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- Contrary to popular belief, the presence or absence of certain sex organs isn’t the determining factor when deciding which animal of a species is female and which is male. In fact, biologists don’t use sex chromosomes either. They actually rely on the size of an animal’s gametes — sperm in males and oocytes in females. As the rule goes, females are the sex that contribute the largest gametes, whereas males are the sex that contribute the smallest gametes and therefore expend the least amount of energy on producing these cells. So, in this particular instance of sex-role reversal, the convention still applies: the female in these species of insect produces the largest gametes — egg cells. She simply also happens to sport a penis that she introduces into the male’s vagina during copulation.