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Projectile weaponry is not unheard of in the animal kingdom - cobra venom, venomous spider silk, velvet-worm slime, scalding water from bombardier beetles, archerfish spitting water, penguins expelling feces and petrel vomit are a few examples that I think of. However, these are all liquid projectiles, and I'm wondering about solid ones. Those are also seen in animals, but are usually limited to either fecal matter or objects from the environment.

Some cnidarians also have nematocysts/cnidocysts, which is probably the closest real-life thing to what I'm getting at.

My question is: could an animal, or animal-like alien, generate a solid projectile (Solely for the purpose of being fired) inside its body and then expel it with speed and force as a means of offense and defense?

If you have any queries, or wish to notify me of an error, please say so in the comments.

marked as duplicate by James Aug 16 at 5:52

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  • We have fish that spit water accurately. Maybe it has scales that constantly fall off and regrow, so it can spit those scales. If you want truly internal projectile, go with kidney stones, made from excessive minerals that animal consumes (eating snails whole?) – Bald Bear Aug 16 at 19:19

10 Answers 10

up vote 2 down vote accepted

Easy projectile - gastroliths. Many animals swallow small stones to aid in digestion. Maybe some sort of bodily function that generates stones thanks to a high calcium diet (or another mineral).

As for the mechanism, as others have said, muscles probably won't cut it. An interesting solution is found in the British planthopper, an insect that has gear teeth on the tops of its legs. They can wind into place, and allow the insect to jump some good distances with great coordination. Some adaptations could allow for similar structures that could process and launch these generated stones.

Take a crested porcupine

If continually bothered, the crested porcupine will stamp its feet, whirr the quills, and charge the disturber back end first trying to stab the enemy with the thicker, shorter quills. These attacks are known to have killed lions, leopards, hyenas, and even humans.

Give it a longer tail, and some muscles to detach quills at command.


When the animal wants to launched the quills, it just has to swing the tail and activate the muscles with the right timing. The quills will fly like projectiles.

Those quills are known to be extremely painful to extract, due to the conformation of their surface.

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    The proposed modifications to the tail would turn a Stegosaurus's Thagomizer into an atlatl (or similar). – Codes with Hammer Aug 15 at 15:06

Urticating hairs

Urticating hairs are grown by tarantulas, and can reach a couple millimeters in length. They may be barbed, and have the potential to pierce the skin and cause serious problems for attackers. Allergic reactions have occurred in humans, and require immediate treatment The tarantula launches the hairs by rubbing some of its legs together, which releases a bundle of hairs; this increases the odds that at least one hair will hit the target.

This page has diagrams of several different hair types (copyrighted). Here's a photo of some on a Grammostola rosea tarantula, showing a "bald" patch where the hairs are grown (image in spoiler tags for arachnophobes!):

enter image description here
Image courtesy of Wikipedia user Sarefo under the Creative Commons Attribution-Share Alike 3.0 Unported license.

Humans have been attacked in this manner, causing eye damage. Here's a video of what the ejection looks like, in real time.

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    I just finished an answer, along the same lines, that I started before there were any answers at all and the page refreshed when I posted it, suddenly there's a spider taking up half the screen, I just about jumped out of my skin. – Ash Aug 15 at 13:25
  • @Ash I've placed the image in spoiler formatting for now; I'm having issues resizing the image, but will do so when I can. – HDE 226868 Aug 15 at 13:27
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    I wish everyone would use that formatting for anything to do with spiders to be honest, the suddenness did not help the fact that I normally have a bad reaction to anything with more than 4 limbs anyway and spiders in particular. – Ash Aug 15 at 13:30
  • Spoiler alert: HORROR. I found it funny when I saw it, knowing that people might get jumpy about that. – Battle Aug 15 at 13:48
  • Urticating hairs are a skin irritant, and can damage soft tissues like mucus membranes or eyes, but I don’t think I’ve ever heard of them piercing skin, causing infections, or causing death (!). Could we get citations on those by any chance? – KRyan Aug 15 at 21:27

The nettles of jellyfish are quite like projectiles being shot into the body of any animal touching it. But they are projectiles on a tiny scale.

Other animals use spikes and quills as defence mechanism. The quills of porcupine and sea urchins are even designed to stick in the flesh of attackers, so they are already detachable.

