Invertebrates Molluscs 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.
Arachnids 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. 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.
Millipedes 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.
Insects 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.
Vertebrates Fish of a
black molly (
Poecilia sphenops). 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. 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
lizards and
snakes, males possess paired
hemipenes, 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
erect, 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. In some
turtles,
crocodiles, some
birds, and in all
mammals, 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
amniotes. In male
caecilians, the intromittent organ is called the
phallodeum.
Mammals of a
dog (
Canis lupus familiaris). All male mammals have a penis.
Eulipotyphlans,
bats,
rodents,
carnivorans, and most
primates (but not
humans) 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. Unlike
marsupials and
monotremes, male
placental mammals expel
urine from the
bladder through the penis via the
urethra. Both monotremes and
marsupial moles are the only mammals with internal penises, located on the
cloacal wall instead of outside of it as in other mammals. Most marsupial penises are variously forked or split into two in such ways that they resemble
hemipenes; in different marsupial species their forms are characteristic enough to be
taxonomically important.
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
cloacal contact between the male and female, in a maneuver known as the "cloacal kiss". Male
ostriches 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. Male
ducks have a penis that is coiled along the ventral wall of the cloaca when flaccid and which may have an elaborate spiral shape when erect. Waterfowl intromittent organ variation is most likely due to an intersexual arms race resulting from a
mating system in which forced
extra-pair copulations are frequent. ==References==