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Basically, giant crabs ridden by humans as cavalry, I just don't know if it's a good idea, and I could use some answers on how to make it work.
The setting is medieval fantasy.
The Giant Crabs won't really have a diverse diet, so ration problems, but then again they are giant crabs, with shells and claws, so I think this might be a good idea.
Crabalry is a good name.
Before I proceed any further, remember the Rule of Cool:
The limit of the Willing Suspension of Disbelief for a given element is directly proportional to its awesomeness. Stated another way, all but the most pedantic of viewers will forgive liberties with reality as long as the result is wicked sweet or awesome.
So if you want a crabalry, have a crabalry. But if you want realism:
The square cube law ensures that no crab like those from our world would ever be big enough for a human to ride. Your crabs would have to be some other kind of creature that evolved differently, and carcinized (yes, this is a word and a thing) at some point.
Crabs need water. Even those who live on land need to be moist all the time. Horses can deal with dry conditions much better. This does not make a crabalry impossible, but makes it very limited to some areas.
Being cold-blooded, your crabs won't be of much use in snowy conditions. Sure, crabs can move around in arctic waters, but have you seen how slow they are?
Crabs need to molt every now and then. They will be useless while molting.
Crabs have a very... primitive nervous system. You might think that a big crab will have a big brain, but the truth is that there are crabs larger than dogs (almost 4 meters/yards from foot to foot when stretched), yet they still have small brains. That's because a bigger brain wouldn't be helpful where they evolved. Your crabs might be smart though, since their evolution is up to you.
But again: Rule of Cool. If you think it's cool, then lots of other people might think so too. Go for it. It would be cool to ride a beast that can run sideways and tear limbs from your foes with its pincers.
Crab cavalry aside, giant crabs would be great mounts for uneven terrain due do the amount, shape and layout of their legs. They would also be like elephants in that besides serving as mounts, they can pick up heavy stuff from the ground on their own and place it somewhere else. So rather than direct combat use, they might be great for construction and as beasts of burden. This still has a place in the military, as well as being useful in a civillian way.
I’ll treat the Crabs like any medieval fantasy creature. There’s no point in watching something like LotR and then saying “wraiths don’t exist” or “those Trolls would collapse under their own weight”.
These are big horse-sized or bigger crabs, people can ride them, they are domesticated enough to function as war animals.
If you ask if Crabs could be Cavalry you run into the same problem as when people ask questions about mechs versus tanks. Of course it can’t compete or do the same things because they are different. So instead of asking if Crabs can be Cavalry, ask what function a Crab could have within an army! There are likely ways Crabalry can be its own thing!
Crabs won’t have the speed of horse Cavalry, and that’s OK. Because unlike horses your Crabs would be far better protected against spear and pike formations. Whenever the horse Cavalry has to stop or risk running into a prepared formation, your Crabalry can move up. You wave a spear at them? They can bat them away, attempt to grab them relatively safely and have natural armor against it. Which any military mind would add some more armor on, reinforced leather most likely as club-like weapons would be the best against crabs.
Crabalry would fill its own niche. It would likely function as heavy infantry that is highly resistant to Cavalry charges. It would also accelerate the evolution of Pike formations as the hammer/axe part would be one of the best tools against Crab shells that large.
Giant land crabs already exist. (Un)fortunately they are still too small. The largest crab is the coconut crab:
This is not only the largest crab. In fact this is the largest terrestrial arthropod. These things weigh about 5kg and are about a metre long. For comparison:
It seems the largest known extinct land arthropod is a 2 metre long millipede that is estimated to weigh about 50kg. That's the weight of a St Bernard dog. Still too small to ride. At least if you're a human:
The crab is too small for Ripley to ride without smooshing it with her rock-hard butt. The crab is even too small for the little girl she rescues in that film.
Since everything eventually turns into a crab the assumption is that if a bigger crab was possible we would have seen one by now. So a crab big enough to ride is hard to believe.
To solve this problem I suggest your giant crabs be normal crabs who had Enlarge Crab cast on them. This is the household name for modification II.a4 of Enlarge Monster designed specially to work on crabs. This spell is known to ignore such trivial considerations as whether the target would be too heavy to move after being scaled up. A wizard did it.
