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Given the perfect environment, denser atmosphere, oxygen rich, maybe lower gravity, how large can a flying insect get and still be able to walk upside down on a surface?

This question starts to tackle it but doesn't give a definitive answer plus the OP states a particular weight which may be too heavy, Large spider walking upside down

As this is an alien species it doesn't necessarily need to have the same sticky method as insects with tiny hairs, it could be something like a gecko or something entirely different as long as its not too sticky they get stuck.

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    $\begingroup$ YOu should know that the formulas for calculating the surface gravity and teh escape velocity of a world are not the same, and thus those two different factors do not change in lock step. You need your planet to have a surface gravity as low as necessary for the large creature to walk on the ceiling, while having the escape velocity as large as necessary for the planet to retain a dense atmosphere. $\endgroup$ – M. A. Golding Jan 1 at 18:41
  • $\begingroup$ The density and oxygen content of the atmosphere are surely irrelevant here? I would think that gravity is all that matters. $\endgroup$ – TonyK Jan 1 at 23:53
  • $\begingroup$ @TonyK from my understanding insects need higher levels of oxygen and a denser atmosphere to grow large, well that's going off extinct large insects on earth $\endgroup$ – RandySavage Jan 1 at 23:56
  • $\begingroup$ I thought your question was how large an insect could get and still be able to walk upside down. Are you really asking two questions in one? $\endgroup$ – TonyK Jan 1 at 23:57
  • $\begingroup$ @M.A.Golding thanks, I didn't know that. $\endgroup$ – RandySavage Jan 1 at 23:58
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It depends on the mechanism it uses to stick to the ceiling (and on the ceiling material).

Using the multiple-tendril mechanism of gecko feet, you can bear a weight of about ten Newton per square centimeter (a mass of one kg in Earth gravity; six on the Moon, and so on). This means that theoretically, and if the ceiling plaster doesn't give way making it fall down, a human-sized insect with webbed hands and feet could easily walk upside down in Earth gravity. If it had more than four legs, all the better.

Using simple adhesion ("stickyness") you probably can't go heavier than about five hundred milliNewtons per square centimeter, but with reduced gravity and feet enough, human size is again doable. Even more so if the insect is less dense than a human being.

Chemical bonding allows even higher weights; the limit is the tensile strength of the ceiling material, provided that it is amenable to that specific kind of bonding (you woudn't be able to walk on a smooth PTFE ceiling). But you'd leave large tracks, you'd need to go comparatively slowly and the need to secrete the appropriate "superglue" would exact a very heavy biological toll. Some marine gasteropods do something similar though, so an alien insect also might.

An intermediate solution would be to use heat glue - you'd almost surely need an omeothermic creature for that, which Earth insects aren't. The insect would be capable of "heating" its appendages, thereby making the organic "glue" secreted by its extremities semiliquid; once the foot has taken hold, it would cool, and the glue would set guaranteeing a good grip. In Earth conditions this would only allow a really slow gait (a sort of insectile sloth, maybe). The foot would naturally gather dust and other debris, so the creature would need to periodically fling the dirty glue away.

Or perhaps - depending on the nature of the "ceiling" - an insect might evolve the equivalent of harpoons or drills to anchor itself. The peacock mantis shrimp is capable of cracking small rocks and "drilling" cavities in shells and burrows in concrete and live rock by bashing its appendages against it; a similar method could allow to quickly drill small cavities which, filled by an expanding palp, would allow "walking" on vertical walls or ceilings to a creature of almost any size ( again, provided the walls or ceiling did not undergo structural failure).

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  • $\begingroup$ @LSeri brilliant, thanks. $\endgroup$ – RandySavage Jan 1 at 16:30
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    $\begingroup$ One kilogram-force is about ten newtons. Ten thousand newtons is about one tonne-force. $\endgroup$ – AlexP Jan 1 at 17:08
  • $\begingroup$ @AlexP thanks, I had started copying down the table in grams and had left a "thousand" in. Yeah, you aren't likely to get one tonne-force of traction per square centimeter, unless the insect uses nuts and bolts :-) $\endgroup$ – LSerni Jan 1 at 19:45
  • $\begingroup$ What about the mechanism that actual insects use, whatever that might be? $\endgroup$ – nick012000 Jan 2 at 3:21
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    $\begingroup$ @nick012000 they either use minuscule grappling hairs or hooks, so that is limited to a few grams-force and don't scale very much: the largest millipedes (shongolos) can have 250+ legs and yet do not seem to climb very much. Adhesives seem to fare a little better, but those are used by molluscs, not arthropods. $\endgroup$ – LSerni Jan 2 at 3:39
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I think you would need to specify the force of gravity at the very least to get anything close to a specific answer. Insects grew much larger during the Carboniferous period presumably due to the richly oxygenated atmosphere, but gravity is really the determining factor. The giant bugs seen in classic sci-fi could not exist because they would collapse under their own weight. Their exoskeletons could not support their weight. Before somebody calculates sizes, assume that your planet is less massive than the earth, that the insects’ exoskeletons are much thicker than just a scaled-up version of our own, and that they might have evolved some other sort of internal support (I.e. bones) to ensure their structural integrity.

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  • $\begingroup$ I agree, I realised after writing it that saying low gravity is a little vague and could allow upside down standing easily, thanks I will update the question with more details $\endgroup$ – RandySavage Jan 1 at 15:23
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If the creature is not from Earth and forces at work are up for discussion, you have carte blanche really. What if the evolutionary path on that planet dictated very strong and light materials making up the exoskeleton and connective materials? If it flies under it's own strength, it's probably already an optimized weight for the size/gravity ratio.

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