Sexual antagonistic co-evolution is the relationship between males and females where sexual morphology changes over time to counteract the opposite's sex traits to achieve the maximum reproductive success. This has been compared to an arms race between sexes. In many cases, male mating behavior is detrimental to the female's fitness. For example, when insects reproduce by means of traumatic insemination, it is very disadvantageous to the female's health. During mating, males will try to inseminate as many females as possible, however, the more times a female's abdomen is punctured, the less likely she is to survive. Females that possess traits to avoid multiple matings will be more likely to survive, resulting in a change in morphology. In males, genitalia is relatively simple and more likely to vary among generations compared to female genitalia. This results in a new trait that females have to avoid in order to survive. Additionally, sexual antagonistic co-evolution can be the cause of rapid evolution, as is thought to be the case in seminal proteins known as Acps in species of Drosophila melanogaster. While Acps facilitate the mutually beneficial outcome of increased progeny production, several Acps have detrimental effects on female fitness as they are toxic and shorten her lifespan. This leads to antagonistic co-evolution, as the female must evolve in order to defend herself. When female Drosophila melanogaster are experimentally prevented from co-evolving with males, males rapidly adapt to the static female phenotype. This male adaptation leads to a reduction in female survivorship, which is mediated by an increased rate of remating and increased toxicity of Acps in seminal fluid. Since non-reproductive proteins do not feel the same evolutionary pressure as Acps, they are not evolving nearly as quickly. Consistent with the arms race theory, DNA analyses reveal a two-fold increase in Acp divergence relative to non-reproductive proteins. Female co-evolution For many females, reproduction can be very dangerous and disadvantageous as in the case of bed bugs mentioned previously. Therefore, females who possess traits where they can lessen the impacts of male behavior are the ones who will survive and go on to reproduce. There are many ways a female can "defend" herself to the onslaught of potential mates. Spermatheca/pseudospermatheca Females have a very complex and an extremely variable reproductive system, commonly known as a spermatheca. Some species do not have a spermatheca in the traditional sense, but do possess pseudospermatheca. Both forms play an