Traumatic insemination, also known as hypodermic insemination, is the mating practice in some species of invertebrates in which the male pierces the female's abdomen with his penis and injects his sperm through the wound into her abdominal cavity (hemocoel). The sperm diffuse through the female's hemolymph, reaching the ovaries and resulting in fertilization. The process is detrimental to the female's health. It creates an open wound which impairs the female until it heals, and is susceptible to infection. The injection of sperm and ejaculatory fluids into the hemocoel can also trigger an immune reaction in the female. Bedbugs, which reproduce solely by traumatic insemination, have evolved a pair of sperm-receptacles, known as the spermalege. The spermalege reduce the damage to the female bedbug during traumatic insemination.

The evolutionary origins of traumatic insemination are disputed. Although it evolved independently in many invertebrate species, traumatic insemination is most highly adapted and thoroughly studied in bedbugs, particularly Cimex lectularius. Traumatic insemination is not limited to male-female couplings, or even couplings of the same species. Both homosexual and inter-species traumatic inseminations have been observed. Traumatic insemination has been likened to human sadomasochism, stabbing, and rape behaviors. However, such coercive sex practices are common in nature and provide sperm competition, enable bypassing the mating plug, and overcome female resistance to being mated.