The question should not so much be "COULD" you ride a crab as "is this the best use of giant crabs on the battlefield?"
There is one historical unit that giant crabs would replace much better than cavalry: war dogs. The point of a warhorse is to deliver a deadly warrior as fast as you can to (and from) the enemy lines. But giant crabs will be much slower than horses, and would be so well equipped with natural weapons and armor, that putting a knight on top of one would be a waste of a knight. War dogs were different. They were used to attack the enemy formation on their own using thier natural weapons. This created panic and disorder in the enemy ranks in preparation for an infantry or cavalry charge.
War crabs would fill this role even better because crabs have a resemblance to spiders causing humans to have a natural fear of them. They also have thick armor and powerful claws that put dog fur and teeth to shame. So put those crabs on the front line and save your horsemen for what cavalry does best: Skirmishing, out-flanking, and chasing routs.
It would be a terrifying and effective mount if it was trainable. Which normal crabs aren't.
The biggest drawback is how much room it would take. you couldn't have close formations, the legs would tangle and it just takes so much more room than a horse. And formations can be critical in medieval battles.
I'd use them like tanks, stick a box on their backs with some archers or artillery and you have mobile, terrifying and self-defending troops which may well be invulnerable to heavy calvalry charges.
Yes and No.
So - On the Yes side - a Giant Crab (able to be ridden by a Human and let's put aside the exoskeleton weight limit thing for a moment) is going to be big (duh!), is going to be armored and has 2 offensive weapons.
This would mean that in the role of say Heavy Cavalry, they would fair pretty well - able to smash apart an enemy line, the terror factor of a giant pincer and being able to hoist a man up and cut them in half is not to be underestimated in terms of psychological impact.
However...
On the No side - What made Cavalry such an effective tool is speed a Galloping horse is faster than a Human. Even a nice steady canter is much faster than a mans Jog - and this is where Cavalry came into it's own - the ability to outflank and outmaneuver infantry. This also includes your scout cavalry - being able to go a great distance at speed and report what you saw and come back.
Then you've also got the harassments of your enemies supply lines - There's some great examples of this in the Sharpe TV series that shows how that works - suddenly a light cavalry division comes around the corner, engages a slow moving supply train slash, Slash SLASH! then disengages before they can rally to a defense and return fire.
In this area, even a Giant Crab, assuming the same speed of the fastest crab is only capable of 3-4 m/s, compares to a horses' 14 m/s.
So this form of Cavalry a Crab wouldn't be effective.
Crabs walk sideways. This will make it extremely inconvenient to attack enemy units directly in front of your formation. Either the crabs will need to turn to one side in order to approach the line of contact, causing each crab and rider to turn their backs to half the enemy, or the crabs will need to sidle across the battle line, greatly reducing their effectiveness as shock troops.
On the other hand, no formation could be more exquisitely suited to a flanking attack then a company of lancers mounted on battle crabs.
Lobsters, now...
I don't know what the naysayers are on about. This is awesome.
Crabalry is a great idea. Terrifying. Pincers. They eat you while you're alive. Armored. Fast. Fearless. Hungry.
Realism? If I wanted realism I'd work some unpaid overtime.
How to "make" it work? Don't bother, just say it does. Or better yet, just show it working: "Sir Bob steered mighty Gorgonoplex to the left into the King's retainers. While Bob struck down a bodyguard, Gorgonoplex grabbed the Duke of Edinburgh in his mighty crushing claw. Tough break, that ain't gonna buff out!"
Light Warcrabs (say, one metric ton, or about 2200 lbs) are saddled for an armored man wielding a flail.
Medium Warcrabs can easily carry two men. One of them could be an archer.
Heavy Warcrabs could weigh in at 5 metric tons and sport a tower of wood on their backs large enough to provide cover for 4 men.
I can’t really say much about the speed of the crabs, it would probably depend on lots of different factors, but they would definitely have one major advantage: the ability to sidestep and basically move in any direction. This added maneuverability could prove very helpful for evading projectiles, especially if this breed of war crab has fast reflexes. Also, the hide could be much tougher than that of a horse, possibly making them better equipped against anti-cavalry weapons such as lances.
So in summary, yes, I think crab cavalry or “crabalry” would work.
