Intromittent organ

An intromittent organ is a general term for an 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 aquatic species fertilize their eggs externally, although there are exceptions. For many species in the animal kingdom, characteristic of internal fertilization.

Male cephalopods have a specialized arm, the , 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 his semen 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.

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 pair 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 females body. Gonopods vary greatly among millipedes, and are often used to identify species.

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