1. Adult Calopteryx splendens xanthostoma are infected by the trophozoites of a eugregarine parasite that attaches to the mid-gut epithelium.
2. The results of this study show: (1) that eugregarines were the predominant parasite in the isolated study population, (2) they were not present in imagos that had just eclosed from larvae but were present in significantly higher numbers in older adults, (3) animals with a eugregarine burden during the prereproductive fat-accumulation period of their adult development (the teneral stage) had significantly lower fat content, and (4) males infected artificially with eugregarine oocysts showed no correlation between eugregarine burdens and postmanipulation fat content.
3. Infection with eugregarine trophozoites during the teneral life-history stage affects the ability of damselflies to accumulate fat, and consequently reduces their ability to fight for, and maintain, a territory when they become reproductively active.