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I'm writing a species of dragon-sized butterflies¹ as flying mounts into a low-magic fantasy setting², and would like to know if these creatures have to be imbued with magic, or if they could just be mundane through some contrivances. What stands in the way of having an insect of this size, and how can we get around it?

I recognize that this question deals with the square/cube law, and the implications it has on creatures of this size - I was just wondering how it differs in the context of insects (Lepidoptera specifically).

¹ I'm looking for sizes ranging from 10-22 feet in height, and 40-150 feet in wingspan, varying by species. I don't mean to have Kaiju-sized butterflies.

² Assume conditions analogous to middle-age Earth (with the addition of magic)

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    $\begingroup$ Uh... well i mean if you have magic you can have whatever sized butterflies you want. Otherwise, from a science based perspective, you can't. $\endgroup$ – Aify Apr 18 '17 at 0:48
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    $\begingroup$ have very small dragons. $\endgroup$ – Xenocacia Apr 18 '17 at 1:01
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    $\begingroup$ @SRM. Good one. I was thinking of butterfly-sized dragons flitting around gardens, backyards and in nature. With tiny bursts of flame, and kids getting burnt by interfering with the mini-dragons. They are, possibly, scientifically feasible too. Quite unlike their gigantic cousins. $\endgroup$ – a4android Apr 18 '17 at 4:34
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    $\begingroup$ Hey, I like that! What if dragons (without magic) are really tiny, and a “scale magic” is used to make them big? The people learn to apply the same scale magic to butterflys, as they make better domesticated mounts. $\endgroup$ – JDługosz Apr 18 '17 at 4:51
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    $\begingroup$ @JDługosz Story title: "Dragons Have Scales". Bonus points for working in the musical definition into the resizing spell. $\endgroup$ – SRM Apr 18 '17 at 10:47

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On an Earthlike World they would need magic. Butterflies like all insects are constrained in size because they don't breathe like us.

Basically they have small holes and tubes all along their bodies so air can get in and be absorbed directly into the tissues. This is fine at small scale, but becomes exponentially more inefficient at larger sizes.

Here is a link to explain how their gas exchange system works Link

Getting around this problem would mean a complete redesign of their gas exchange system... or..... magic.

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    $\begingroup$ Or much higher atmospheric oxygen content. Brief search suggests that ~30% can produce en.m.wikipedia.org/wiki/Meganeuropsis dragonflies 40 cm long, with 70 cm wingspan. You could probably get 2m butterflies without magic. Though sideffects on rest of biosphere would be considerable. $\endgroup$ – M i ech Apr 18 '17 at 1:57
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    $\begingroup$ Didn't have that oxygen content in medieval times as far as I know 30% is a heck of a jump, but there were large insects in the early fossil record including a millipede thought to be around 2m, so at some point it was possible although the big ones weren't flying. A 2m dragon isn't very big though, crocs grow much bigger than that. $\endgroup$ – Kilisi Apr 18 '17 at 2:58
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    $\begingroup$ @Kilisi the only modern day dragons known to man are the Komodo Dragons, they grow to about 3m. Interesting creatures, but of course neither flying nor firebreathing :) $\endgroup$ – jwenting Apr 18 '17 at 6:12
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    $\begingroup$ @jwenting na... I saw a documentary not long ago, How To Train Your Dragon, there's lots of different sizes up North somewhere. $\endgroup$ – Kilisi Apr 18 '17 at 7:14
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    $\begingroup$ IIRC there is some fossil evidence of very big butterfly-ish creatures (possibly even ancestors of extant species?) at times when the Earth was much hotter+moister+carbon-dioxidy... but "very big" here doesn't mean 2m... $\endgroup$ – errantlinguist Apr 18 '17 at 14:41
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The dynamics of flight depend on the scale, too! Even if you redesign the animal to exist in a large size, and make it large and strong enough to lift a human as well, you will then need suitable wings.

Look at how bird wings differ by size: and that’s just within birds. You simply could not make a paper-thin wing articulated with a tiny joint, in that scale.

Besides the biology being nothing like an insect inside the body, the flight surfaces will look nothing like a butterfly.


What if dragons (without magic) are also really tiny, and a “scale magic” is used to make them big? The people learn to apply the same scale magic to butterflys, as they make better domesticated mounts.

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    $\begingroup$ "I don't like math. I'm going to be a writer so I don't have to learn this number stuff!" -- said no sci-fi author ever. $\endgroup$ – SRM Apr 18 '17 at 10:49
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    $\begingroup$ The author of the Star Trek transporter certainly has problem with E=mc^2 … $\endgroup$ – WGroleau Apr 18 '17 at 11:36
  • $\begingroup$ Thanks for pointing out the flight aspects - it didn't even cross my mind how janky that would look. $\endgroup$ – user19838 May 13 '17 at 0:45
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Besides breathing and flying, another issue is the diet.

