Meet the animal kingdom’s first-known female penis. The journal Current Biology announced on Thursday that scientists have discovered four species of Brazilian, winged insects (called Neotrogla), in which females possess a penis and males possess a vagina. Why aren’t the ones with penises considered males, you ask? Scientists actually rely on the size of an animal’s gametes (sperm for males, oocytes for females) to determine their genders. Females produce the largest gametes, a rule that holds true for this species of insect, in which the female still produces egg cells—they just also happen to have penises. The insects were discovered in Brazil’s Peruaçu River Valley. Mating behavior lasts 40 to 70 hours, during which the female penetrates the male and gathers large quantities of sperm with which to fertilize her eggs.