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The flight-mechanisms and exoskeletons of insects are only practical at their current scale -- the wings interact with the air as more of a 'fluid' medium at their scale, than gas as birds experience it (I may be wrong on this). Second, their exoskeletons are less cumbersome (and indeed helpful) at the micro level they exist at -- whereas on a larger scale the dynamic would be very different in both how their muscles work and the efficiency / weight of their construct.

So to really create a believable creature that is a genuine insect (or better said, arthropod, sea or land) what changes would be necessary to create a creature that is strong, robust, and threatening?

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    $\begingroup$ Definitely related: Can you simply scale up animals? I seem to recall that we had a similar question specifically about insects, but I can't find it right now. $\endgroup$ – a CVn Dec 18 '15 at 20:25
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One of the biggest limitations is actually breathing and circulation.

See here: https://www.quora.com/Entomology/Do-any-insects-have-hearts-and-lungs-or-do-they-have-a-completely-different-system-of-breathing

Hearts: Sort of. Insects have an open circulatory system, meaning their organs just float in blood. It's similar to how human organs float in lymph, so we call insect blood "hemolymph." However, insects do usually have a tube on their back that has holes to take in blood and pumps it up to the front of the insect. This sets up a current of hemolymph flow in the body. So, yes, insects have a "heart" and maybe an aorta... but no other blood vessels. Also, large insects have smaller, muscular "hearts" by the starts of their limbs or wings to help pump blood in those body parts as well.

Lungs: Nope! Insects have a completely different system of breathing. They have a system of tubes called trachea leading from holes along the sides of their bodies called spiracles to their organs. Air flows in the trachea as it does in our lungs. You can think of the trachea as being blood vessels for air only, carrying oxygen and taking CO2 from every tissue directly. Insect hemolymph, likewise, does not carry oxygen the way our blood does. Small insects just have trachea, while large insects can pump air through them using their muscles or even air sacs.

This design does not seem to scale well and is one of the main reasons that prehistoric insects could grow considerably larger than our insects today. The higher oxygen concentrations in the air then allowed it.

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  • $\begingroup$ This makes me wonder, then, if the rising CO2 levels modernly will (eventually) lead to even smaller insects (hypothetically, take the setting of Rainbow Mars, where the CO2 content was even higher: would that lead to the extinction of the larger beetles?). $\endgroup$ – Draco18s Dec 18 '15 at 20:20
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    $\begingroup$ @Draco18s as I understand it, the issue is not CO2 levels (which are still a small fraction of a percent) but O2 levels (which were way higher in other biological eras). $\endgroup$ – SJuan76 Dec 18 '15 at 21:14
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    $\begingroup$ @SJuan76 True! Although presumably as CO2 concentration rises, O2 concentration falls. $\endgroup$ – Draco18s Dec 18 '15 at 21:15
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    $\begingroup$ @Draco18s If the CO2 concentration of Earth's atmosphere doubles, it goes from 0.034% to 0.079%. That doesn't really make a very big dent in the concentration of O2, which is 20.946% at present. If the CO2 concentration increase were to displace oxygen in the atmosphere, O2 would go from 20.946% to 20.901%, which is an about 0.2% change (20.901 / 20.946 = 0.99785...). All current percentages from Wikipedia, based on volume figures. $\endgroup$ – a CVn Dec 18 '15 at 21:22
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    $\begingroup$ It's also important to understand that even (currently rising) concentrations of $CO_2$ are close to the absolute minimum known concentrations of $CO_2$ over the history of the Earth. $\endgroup$ – Jim2B Dec 19 '15 at 14:40
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Tim B had an excellent answer (take a second to up vote that now).

There's also the mechanical limitation of the exoskeletal structure.

Using chitin as the structural material and exoskeletan as the structural technique, Terrestrial insects are limited to about a 3 ft diameter size. Any larger and they couldn't move because they'd weigh too much for their structure.

The largest size an insect could be is the smallest of all the limiting factors. So it's important to know that even insects breathing pure oxygen could never grow to a size much larger than the stated 3 feet.

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    $\begingroup$ Next time I see a spider/roach/mosquito that is 3 feet I diameter, let me assure you, I will take little comfort in the fact that that is about the upper size limit. $\endgroup$ – Gary Walker Dec 19 '15 at 15:52
  • $\begingroup$ Both answers are good - I appreciate this one for the details given as well. $\endgroup$ – Pipsqweek Dec 20 '15 at 1:24
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The main limitation to insect size is gas exchange. An insect does not possess any lungs or gills; instead it possesses various holes (spiracles) on the outside of its body, which lead into tracheae that branch out all over the body and supply oxygen directly to the animal's tissues. Insects don't have blood in the traditional sense, because they don't need it, thanks to the fact that gasses just diffuse right into their cells instead of having to be diffused into the blood first.

Now here's the kicker: in places where the exoskeleton pinches and becomes narrow (i.e: joints on the extremities), the trachea still needs to pass through. In smaller insects, this is not so much of a problem, since their tracheae don't take up much space and can be very slim; able to pass through their tiny joints. As an insect gets larger, however, the trachea needs to grow disproportionately large to accommodate gas exchange into the extremity. Once the trachea takes up 90% of the space within the joint, the insect cannot physically grow any bigger, because it still needs to fit in things like tendons and ligaments. For an insect to grow to human-size or larger in our type of atmosphere (low oxygen density), it would need to change its entire respiratory and circulatory system. Doing so would render the question moot, since if it were to alter its biology at such a fundamental level, it wouldn't really be an insect any more.

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