Tiny spike 

This looks like a medieval weapon, but it's the penis of a seed beetle (Callosobruchus maculatus).

"They have penises that are armed with spikes and structures that probably evolved originally as ways of anchoring inside the female and keeping hold of the female during sperm transfer," Bruno Buzatto, a lecturer in terrestrial ecology at Flinders University, says.

Studies have found that seed beetles with particularly long spikes father more children. 

But it comes at a cost for female beetles.

"Often those things really hurt the female badly, and mating can diminish their life span because it injures them so badly inside."

He says studying how seed beetles behave, where the interests of the male and female do not align, can help answer broader questions about sexual conflict.

"The priority here for males is 'Let's maximise how many eggs I can fertilise', even if that comes at a cost of female life span."

Oh, and when we're talking insect genitalia — "penis" isn't really the correct term.

An aedeagus is a modified body part that was not originally related to the genital or reproductive system.

In the seed beetle's case, its aedeagus is a modified plate from the end of their abdomen.

Forked tail

This is an Indian python's (Python molurus) penis.

Well, penises. Snakes have two, called hemipenes, usually found in the tail.

"[Hemipenes] are quite diverse, and they are fairly spiny … but the spines are soft," Andrew Durso, a wildlife biologist at the Florida Gulf Coast University, says.

"Sometimes they seem like they get dragged around in the dirt, so they must be pretty resilient to punishment."

Dr Durso says snakes are under-studied but probably only pop their hemipenes out when they are about to mate.

"And when they use them they evert it ...  it's like a sock when you take it off your foot. 

So the pocket becomes the exterior of the hemipenis and the inside is filled with lymphatic fluid."

Some hemipenes are forked and look like a pair of Ys.

Regardless of penis forks, snakes only use one hemipenis at a time.

So why do they have more than one? Having a second penis, complete with an undepleted batch of sperm, could be handy should they get two mating opportunities in rapid succession.

It's thought that the sperm produced in the left testis come out through the left hemipenis, and those in the right testis through the right, Dr Durso says.

Experiments with lizards, which also have hemipenes, found they tend to alternate sides when mating multiple times in quick succession.

Extreme monotreme

This fantastic phallus belongs to a short-beaked echidna (Tachyglossus aculeatus).

A 2021 study detailed the anatomy and workings of the short-beaked echidna's penis, which happens to be bright red … and has four heads.

Lead researcher Jane Fenelon told ABC Hobart that the echidna penis was "very distinctive".

"It looks like one single tube, and at the end it has … four rosette-like glands on the end," she says.

"What's even more unusual, particularly for mammals, is that they only seem to use two of them at any one time, and then they alternate between the two sides."

The echidna's blood supply, which courses down one side of the penis or the other, controls which pair of heads is used during mating.

Echidna sperm cells cooperate with each other. They form torpedo-like bundles of up to 100 sperm which lets them swim faster than if they were on their own, she adds.

"Then when they get to the egg, it's every sperm for himself."

Final buzz

This is the genitalia of a European honey bee (Apis mellifera) drone.

This "endophallus" is usually tucked up inside the body of a male honey bee or drones.

When it's time to mate with a queen bee, which takes a few seconds and is done mid-flight, the drone contracts abdominal muscles that turns their endophallus inside out and "inflates" it inside the queen.

His ejaculation is so explosive that it blasts his endophallus off, leaving it inside the queen, and the drone dies shortly afterwards.

Ballistic bird

This corkscrew penis belongs to a mallard or wild duck (Anas platyrhynchos).

Scientists have actually filmed the corkscrew shooting out at 121km per hour.

"The swirling duck penis is like a ballistic sperm delivery system that inverts itself at high speed out of the mallard in question," says biologist Diane Kelly of  the University of Massachusetts Amherst.

It is unusual, not just for its shape, but its rarity. Only around 3 per cent of bird species have a penis.

The rest haven't exactly lost their penis, but rather their penis fails to develop.

