Insects have been evolving since the Devonian, and over 1 million species have been documented, with more to come. Their diversity and success is unlike any other group of animals. And yet, we see no large insects, none even comparable to usual tetrapods. With this many species, one would expect to have tetrapod-like insects, and yet nothing like them exists. There are three main reasons for this, and these are the lack of an internal skeleton, their exoskeleton, and their mode of respiration.
The thing that has made tetrapods so successful is their bony skeleton, rather than their limbs, or anything along that line. For example, take the megasquid, from The Future is Wild. It lacks a skeleton, being a cephalopod, and yet is the size of a large mammal. According to the show, muscle would be able to hold it together, but in that case, why are no living land invertebrates this size? Obviously, the megasquid, and any invertebrate even remotely close to its size would collapse. This lack of a rigid skeleton is one thing putting a lesser upper size limit on terrestrial invertebrates, including insects.
Secondly, the exoskeleton is hard to retain at large sizes, and yet is necessary for an arthropod to survive. This applies only to arthropods, and not to other invertebrates. Exoskeleton thickness grows relative to size, but this relativity is not proportional, but increases but the square of the length, making for giant insects having exoskeletons a meter thick or more. Unlike one would suspect, the exoskeleton is very heavy, possibly more massive than bone by measure of volume. Once again, any insects bearing this would simply collapse.
Although some may say the exoskeleton is more important to an insect's size than is respiration, respiration is also quite important. This becomes somewhat evident with the Carboniferous showing high oxygen levels coinciding with large insects. Although the lack lungs are another thing limiting insect size, the theoretical limit of insect size due to respiration is less than the observed size of insects in the fossil record. Thus, either respiration is irrelevant in insect size, or these insects had something like lungs.
Despite their diversity, clearly, insects will never rule the world. With all of these limitations, even in a high oxygen world tetrapods will remain dominant. Four hundred million years of evolution in insects has still resulted in no more than a background group of animals, always far behind the ever superior tetrapods.