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There are a lot of giant insects and arachnids in fantasy and sci-fi. The understanding of why this is impossible on a world with Earth-like conditions is pretty universal: ineffectiveness of blood-circulation and breathing systems as well as exoskeleton becoming more of a hinderance and disadvantage at bigger sizes.

But what about a creature/taxon that would look like an insect? Doesn't have to be an Invertebrate per se but must:

  1. be a macro-sized lifeform(let's say a range between a Sparrow and a Blue Whale);
  2. have very "insect-like" features - enough for both uneducated peasant and an early Naturalist to classify it as an insect.

On a planet with conditions similar to Earth's:

  • Why and how would such creatures evolve?
  • What obstacles and how they'd have to overcome?
  • Could some of them form Eusociality and how they would interact with human-like species?

For some of the more "recent" examples of such creature look for "Kikimore" and "Endrega" from the Witcher franchise.

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  • $\begingroup$ I don't get how you can ask this question when you've already researched the leading the hypothesis on why there aren't giant insects....... just improve their respiratory and circulatory systems, would have no impact on the outer appearances.....Also there are giant spiders in existence that are bigger than many birds (even though they aren't insects). $\endgroup$
    – anon
    Dec 11, 2017 at 16:37
  • $\begingroup$ @anon that is exactly why I labled the creatures as "insectiods" instead of "insects" and used word "Giant" in quotes. I am not interested in going in circles about the afformentioned issues of why human-sized insects are impossible on our planet, but rather would hear out people's reality check for a case where such a creature would be a mis-conception to be labled as insect(ie has insect-like features but isn't insect on itself). Maybe I should add clarifications of why covergent evolution of multiple limbs, different body structure and exoskeletons would be sound within Earth evolution? $\endgroup$
    – Nick Dzink
    Dec 12, 2017 at 17:49
  • $\begingroup$ insect is already an ambiguous classification as are most of the top level categorizations. Give your creature 6 limbs a head abdomen and thorax and ta-da you've started a scientific debate. The old classification system is constantly under attack with new organisms being discovered that blur the lines. $\endgroup$
    – anon
    Dec 12, 2017 at 18:07

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So you want a hexapod/octopod (6/8 legs) creature which only looks like an insect.

As each leg increases the metabolic needs, there must be strong evolutionary pressure and reward for developing many legs. The advantage of hexa-/octopods is climbing ability.

As four-legged animals could be also excellent climbers (ibex, chamois, snow leopard), I suggest that the species lives overhead. And the reason is a symbiontic relationship with a nutritious fungus which started to grow overhead as protection against predators. First it was branches in forests. The spider-like organism fed on the fungus, hatched their eggs inside it (the fungus is sticky and poisonous) and had spores on their feet and therefore helped the fungus to spread. On the other hand the spider-like organism had a monopoly for the nutritous fungus and protected it feriously against any intruders.

As the fungus was so nutritious that it allowed the organism to grow, they developed lungs, endothermic temperature regulation and an internal skeleton in convergent evolution to overcome the size restrictions. The legs were strong, muscled with smooth muscles to allow almost unlimited hanging and had sharp ends with hooks to allow navigating ceilings. Finally the fungus did not grow only on wood, but also on cave ceilings or overhangs because they were fed by the spiders. The poison of the fungus was concentrated in the sharp end of the legs and allowed the spiders to kill prey. Then the prey was lifted up to the fungus which developed to utilize the preys nutrients.

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Limiting factors on sizes of real insects may be O2 concentration and preditors

To identify the factors that need to overcome it is useful to study a time in Earth's history when insects were larger than sparrows.

1) There is evidence that O2 concentrations limit the size of insects (see references below) so a larger insectoid would need a difference respiratory system and probably circulatory system (or equivalent for insects).

2) Another theory that has been advanced is that the evolution of birds as a predatory drove down insect size (see references). So lack of overly effective predators might be a factor.

references:

https://paleozoicgiantinsects.weebly.com/index.html

http://www.sci-news.com/paleontology/article00379.html

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