behaviour consists of three to seven cycles, each of which includes one intra-male sperm translocation, one stage I and one stage II (i.e. the complete copulatory sequence typical of zygopterans). The duration of pre-copulatory tandem and cycle 1 of copulation was negatively correlated with time of day and positively with male disturbance. Males captured during stage I of any copulatory cycle always had the seminal vesicle full of sperm, thus indicating that they do translocate sperm during sperm translocation behaviour. Females were inseminated at stage II. The volume of ejaculate in females in the held interrupted during stage I of the first copulatory cycle was not significantly different from the volume stored by pre-copula females indicating that males cannot remove a significant amount of sperm from the female's genital tract at this stage. Spines on the horns of the penis, which in other damselflies help remove the sperm stored by females, are, however, absent in C. scitulum. Experiments with virgin females that received up to five inseminations indicated that the sperm are progressively packed, and therefore the volume is not directly proportional to the number of inseminations. Male C. scitulum thus has a poor sperm removal ability and multiple intra-male sperm translocation and insemination of the female during the same copulatory act seems to be a mechanism of sperm competition by which the male achieves a greater fertilization success.