What is missing is a mechanism to launch them in the direction of an attacker. Muscles are probably not fast enouch for that purpose, so you need a lock mechanism that is under constant strain and can be released explosively. Think of it like a bow and arrow: you put constant tension on the string while aiming the arrow, only to release all the force at once by releasing your fingers. Although you excerted all this force with your own muscles, your arm cannot release this force fast enough to throw the arrow.

In an animal you could attach tiny tendons to the quills that are connected to tiny muscles. Aggrevation causes the animal to tense up, straining the tendons. If the tension gets too much, the tendons rip, catapulting the quill straight from the skin.

The disadvantage (and probably the reason why nature prefers liquid projectiles) is that quills, muscles and tendons take much time and energy to regrow. Reproducing a liquid requires much less time and resources, thereby offering higher chances of survival for the animal.

Pellets could be an option, mostly birds just cough them up as necessary and discard them but they could be spat at a predator/prey/intruder.

Another option that I find personally disturbing, because of the creatures that use it on Earth, is hairs/skin grown spikes. If you are arachnophobic don't follow this link. Tarantulas have specialised hair like growths that they kick off their exoskeletons at predators, or anything else that intrudes on their territory, these are deliberately sharp and barbed so they stick in and irritate the skin, they also tend to get infected where they stay embedded, not sure if that's by design but there are ways to make it so it is.

I can present you thoughts, but not a clear answer:

  • The amount of matter exerted could mean a too big loss for the host body. If it's not worth and causes higher overall resource costs than what it yields on average, it's not feasible.
  • The projectile must be lethal. Meaning it needs sufficient power and venom. This may allow it to kill even greater prey.
  • The body structure has to be fit for that purpose while considering digestive systems. Plus it would be hard matter which may be clunky, slow down the creature, make it less flexible or disturb reproduction.
  • The creature may require unusually high intelligence, which is a rare good in the animal world, and may only be really feasible at extraordinary magnitudes, in addition to experience and inherent accuracy in order to aim and consider ballistics at distances in which its purpose would actually come to shine, plus consider ammunition.
  • What capabilities would remain if running out of "ammo"? Is that a death sentence, given the food input might be put on hold? If it's only for defensive purposes, for let's say a herbivore, the requirements and resource demands may be simply too high in order to compete well enough.
  • Doesn’t need a lot of power if it administers a quick acting poison on contact. – WGroleau Aug 16 at 3:55
  • Sure, but it still needs to pierce the skin (sufficient mass, fitting structure, sufficient force), have a sufficiently potent poison and dose. If the prey runs away or the predator is still able to fight, it may not be enough. And consider that every trait has a demand in an evolutionary perspective, otherwise everything could be venomous or poisonous. Oh, what comes to my mind are Hydralisks from Starcraft. – Battle Aug 16 at 5:20
  • Not necessarily. Mere contact with certain plants can kill if treatment is not provided. But OP didn’t ask for lethality, just for a means of offense/defense that is a solid projectile. – WGroleau Aug 16 at 5:44
  • @WGroleau - I assume that it has to be lethal in order to be evolutionarily feasible. If it does not fulfill that requirement, neither would it come into existence, nor would it remain for long. – Battle Aug 16 at 5:56
  • Many species have traits that are not lethal but nevertheless protect them. – WGroleau Aug 16 at 6:07

Not animals, and not for offense, but there are plants with projectile seeds, e.g.

Here's an evolutionary approach. Originally an egg-laying species could expel eggs as a way of distracting and escaping predators who would then feast on the egg.

Over millennia this egg-expulsion could become faster and more powerful. The egg would be projected into the predator's face thus temporarily blinding it.

Ultimately the shell would become thicker and thicker until solid eggs were produced and 'fired' at high velocity that would render the predator unconscious or injured.

There would be two oviducts - one for creating viable eggs and the other for creating projectiles.

Cone Snails fire venom-filled, barbed harpoons at prey/threats!

This is the closest real-life example I know of.

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With venomous snakes, their teeth fold back into the mouth, and if they break they grow new ones. It could be that the creature has a similar mechanism, but that the "fangs" launch instead of just folding out to bite. If the projectile is in the mouth, it may be easier for the creature to aim?

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