A butterfly does not "eat", it only drinks, by unfurling a curled up tube called a proboscis that acts like a straw. It feeds on anything that can be dissolved in water, including plant nectars and saps, pollen, rotten fruit and even dung and human sweat.

I can see several issues with this:

  1. A 20 foot tall butterfly would probably need a proboscis of at least 10 feet, which may cause problems with sucking up fluids through such a tube, no matter how wide it is;
  2. Where is the butterfly going to find enough nutritious liquids to sustain itself? It could probably slurp down an entire vat of fruit juice and still be hungry;
  3. How is the butterfly going to access the fluids? nonmagical plants probably aren't going to be able to provide a gallon of nectar a day, and even if they do, it'll have to be in a way that the butterfly can access.
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    $\begingroup$ Vampire butterflies gracefully fluttering down to drain a cow of blood a couple times a day :) $\endgroup$ – JollyJoker Apr 18 '17 at 11:25
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    $\begingroup$ @JollyJoker I actually was considering including a paragraph to that effect, but "bloodsucking butterflies" does not seem like the kind of thing that someone would want to ride, especially if they wanted a kind looking creature to ride. $\endgroup$ – Nzall Apr 18 '17 at 12:04
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    $\begingroup$ @JollyJoker it's not an insect, it can't fly, but here on Earth we already have the Amazon Giant Leech. Eighteen inches long, two pints of blood per bite ... shudder. $\endgroup$ – nigel222 Apr 18 '17 at 13:07
  • $\begingroup$ Really big flowers. $\endgroup$ – Mazel Apr 18 '17 at 14:57
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Instead of giant butterflies, you could have tiny humans. The butterflies and dragons, while small compared to normal sized humans, would appear several times bigger than very small humans. Of course then you have to deal with humans living in a much larger environment surrounded by giant flowers and other such things, but that could make for a unique premise as well. This would help deal with how the butterflies feed, breath, and fly since the flowers would continue to be normal sized and their bodies would function as they would now.

You wouldn't run into the common square cube law issue, but as indicated by this answer, at such tiny sizes you run into the issue of humans overheating. You could gloss over that a bit easier by claiming humans in this environment evolved better heat dispersal or perhaps were artificially created or enhanced to disperse heat better. If you actually have dragons in this environment as well, you already have an existing creature that has a highly refined body heat management system. Either way it's a bit easier to handwave than the square cube law.

Basically, the dragons are butterfly sized, but the humans are tinier.

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How can we have dragon-sized butterflies?

By stipulating butterfly-sized dragons.

Or, if it's the size of butterflies relative to the rest of your cast you're interested in, by scaling your cast down accordingly.

What stands in the way of having an insect of this size, and how can we get around it?

  1. Respiration: increasing atmospheric oxygen content, per Mitch Connor's answer, gets you to seagull-sized, but past that you'd probably need to give them a not-very-insect-like pumped respiratory system. Either that or they're going to be so perforated for gas diffusion, they'd be more like a terrifyingly fragile elaborate chitinous filigree than a solid body, at least when you get close.

  2. Structural strength: the general square/cube problem with exoskeletons remains even with higher oxygen content. The filigree solution might work here too, actually, but I'm not sure how similar it'd really be to a butterfly

    • structural strength of the wings is probably more of a problem, because they need to be at least solid enough to displace air. A highly-perforated body (for strength:weight and respiration) also has higher drag, which probably doesn't help with flying. Maybe they could just glide on thermals or something?

On reflection I quite like the filigree approach, but it'd look more like a roughly butterfly-shaped bio-mechanical steampunk contraption than an insect.

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Make the air density greater than it is on Earth. Aluminum foil can float on sulfur hexafluoride. With greater air density, you can have greater buoyancy.

https://www.youtube.com/watch?v=1PJTq2xQiQ0

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You can start by making them light as the atmosphere around them. For example assume they consume helium from a none atmospheric source; and store it in their cells, making each cell into a tiny Helium balloon.

There is still the question of inertial mass - so make them slow moving. Maybe lowering atmospheric density will also help, by lowering friction.

If you want them to carry things, such as passengers, they'll have to be able to compensate for the additional weight, by regulating the amount of helium in their cells. Another option is to make them lighter than the atmosphere around them, and carry some substance as basalt, again, on a cellular level.

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  • $\begingroup$ Then again, decreasing atmospheric density diminishes buoyancy and aerodynamics. $\endgroup$ – Peregrine Rook Apr 19 '17 at 1:34
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Interesting thought: Does the dragon-sized butterfly need to look EXACTLY like a real-world butterfly?

