the opposite sex. Responses in insects can vary in both genitalia and sperm structures, along with variations in behavior. Spiny genitalia Male genitalia evolve more rapidly and divergently in animals. Spiny genitalia can aid in male-male competition. In seed beetles, spiny genitalia help with anchor during copulation and allow a rapid passage to the female's reproductive tract, thus overcoming female barriers to sperm. Females suffer costs as a result of injuries, but males do not benefit directly from harm inflicted on their mates. Damage, such as scarring, increases in the female tract with the number of matings. In seed beetles, a positive correlation exists between the degree of harmfulness of the male's genitalia and the thickness or reinforcement of the wall of the bursa copulatrix in the female's reproductive tract. As a result, females’ connective tissue in the copulatory tract increased in thickness. However, females with a thicker copulatory tract correlated positively to the amount of scarring, suggesting that scarring is a poor measure of costs for females. Females have evolved in other ways such as investing in immunocapacity to help with trauma associated during copulation. Copulation Male bed bugs have a unique way to copulate called traumatic insemination. Males use their intromittent organ to stab and inseminate females through their abdominal wall even though females contain a genital tract. Male bed bugs can also adjust their ejaculate volume and time of copulation through the presence of ejaculates in females to conserve sperm and determine paternity outcomes. Females have evolved a paragenital system to counter traumatic inseminations. The paragenital system contains a mesospermalege where sperm is deposited. The sperm migrates through the blood to the sperm storage site and oviducts, and then to the ovaries to fertilize eggs. Female bed bugs have also evolved physiological by the presence of phagocytic cells in the mesospermalege that ingest sperm after mating. Development time Selection on development time is often sexually antagonistic. In seed beetles, populations differed in development time and growth rate between sexes. Population fitness is not significant to either body size or growth rate, but variation in development time was significantly related to population fitness. In females, genes associated with long development time lead to high fecundity and mate immediately upon eclosion. Males have shorter development time and emerge early (protandry) resulting in greater fertilization opportunities. Sperm tail length Competition between differing male phenotypes also exists at the microscale level.