essential role in sperm storage and fertilization. In the family Tingidae, pseudospermatheca are located at the base of the oviduct and are hypothesized to have functioned as spermatheca at one point in time. They now serve as storage units for sperm, where a female can introduce the stored sperm to her eggs when she finds it optimal. It is this factor that has put females in the driver seat of evolution. These organs give females the ability to pick and choose which sperm they will use to fertilize their eggs. Males now have another factor they need to overcome. In the case of D. melanogaster, females will mate multiple times and then expel the excess sperm that she does not need. However, neither the first or second mate know if it his sperm that was dispelled, because at any postcopulatory moment a female can store the sperm of more than one male. Enzymes secreted by females Enzymes secreted by female reproductive tracts may also play a role in sexual antagonistic coevolution with males. In Drosophila species, a large group of enzymes known as serine proteases have been associated with female sperm storage organs (most notably, the spermatheca) through genetic sequencing and analysis. It is hypothesized that these proteases break down various proteins in male seminal fluid. This would result in females choosing for males that can overcome these digestive enzymes, whether through genetic variation or physiological ability to produce greater quality or quantity of sperm. Behavior Before a male even has to begin worrying if the female will use his sperm or not, he must mate with her, which can be a problem within itself. Potential mates often play a game of persistence and resistance. In the case of water striders (genus Gerris) males will harass females and try to grasp them by chasing and lunging at them. Females can be extremely evasive and often fend off these aggressive attacks. Even when a female is finally grasped she continues to struggle. However, this type of avoidance is very costly to a female, so she ends up having to balance the cost of mating and the cost of resistance. However, in species with singly mating females like the fly Prochyliza xanthostoma, the cost of resisting mating is low relative to the benefit of evading a low quality male. Male co-evolution Like females, males have developed responses to counter evolutionary adaptations of