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1. Number of sperm and its relationship with larval rearing density were investigated in the armyworm Pseudaletia separata. Males that emerged from crowded larvae produced significantly more apyrene sperm than those from solitary larvae (375 700 ± 116 600 and 290 300 ± 99 600 at a mating with a 3-day old virgin, respectively), with no significant difference in number of eupyrene sperm between the two types being observed.

2. For both solitary- and crowded-type, the amount of fertile sperm the males produced at a mating exceeded the number needed to fertilize all of a female’s eggs, suggesting that sperm competition may be a major selective force for keeping sperm numerous. The production of more apyrene sperm by crowded-type males may be an adaptation to cope with the increased sperm competition from rival males at high density.

3. The relationship between number of sperm and spermatophore size was also studied using solitary-type moths. Large spermatophores were found to have more eupyrene and apyrene sperm than small ones.