An adaptation is a physical trait that gives an organism a survival advantage in an environment that it would not ordinarily have. In the case of spiders, spiders have a specialized set of pedipalps, the end section housing a special palpal bulb used to deliver sperm to the female for egg fertilization.
Shrimp similarly have a protruding penes on the male mantis shrimp. The purpose of the penes is to deliver sperm to the genital slit of the female shrimp during copulation.
Male millipedes have a secondary sex organ called a gonopod. The purpose of the gonopod is to store and deliver sperm to the female millipede when the millipedes copulate.
Flies have a torsional difference between the sperm duct of the male and the genital orifice of the female. The male must face an opposite direction for the necessary alignment. This gives flies a faster reproductive rate than most other insects.
Mosquitoes seem to be the only specimen in the group without a specialized trait for reproductive adaptation. In all the other groups, the existence of a structure to help deliver the sperm to the female seems to be the common thread in terms of a reproductive adaptation trait.