Guppy (Poecilia reticulata), small fish of northern South America and the Caribbean named for the Reverend Thomas Guppy, who discovered it in Trinidad in 1866. It is also called the rainbow fish, a more appropriate name, because the male is brilliantly colored. The females are usually drab and grow up to 2 in (5 cm) long—twice as big as the males. Guppies are popular aquarium fish. The young are born alive and have to take a gulp of air before they can swim properly. Guppies feed on algae and insect larvae. For this reason they have been released into ponds in many parts of the world to control mosquitoes.