An intromittent organ is any external organ of a male organism that is specialized to deliver sperm during copulation.
Species with intromittent organs
Invertebrates
Molluscs
Male
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.
Arachnids
In
, the intromittent organs are the male
, 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.
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 Opiliones penis, which is not present in other .
Millipedes
In most
, 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 female's body. Gonopods vary greatly among millipedes, and are often used to identify species.
Insects
Male insects possess an
aedeagus, whose function is directly analogous to that of the vertebrate penis. Some insects also have
valva. 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.
Vertebrates
Fish
In male members of
Chondrichthyes (
and
Batoidea), as well as now-extinct
Placodermi, the pelvic fins bear specialized
valva. 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.
Members of Poeciliidae are small fishes that give birth to live young. In males, the anal fin is shaped into a grooved, rod-shaped organ called a gonopodium used to deliver sperm to females.
Tetrapods
In
and
, males possess paired
hemipenis, each of which is usually grooved to allow sperm transport and spiny or rough at the tip to allow firm attachment to the female.
To become
erection, a hemipenis is evaginated (turned inside out) through muscle action and engorgement with blood. Only one is inserted into the female's cloaca at a time.
In reptiles, the phallus has an open sulcus instead of a closed urethral tube.
[Gredler, Marissa L., et al. " Evolution of external genitalia: insights from reptilian development." Sexual Development 8.5 (2014): 311-326.]
In some , , some , and in all , males possess a penis centered along the midline of the body. During copulation, it becomes erect due to engorgement with blood or lymph, though in many animals it also contains a stiff or even bony support structure. When not in use, its soft penile tissue is usually flaccid, and depending on the species, may be retracted into the body. The anatomy of the penis varies widely according to species. However, the penis evolved only once in the evolutionary history of Amniote.
Mammals
All male mammals have a penis.
,
,
,
, and most
(but not
) have a bone called the
baculum or
os penis that permanently stiffens the penis. During copulation, blood engorges the already-stiff penis resulting in a full erection.
Monotreme penes are variously unusual; the platypus has a penis with a two-lobed (bifid) tip though the whole shaft is inserted in mating, possibly to engage both of the uterus branches, but the echidna's penis actually has four heads, only two of which function at a time.[Gadow, H. On the systematic position of Notoryctes typhlops. Proc. Zool. Soc. London 1892, 361–370 (1892).]
Most are variously forked or split into two in such ways that they resemble hemipenes;
Birds
Although birds reproduce through internal fertilization, 97% of males completely lack a functional intromittent organ.
For the 3% of birds with an intromittent organ, copulation occurs through brief insertion of the male organ into the
vagina before ejaculation.
Alternatively, for the vast majority of birds—a group comprising nearly 10,000 species
—sperm transfer occurs by
contact between the male and female, in a maneuver known as the "cloacal kiss".
Birds are one of the only groups which reproduce through internal fertilization but have repeatedly lost the intromittent organ.
Male have a conical shaped penis that is wider at the base.
A functional intromittent organ is known to be present in most species of Paleognathae and Anseriformes.