Many realistic dragons have strengthened and thicker wings, joints, and bones. In a world where a 20 foot tall butterfly can exist, the butterfly wasnt magically enlarged from a smaller one. It grew and evolved over time, of course assuming the necessary food sources are there.

So if you find and eliminate all constraints to nourish and feed a giant butterfly, then there is no reason the biology of the butterfly can't evolve to make use of that.

Also to note, for an environment with the proper oxygen levels and large flowers with large nectar stashes that can sustain a butterfly, there must also be a either a much larger predator for the defenseless butterfly or a defense mechanism that makes it something like an elephant, giraffe, moose; not an apex predator, but enough strength to defend itself from one.

EDIT: Didn't see the second footnote regarding middle age like world. Umm, make them native to a patch of magical forest that has the necessary oxygen levels and nectar resources. They can stay alive for a number (24?) of hours in a normal oxygen level. The food can be resolved either with a magical bottomless bag with lots of nectar stored or some sort of high density / high energy nectar created by magic.

Basically you'd have a naturally large butterfly that is only sustainable through a certain location or with magic.

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  • $\begingroup$ I like it being only one location where they can survive...like maybe there is a cliffside where the updraft is enough to keep the butterfly flying or it only lives for a little while....or maybe its a evolutionary thing to keep predators down...get a huge butterfly for every hundred little ones and the big one kills predators or something. $\endgroup$ – FreeElk Apr 18 '17 at 13:48
  • $\begingroup$ @FreeElk - The thing about having one massive one is the resources needed to keep it alive. There has to be a supporting environment to allow it to grow, hence the oxygen-rich forest and gigantic flowers/nectar. $\endgroup$ – Ni Yao Apr 18 '17 at 14:15
  • $\begingroup$ Also the OP wants to use them like mounts, so they need to fly normally. $\endgroup$ – Ni Yao Apr 18 '17 at 14:16
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Can your world survive caterpillars the size of school busses?

You need two things especially:
(1) increased air density relative to gravity.
(2) An ecology that can support the huge, ravenous CATERPILLARS implied by your butterflies.

Increased air density, relative to local gravity is key to scaling up your butterflies, as has already been mentioned. Does your world have to be a planet? Consider alternate venues, such as in a closed, albeit huge environment, such as the interiors of the living Titans in John Varley's Gaea Trilogy. See:
https://en.wikipedia.org/wiki/Gaea_trilogy

or in the no-solid-ground 'smoke ring' of Larry Niven's Integral Trees: https://en.wikipedia.org/wiki/The_Integral_Trees


You also need an ecology that can feed both the larval butterflies (caterpillars) and your protagonist species. Doing so may not be so easy, with realistically voracious giant caterpillars.

Caterpillars are voracious consumers of leaf matter. Is there a mechanism that allows humans (or other, non-caterpillar/butterfly protagonist species) to have something to eat, with those giant plant vacuums all over the place?

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Well... when you consider that dragons themselves are unscientific... i.e.:
it does not make sense for them to be able to fly with their size and weight. Most stories have them flying with the power of magic. With magic as the answer, any answer can make (non)sense :-)

That translates to: yes, you can have butterflies the size of skyscrapers, as long as you use magic to answer how it is possible.

Edit: Well, I am new to the arena. So I do not think I can really provide anything more scientifically to the answer. Secespitus is right.

My answer has already been provided. And as someone who knows jack all about insects and how they breathe, I cannot provide any more useful insight.

But let me try to add on to what has already been provided, the only missing ingredient: how magic could help.

If insects do breathe using micro holes, have their breathing be handled completely through the use of magic - with magic being used to act as a black hole of sorts for air.

If the holes in the insects body increase in size proportionally, you could even have it such that anything that gets sucked into the hole, gets broken down into energy/required material (such as protein/ vitamans/ etc.) - which could even be used for their protection.

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  • $\begingroup$ The question asks for a reality-check and therefore answers based on science are preferred. "No, you would need magic." is a valid answer in this context, as the currently most upvoted answer shows, but the author of the answer is giving readons as to why this wouldn't be possible without using magic. You mention that size and weight would be limiting factors, but you don't explain why they would be limiting factors. As such I think your answer needs a few edits to provide more details and differentiate it from the existing one $\endgroup$ – Secespitus Apr 19 '17 at 12:57
  • $\begingroup$ I like your edit. It's an interesting way to use the magic and to add something to the insects. It may not be as scientifically as other answers but okay for this sort of question and might be an interesting thing to read for future readers who are interesting in scaling butterflies in their story. You get a +1 from me. $\endgroup$ – Secespitus Apr 20 '17 at 13:09
  • $\begingroup$ This is just a poorly worded form of Kilisi's answer. $\endgroup$ – T. Sar Apr 20 '17 at 13:26

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