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Hypothetically, if we remove the gas/liquid medium, and the insects were to evolve on out planet in the vacuum, would they need the symmetry?
They would need symmetry of propulsion, just like all spacecraft. A daddy long legs with 1 foot on one side and 10 on the other would just move around in circles. Not very effective. A cheetah with 3 feet on one side and one on the other would do the same. Not only do we have to factor in fluid dynamics but we also have to look at propulsion and weight balance. Even the basic design of our automobiles are symmetric due to this factor. Four contact points make for a very stable and even propulsion, allowing for strong manuevering and balance.
Also, the ancestors of all terrestrial insects are aquatic archopods. You know, like shrimp, lobsters, crabs, etc. So there is a basis for fluid dynamic adaptations.
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