"[These] birds start to make it ... but the urethra eats the entire penis," Dr Kelly explains.

They rely on a "cloacal kiss" for mating, or the meeting of two orifices called cloacas, between which sperm is transferred.

Scientists don't know why most birds have evolved without a penis.

But this doesn't explain why ducks have penises, says Patty Brennan, an evolutionary biologist at Mount Holyoke College in the US.

"Ducks are some of the longest migrating species and they have huge penises, so that doesn't really work," Professor Brennan says.

Another hypothesis is to reduce the chances of getting a disease, because most animals haven't separated out their digestive outlets from their sexual inlets.

"Birds have all sorts of stuff in their cloaca — they get organisms that can cause disease, and it it possible that by eliminating the penis they are reducing their incidences of sexually transmitted diseases."

Bungee croc

This is the penis of an American alligator (Alligator mississippiensis).

When crocodilians, such as alligators and crocodiles, are not in mating mode, their penis sits inside an orifice called a cloaca.

And the penis is always stiff, according to Diane Kelly, who studies copulatory systems at the University of Massachusetts Amherst.

"Where we [humans] would have that central fluid space [in a penis], a crocodilian has that space packed with so much collagen that it becomes effectively permanently erect," Dr Kelly says.

So how does a crocodilian pop his penis out when it's time to mate, and back in again once the deed is done?

There are no muscles attached to the penis itself. Instead, muscles in the cloaca, which form a sling around the penis's base, cause it to shoot out.

Then a pair of tendons fling it back inside.

"Those two tendons are like big elastic bungee cords, and they get put into tension as the penis [emerges from the cloaca]," Dr Kelly says.

"When the muscles that are holding it all in place relax, the whole system flips back into its resting position by elastic recoil."

Pumped papillon 

This is the inflated penis of a Helicoverpa moth.

This image and video were made possible thanks to an invention called the "phalloblaster", developed by the CSIRO.

Many insects look the same under a microscope and tiny differences in genitalia are often the best way to tell two species apart.

According to a CSIRO scientist, "the phalloblaster inflates the penis with a stream of pressurised alcohol to create the same shape as when the insect was alive.

"The alcohol dehydrates and hardens the structure, so that once the process is over, the genitalia remain inflated rather like miniature balloons. It makes the genitalia much easier to study."

Some butterflies can, in a way, "see" with their genitalia, thanks to light-sensing receptors.

Such "hindsight" is thought to help with mating, guiding the male to successfully dock with a female.

In 1980, insect vision researcher Kentaro Arikawa, then a graduate student, accidentally discovered the light-sensing-genitals of the Chinese yellow swallowtail butterfly (Papilio xuthus).

If the genitalia were misaligned, Professor Arikawa writes, "some space would be available through which light could enter", and the male would try to reposition himself.

Hanging out

You're looking at Lyle's flying fox (Pteropus lylei).

As is the case in insects, bat genitals are often used by scientists to distinguish between smaller, similar-looking species.

The southern free-tailed bat (Ozimops planiceps) and inland free-tailed bat (Ozimops petersi) are closely related, and sometimes called the "long-penis" and "short-penis" forms of free-tailed bat respectively.

And some bats hang with their penis out … literally.

Diane Kelly from the University of Massachusetts Amherst says most male mammals don't keep their penis out in the open the way that humans do.

"Most mammals keep their penises tucked under their belly in a specialised sheath called a prepuce, but for some reason some bats and primates have lost [most of their prepuce]."

The flipper 

This is a penis on a common bottlenose dolphin (Tursiops truncatus).

Like crocodilian penises, dolphin penises contain collagen-rich erectile tissue, which means they don't ever fully soften.

Diane Kelly from the University of Massachusetts Amherst says male bottlenose dolphins can force females into mating, but females have a trick up their, er, sleeve.

Bottlenose dolphins have a "complex vagina", full of internal folds and flaps.

"If she rotates her body a few degrees, he can be shunted off into the blind alley of her vagina and not be able to inseminate her," Dr Kelly says.