Intromittent organ

An intromittent organ is any external organ of a male organism that is specialized to deliver sperm during copulation. Intromittent organs are found most often in terrestrial species, as most non-mammalian aquatic species fertilize their eggs externally, although there are exceptions. For many species in the animal kingdom, the male intromittent organ is a hallmark characteristic of internal fertilization.

Male cephalopods have a specialized arm, the hectocotylus, which is inserted into the female's mantle cavity to deliver a spermatophore during copulation. In some species, the hectocotylus breaks off inside the female's mantle cavity; in others, it can be used repeatedly to copulate with different females.

In spiders, the intromittent organs are the male pedipalps, even though these are not primarily sexual organs, but serve as indirect mating organs; in the male the pedipalps have hollow, clubbed tips, often of complex internal anatomy. The sexually mature male typically deposits semen from the palpal bulb onto a specially woven silken mat, then sucks the emission into his pedipalps. In mating, he inserts the openings of the pedipalps in turn into the epigyne, the female external genital structure.^{[citation needed]}

In Solifugae, sperm transfer is also indirect; the male deposits a spermatophore on the ground, picks it up in his chelicerae, then inserts it into the female's genital opening.

In Opiliones (harvestmen), males have a structure called a penis, which is not present in other arachnids.

In most millipedes, sperm transfer is performed by one or two pairs of modified legs called gonopods, which are often on the seventh body segment. During mating, a male bends his body to collect a spermatophore from the genital pore of his third segment, and inserts it into the female's body. Gonopods vary greatly among millipedes, and are often used to identify species.

Male insects possess an aedeagus, whose function is directly analogous to that of the vertebrate penis. Some insects also have claspers. Male moths have an additional organ called the juxta, which supports the aedeagus. These however are generalisations, and insect genitalia vary enormously in anatomy and in application. For example, some insects, most notoriously the Cimicidae and some Strepsiptera, practise traumatic insemination, in which the intromittent organ pierces the abdominal wall and the semen is deposited into the hemocoel.

In male members of Chondrichthyes (sharks and rays), as well as now-extinct placoderms, the pelvic fins bear specialized claspers. During copulation, one clasper is inserted into the female's cloaca, and sperm is flushed by the male's body through a groove into the female.