What Makes a Penis in the Animal Kingdom, and How It Functions Both in Mating and Other Purposes

What qualifies as a penis in many members of the animal kingdom? In this article, the various kinds of "penises" and their functions in and out of mating are discussed.

What Makes a Penis?

For vertebrates, including humans, a penis is relatively easy to pinpoint. Unfortunately, this is not the case for many invertebrates. 

We may define a penis as "something which inserts into female genitalia in copulation and deposits sperm." Using this definition, we can look at some animals' anatomical parts and see if they fit this description. In fairness, though, some reports only classify penises as belonging to higher vertebrates. 

Millipede "Penis"

In millipedes, the eighth pair of legs are used as gonopods or intromitta, meaning feet which copulate. Some millipedes also use another pair of legs near their genitalia to provide the sperm for reproductive purposes.

Because the organ which inserts into the female is different from the organ which provides the sperm, then by definition above, millipedes have two sets of "penises" that complete the definition. 

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Flatworms

In some flatworms species, females have no genitalia at all, so the male uses a protrusible sperm duct with a type of 'stylet' to pierce the female anywhere in her body. The sperm which was injected migrates from whatever it is to go to the female's eggs.

Sperm delivery system in many insects

The aedeagus is a penis-like structure that has different sizes and shapes in various insect species. It is an extension of the rigid plates covering the abdomen, which are equipped with ducts connected to the insect's testes that serve as the sperm delivery system.

Many human males may like the idea of having an armored penis, as long as it does not diminish the sensation during copulation. 

Such armored intromitta, depending on the animal, maybe a spiral and could have flaps, hooks, claspers, or valves designed to grab the female. 

Arachnids' Spermopositor

Mite harvestmen of the Cyphophthalmi suborder is a group of tick-like, spider-like, moss-dwelling animals measuring only a few millimeters. They have genitalia that flips out or evert.

The eversible organ pokes sperm packets into female genitalia, but the actual penile-like structure is not inserted. This kind of sperm deposition makes them "spermopositors."

For spiders, the penile structure comprises a pair of leg-like appendages called pedipalps, which have elaborately wrapped packets of sperm. The spiders' intromitta are palpal organs with a hard tip that provides the sperm thrust by the pedipalp into the female.

This palpal organ has a similar appearance to a mitten, and depending on the spider species, can be large and hairy, and may or may not have extensions, pointed parts, or folds.

The pedipalp and palpal bulb function as a 'proper' penis according to our penis definition, but interestingly, spiders also use their pedipalps to smell and taste. A part of their pedipalp under the tip of the palpal bulb even makes music during courtship.

READ ALSO: The Mating Story of Anglerfish

Barnacles

Barnacles' intromitta is shaped into a long cylinder with folded, stacked rings, with bristly extensions known as setae: the latter fans out and senses chemicals to detect if a female is present. Barnacle penises are also able to grow during mating season, which is then shed afterward.

These are some of the strange types of a penis present in various animal kingdom members, making for a fascinating study. 

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