polyembryony, a condition in which two or more embryos develop from a single fertilized egg, forming what in humans is known as identical twins. A common phenomenon in many plant and animal species, polyembryony occurs regularly in the nine-banded armadillo, which usually gives birth to four identical young. Striking examples may be found among parasitic insects of the order Hymenoptera; Copidosoma truncatellum, a parasite of certain cutworms, lays a single egg in the body of the host worm from which may develop as many as 2,000 individuals.