Sperm co‐operation in the Fishfly, Parachauliodes japonicus

1. Males of the Fishfly Parachauliodes japonicus (McLachlan) produce sperm in bundles. Each bundle consists of hundreds of sperm with their heads agglutinated. At copulation, on average 500 bundles are packed in a single spermatophore which is attached externally to the female genitalia. The bundles swim forward by synchronous flagellate movements in viscous seminal fluids and finally enter a spermatheca of the female. Females detach the spermatophore without guarding by the male; males guard the spermatophore for 5 h. 2. In this study, the effects of sperm‐bundle size and medium viscosity on their swimming velocities were examined. As sperm‐bundle mass increased, the amount of time it took to move a unit length decreased. As viscosity increased, those sperm‐bundles with greater mass moved faster than sperm‐bundles with less mass. 3. The distance sperm move seems to be longer in P. japonicus than in most other insects that ejaculate directly or place the spermatophore internally in the female storage organs. Thus, large sperm‐bundles may be an advantage for sperm of P.japonicus who move over a long distance in a viscous environment.