I'm starting work on a short series set in a standard fantasy sky world: floating islands and so forth - with the added element of insects and bugs large enough to move humans around. Bees the size of horses, beetles as big as an elephant, etc. Now I'm fairly attached to the premise, so I'm more than willing to have magic fill in the gaps where real biology can't, but I'm curious: How possible is this? There are also a couple of environmental factors that might affect things. Outside of specially protected human settlements, the concentration of oxygen in the atmosphere is much higher, around %80 as a starting "not sure how this affects things yet" number. (Suffice it to say that it is too high a concentration for humans to survive without breathing equipment.) The gravity outside of human settlements is also much lighter, about 1/5 what it is here in reality. So, with these factors in mind, how well would these massive bugs do?
For most (maybe all, but I'm not an expert) insects it's impossible; they have a critical mass where their surface area to internal area ratio becomes unsustainable. There were insects much larger than modern ones in ancient eras, but none anywhere near human sized. "Bugs" gives you more leeway, depending on how you define it. There were centipede-y things a metre long eons ago, for example. Size is one thing, but strength is probably even trickier. You could create a seven-foot centipede but its structure wouldn't withstand a six-foot human on it. I think magic is the answer!
80% oxygen sounds like a good enough start on it's own http://news.nationalgeographic.com/news/2011/08/110808-ancient-insects-bugs-giants-oxygen-animals-science/
Firstly are you insane Horse sized Bee's???? I really couldn't cope with that I hate hate hate bee's and wasps. Secondly, @Tenderiser seems to know his stuff here and his advice for magic is probably the best option. However I would say you could go down a few different options: 1. The insects were bred and genetically modified by mad scientists and dropped on this place you described, so could explain it away with bullcrap science. 2. If the gravity is much muss less than earth then the critical mass ratio wouldn't be applicable (or at least need amending) and see where that leaves you. 3. Could the insects not have synthetic exoskeletons bringing them up to the size you need? 4. It was all a dream
Insects breathe through a network of "tracheae". The air travels through these pipes via diffusion straight to the body's cells. Some insects do have a system for pumping in air into the larger pipes, but it still has to diffuse into the air capillaries in the body. Diffusion is a really slow process, which drastically limits the maximum size for insects; too large and the oxygen would have to diffuse over distances long enough that it would take essentially forever. To give a little perspective, if I'm not mistaken it would take over a year for oxygen to diffuse through a human body (from head to toes). As @Tenderiser said, small insects have a much larger surface area-to-volume ratio, and thus there will be more oxygen, and shorter distances for it to reach through their bodies. The larger an insect becomes, the smaller the area-to-volume ratio and the less efficient the transportation of air (and the less air per volume unit). This is one of the main reasons why e.g. humans have such an advanced breathing system (lungs -> blood -> cells); it would simply be impossible for the air to reach the cells through diffusion, and even if it does reach it there wouldn't be enough to keep us alive. The 80% oxygen levels would probably help, since a higher oxygen concentration means faster diffusion, but I believe human-sized insects is still stretching the limits too far. (Biologically speaking, not in fiction.)
I think they would only be possible to be 5x the normal size. Given the fact that giant insects in normal gravity would collapse under the weight of their own exoskeletons (science has figured this one out, look into it further if you like). So magic or some sort of extra ability is required to break the concept theory for your giant insects to function in the universe you have created. Without an internal structure to carry the burden of the exoskeleton, the gooey bug center would be squeezed out like a bug paste as the outer shell collapses in on itself.
I talk about this type of things with astronomer and cosmologist friends of mine all the time. I see some things that aren't as well known that you may want to consider. Biology Insects size are limited by the cube/square law. If you double an insects surface area, you quadruple it's volume. This has two side-effect: weight support and breathing. Insects have exoskeletons, so that's their only means of support. If you increase the mass of the insect, the thicker and more rigid the exoskeleton has to be. You could eliminate this problem in a number of ways. If you increase the density of the atmosphere dramatically, their bodies could be lighter than the air. If they are close to mutually buoyant, they can be as massive as you want. You could also have insects simply evolve internal structures. We aren't supported by only our bones, we have lots of systems that have evolved to fill in problems of our bones, your insects would evolve similarly. You could also simply reduce the gravity of the planet by making it smaller. Insects breath through their skin. So actually, the high oxygen is required to have big insects. In earth's past, insects got huge simply because there was more oxygen in the air. There were twelve foot long millipedes and three foot dragonflies. These evolved to get smaller as the oxygen content dropped. Chemistry Your oxygen content is waaaaay too high. Under those circumstance, an insect would flap it's wings once and the whole forest would burn down. In order to have a sustainable ecosystem, your planet can't be covered in rocket fuel. The biosphere would have to be able to recover from natural events like volcanos, asteroids, lightning. Oxygen corrodes everything, including organisms. In fact, you have a huge number of systems specifically designed to dampen the effects of oxygen atoms in your body. About 20% is as high as you can go and still have a somewhat safe environment. That's PLENTY of energy, the amount of energy stored in an oxygen molecule is more than you'd think. Astrophysics Oxygen is not a naturally occurring gas. Atomic oxygen (one atom) is extremely reactive and will bond with almost anything it touches. Even if a planet formed in a huge cloud of oxygen, there would still be none in the atmosphere, it'd get trapped in iron most likely. Molecular oxygen has to be made in chemical processes and only then does it become "stable." It's still reactive as hell, but at low (in cosmological terms) temperatures, it'll stick around in an atmosphere. The star it's orbiting also is not very oxygen friendly. Solar radiation changes O2 molecules into O3, so your biosphere would have to not be in equilibrium, it'd have to be spewing out oxygen at rates that far exceed the animals ability to use it. This would be tough to sustain long enough for intelligent beings to evolve. You'd have to have an ozone cycle as well, something would have to convert it back into molecules useful to the biosphere. Your planet would likely go through wild fluctuations and you'd have many more mass extinctions than Earth has (we've had 5.) Oxygen is a terrible insulator. Your planet would be freezing unless it's ridiculously close to the star, which would cause it's own problems. Close proximity to the star increases the rate at which you'll lose oxygen, although I suppose a biosphere could exist considering the density of the ozone layer you'd have. You need carbon dioxide or methane to keep your planet warm. You also have inert gasses that aren't going to get chemically bound to the planet itself, and also can't really contribute to the biosphere: nitrogen. Most planets form with a decent amount of it, and there is only a few things that can happen to it. It'll either stay in/on the planet, or the planet will be small enough or close enough to the star for it to get blasted away into space. Those are the only two options for nitrogen.
