Internal fertilization is the union of an egg and sperm cell during sexual reproduction inside the female body.
[1] For internal fertilization to happen there needs to be a method for the male to introduce the sperm into the female's reproductive tract.
[2]: 124–125 Male mammals, reptiles, and certain other vertebrates transfer sperm into the female's vagina or cloaca through an intromittent organ during copulation.
[6] Salamanders, spiders, some insects and some molluscs undertake internal fertilization by transferring a spermatophore, a bundle of sperm, from the male to the female.
These are traditionally classified as follows: Internal fertilization allows for: Some species of fish like guppies have the ability to internally fertilize, this process happens by the male inserting a tubular fin into the female's reproductive opening and then will deposit sperm into her reproductive tract.
The male deposits a spermatophore on the ground and the female will pick it up with her cloaca (a combined urinary and genital opening) and fertilize her eggs with it.