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I'm finally actually participating in the weekly topic challenges, instead of just helping run them.

Let's say that there's a siege around a small series of fortifications that comprise the only defense of a small band of rebel soldiers (the technology level is about that of Europe circa 1100). The surrounding army can't seem to get in because the rebels have chosen to hide amidst old fortifications, like the ones at some places along Hadrian's Wall. They're old even at this point in time, but instead of some of the fortifications at present-day Hadrian's Wall, the fortifications are ten feet tall and about two feet deep.

The soldiers have no weapons that they can use to get in. They can't go over the wall, because there's a two-foot deep ditch surrounding the wall that increases the effective height to about twelve feet. They have no rope, and only bows and arrows, swords and shields. There are no trees of other vegetation which could help them - in fact, no natural materials they could use at all. Yet they need to quickly squelch out the rebels (i.e. within a couple days, at which point they have to leave and fight elsewhere), who have enough provisions to survive for two weeks.

Fortunately, the soldiers know that the surrounding countryside is filled with a certain kind of insect that can eat through stone. They spend all but one of their remaining days catching enough of these insects. They gather them in bags and then let them loose at the base of one of the sections, where the insects manage to carve a hole big enough for an armored man to get through. The rest is history.1

Could these insects exist? If so, would it be reasonable for a large swarm (e.g. 5,000 - the soldiers were busy) to eat a hole of this size in a period of 24 hours?

The surrounding region looks a fair bit like Northeast England, as shown here:

Beautiful, isn't it?


1 Not yet, actually. I don't know what happens next.

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  • $\begingroup$ What sort of stone is your wall made of? $\endgroup$ – apaul Mar 16 '15 at 2:35
  • $\begingroup$ If there were enough soldiers to catch 5000 of these insects to let loose on a wall, you'd think there'd be enough resources to cut down a couple trees to make a battering ram. It's unconventional, I'll give you that, but I don't think it is very realistic. $\endgroup$ – Neil Mar 16 '15 at 14:20
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    $\begingroup$ Having problems resolving how the insect could have these effects in 24 hours, yet still have left the wall standing for hundreds of years. If they could do some serious damage in a 24 hour span and exist in numbers that can be easily gathered from the local country side...wouldn't they have already gone through Hadrians wall hundreds of years ago? I can't think of any good reason why they'd exist, yet avoid the wall until it was nicely handed to them by soldiers (were they waiting for an invite?)...if these did exist, the wall would have been eaten years and years ago, no? $\endgroup$ – Twelfth Mar 16 '15 at 18:05
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    $\begingroup$ @Neil They're nowhere near forests, so there's not an issue. $\endgroup$ – HDE 226868 Mar 16 '15 at 21:50
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    $\begingroup$ @HDE226868 You have to specify that there are no trees then, or your reader might wonder why such an inconventional tactic was used in place of a more practical one. $\endgroup$ – Neil Mar 17 '15 at 9:06
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It seems more likely that they could be used to weaken the wall, to the point where you could take it out with primitive siege equipment.

For example, imagine a hyper-active ant or termite species that builds large underground nests. You could sneak those into the area outside the wall. Over a few days, they'll excavate underneath, weakening the wall's foundation - at that point you can knock it over easily, or finish the excavation manually and cause the wall to tumble down.

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I can find historic references to insects used in siege warfare but only as projectiles. Having a hive land on your house will ruin your day. There are burrowing insects but they prefer the softer mud and mostly just to build shallow nests.

Leaving history behind, is it possible? Difficult to say. To make it believable what you really need is a story of what good this does the insect. Pill bugs dig under stones because it's a very effective way to hide from predators. What advantage would they have if they could dig directly into the stone? Hide from predators that turn over stones?

Another problem is even if you had a swarm of millions when they were turned loose on the wall they'd each make their own little hole. Even if they all dug completely to the other side, for some reason, what you're left with is a stone wall that is still standing. Just with a number of tiny holes in it.

When the number of holes is dense enough the problem will be that the lazy insects would just use the existing holes to get to the other side and not make any new ones.

So either your insects have to be as big as an armored man or you need the wall made out of something the insects like to eat.

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If those insects indeed exist and are common on the region, castle builders would be a tad smarter.

If there was such an insect that could burrow in stone, they would be a pest to the walls. Castle Builders would have already adapted, and probably created some type of coating to prevent those pesky bugs from making nests on the buildings, especially if they are common as you say. Also, you have the problem of coordinating those insects. Why do they dig into the stone? They want to make a nest? If so, they wouldn't eat all the way to the other side. They use the stone as food? Improbable, since stone is not exactly the most nutritive food out there [citation needed]. Would they dig together to make a huge tunnel? certainly not - digging a tunnel big enough that you would be able to fit inside is something that animals do to create some protection, not to breach wall. Your insects, if they were able to dig stone, would be mostly interested in creating several small nests than one big, huge tunnel.

There is the digging mechanism issue, too. For they to dig solid rock, they would need a really complicated mechanism of acid-production. The queratin-based limbs of insects is not hard enough to damage rock, and if your insects had some kind of adaptation to make them stronger, it certainly would be a slow process. Think about how long a human, using modern tools, take to create such a hole in a thick stone wall.

If they produced some type of acid, they would be able to produce just minute quantities of it each time, and would need food and rest between 'discharges'.

Another issue would be hunting those insects. If they can create nests in solid rock, how would your hunters gather them in the first place? The nests would be highly inaccessible without specialized tools - tools that an army would probably not carry with it.