That was an interesting read, I must say. (Though I dislike the expression 'breathe through their skin'.) I didn't even consider the reactivity of the oxygen and the problems that arises from that fact alone. Good post! As for @Lone Vista, I guess the simple answer to solve a lot of these problems would be that 'it's fiction'; I actually think most people would accept it. If there's some kind of explanation to it (like magic or pseudo-science) it might be even more accepted (as long as the explanation sounds reasonable and thought-through, rather than forced).
If this is a fantasy world, could the insects have developed some supporting internal structure that real insects do not have, as well as a more advanced network of trachae such that the surface area:volume ratio isn't a problem?
Go with Steerpike's advice. It is a fantasy, if you can float islands in the sky then bugs can be as big as you need, IMO. You are not working with Earth standards so create your own. As much validity as there is in killjoy newjerseyrunner science always overlooks possibilities until they smack us in the face. What if there was a different isotope of Oxygen, and it wasn't so quick to attach to everything, that is unknown to us right now? Watch the TV show "Zoo" they have vultures carrying human bodies, we sort of just accept it.
Yeah, you're going to need to go with magic here. Even if the insects were large enough for your needs, the likelihood of them being able to carry a human is nigh impossible. This is primarily due to their physical weight than anything else- for example, the largest arthropods- marine crustaceans- can only reach certain sizes before their molts become too heavy to leave, causing them to suffocate in their own exoskeletons. If a giant insect can't even support its own weight, then it couldn't support a human as well. Besides that little snippet, I agree with the above people.
I'm with steerpike. In your world, bugs can have lungs if they need to. Give em' bones and make em as big as dinosaurs. Give'em special muscles unknown to earthlings that make them 100x as strong as earth insects. Best thing is, you probably don't even need to really discuss it. When I read starship troopers I didn't have any problems with the size of the bugs. (slightly larger than human size, I believe)
Yeah, not many people read Lord of the Rings and throw the book at the wall because Shelob is biophysically problematic.
Plus according to science bumble bees can't fly, IIRC. Lone Vista: not sure why no one has specifically mentioned that at one fifth gravity your planet would have difficulty holding an atmosphere, would be like our moon. Newjerseyrunner talked around this, but it is the more well known issue to many readers I believe.
You might be able to do the atmosphere if there is way to replenish it at the rate it's venting off into space.
Good point about the atmosphere! The numbers I have for the world are very much a rough starting point, so finding out more about what makes for a more sustainable ratio is rather helpful. Though, funnily enough, I do have a magical reason for new air to constantly be produced...though I shouldn't be at all surprised if that brought about an entirely new set of problems. As I said, I'm happy to fill in all the physics and biology gaps with magic, (or, as has been suggested, simply not going into it) but I like having an informed sense of what would work on its own to start writing from. Plus, research always gives me new ideas!
If it's far enough away from the sun or has a powerful magnetic field, it will be able to hold onto a heavy atmosphere. Oxygen is fairly heavy, as is CO2. While gravity is a limiting factor for the ability to hold an atmosphere, within the habitable zone, the star has a much stronger stripping effect. While Titan is the largest moon in the solar system, it actually has less gravity than our moon, but the air pressure at the surface is actually about 1.45 times that of Earth.
One more vote for @Steerpike. I would also hazard that as soon as you start to try to explain these fantasy elements through the filter of science and reason, you give the reader reasons to pick things apart. Why even go there? As Steerpike mentions, I don't care that Shelob is totally impossible. Not much in LotR is possible in the real world. That's why we go to Middle Earth to tell the story. In Middle Earth most anything can happen and most anything can exist. We don't question because we come to the story with an understanding that the pedantic exactitudes of science are to be left at the door. It's not that kind of story.
They would have to be specially evolved to be large. With multiple supporting internal structures. Similar but not identical to our skeletal system. As well as some form of advanced respiratory system. In short to be "believable" they would have to depart somewhat from the insects we know of. However what you are asking is not as unfeasible as some have suggested given the extreme high oxygen environment. This would make it easier to evolve an efficient respiratory system requiring less steps. Dealing with oxygen toxicity would be the primary evolutionary hurdle. And given that life exists on volcanic vents deep in the sea. Hardly an insurmountable one. Needless to say having a solid enough internal structure to support their own weight would also give them enough stability to support a human rider. Of greater concern is insects flight. While birds often ride currents and glide insects use brute force to stay in the air. The energy demands and size and efficiency of their flight muscles would have to be truly extraordinary. And they would have to be solid enough and move with such speed that anyone hit by such a wing in mid flight would probably not survive the encounter .. including the rider. I don't think it's impossible if you can somehow fit these concerns into your narratives and modify the evolved insects so much. If you went into your worlds evolutionary history maybe you could find some plague or other catastrophe that seemed to spare only insect life. And so they where allowed to dominate the world and evolve unhindered by the rise of vertebrate life.