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How important is it that the attackers introduce the bugs?

What I mean is, if you happen to figure out some kind of bug that could eat stone, does it have to be introduced to the wall by the attacking army? This is a bug that nests in stone, and here are some walls that have been around for a really really long time, and it's above ground so the bugs don't have to mine for it. Why would these bugs not have already found and weakened the wall. So your attacking army is laying siege, trying to figure out how to get in, and one of them notices bug sign, like how termites will build tunnels up walls to get to wood without being exposed to the sun. So they know that section of wall will be weak, and can batter or dig at it.

As for how they exist, have a predator like an ant eater that is really good at digging up these bugs, but because of super hard and sharp appendages, these bugs working together are able to dig into softer stone given time and that gives them protection from the predator. An old wall would be extra attractive too because loose mortar would give them a way in that virgin rock wouldn't have, and by weakening the mortar they would severely weaken the wall.

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You want some form of modified termite.

Now...this is still going to be a pain. But here is what I am thinking.

How:

  • These modified termites secrete an acid that softens certain inorganic minerals for consumption/extraction. Termites generally stick to fibrous/cellulose materials but they will eat gypsum too which isn't stone but isn't wood either.
  • It would probably be more believable if the stone in question was a bit softer/more porous. Granite would be tough to accept (though granite is pretty porous).

Why (options):

  • The mineral is needed for nutrition and is only available in trace amounts outside the stone.
  • The stone matter is needed to construct their termite forts.
  • Some termites have 'farms' in their mounds, perhaps they use limestone as a fertilizer.

Problems:

  • The rate you are asking for (24 hours) is going to be a stretch. You would have to have way more than 5000. A termite mound in Africa can house MILLIONS of termites.

  • Completely bringing the wall down or eating only a section seems impractical as well, they will spread out to consume. But perhaps they make the wall soft enough that men with war hammers can break a hole in it easy enough.

Course you could just introduce a super termite that is a major plague on the place...but if they consume that quickly why would anyone ever build anything out of stone.

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The insects are explosive.

The miner beetles your soldiers are out gathering don't actually eat through stone walls, per se. While they build their nests in stone by secreting small amounts of explosive chemicals to blast off small flakes of rock, expecting them to burrow through a wall would be silly. If left to their own devices, the insects would spread out and build lots of shallow nests in the surface of the wall, instead of all burrowing in one spot, if they even decided to build nests there at all. They prefer loose, fractured stone for their nests, as construction is easier there, which is why the wall hasn't been slowly blown to dust by their nest building habits over the course of the time it's been standing.

However, these insects possess a powerful defensive mechanism. Their stone cutting abilities come from chemicals they produce and stone in a pair of membranous sacs which explode when combined. When the beetles are killed, these sacs rupture, creating a powerful localized explosion strong enough to highly discourage any animals from trying to eat them.

What the soldiers have done is gather thousands of these normally solitary creatures and stuffed them in a sack. They don't intend for the beetles to leave the sack and eat through the fortifications. They expect them to die. After being placed next to the wall, a single arrow into the sack of beetles will kill a few, triggering a chain reaction and an explosion powerful enough to blow through the wall. While a single, soldier-sized hole slowly bored through the wall would be easily defensible, the resulting explosion will create a large hole as well as killing or injuring any nearby guards with shards of rock blown loose by the blast.

The rest as they say, will be history as soon as someone gets around to writing it.

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  • $\begingroup$ I somehow managed to miss the fact that this question was over a year old... $\endgroup$ – ckersch May 18 '16 at 20:56
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If the wall was made from limestone, and the insects stored quantities of acid (such as formic acid made by ants), then it might be possible to damage the wall slightly - i.e. take the surface layer off it - by throwing the insects at it so that they died and their acid was plastered on the wall.

However, to actually burrow right through the wall would likely take more ants (or whatever) than would be practical. You'd have only slightly more success by extracting the stomach acid from your soldiers, but to dissolve a man-sized hole through a wall even with that amount of acid would be unlikely to be possible, and it would likely take more time than you have. You'd have trouble keeping the acid where it was needed, and you'd have to keep replacing the spent acid with fresh.

There may be insects that generate acid, but that's an awful lot of acid they would need to generate. Acid has to come from somewhere, and that takes energy, which insects probably won't have in the timeframe you mention, unless there are literally tons of the bugs.

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I can see a single option. Suppose your world supports to basic forms of life: silicon-based and carbon-based. Furthermore, the carbon-based plants are outcompeting or have outcompeted the silicon-based plants. Now a species with a similar place in the silicon ecology as termites have in the carbon-ecology could have evolved to eat silicon-rich stone. This would imply that your fortress is built with silicon-rich rocks. You would still need a very large amount of them as they won't eat in the direction you want them to eat. They'll probably follow the silicon-rich parts of the stone and hollow the wall in a way similar to wood-bores. Furthermore, they can only eat so fast, so you might need to adjust your timescales somewhat. Lastly, as we have developed various treatments against wood-bores, people in your world might have developed treatments against stone-bores (like drenching the stone in silicon-based poison).

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I think the best bet is an acid producing silica-based life form like the Horta in Star Trek: The Original Series. The creature was ruining the mining colony and had McCoy say, "Damn it! I'm a physician not a bricklayer!" when he was called upon to help it. Hortas were big but a smaller life form might be feasible.

I think the main objection - already made - is that if there were such insects native to the area they would already have gone for the castle. Perhaps your invading army carry a colony with them in specially treated barrels or such.

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