Ah. Well, I have noticed that some of the designs the Protoss employ seem to resemble insects. Take for example the Reaver, a giant, caterpillar-like automaton. Did the original designers intend to make it appear as some great insect, perhaps to psych the enemy? Or perhaps that design maximizes stability and defenses? Whatever the case may be, it clearly looks and moves like an insect, and I do not believe it to be wholly coincidental.
I have seen on a TV show some time ago, perhaps on the Discovery channel, that archetects are studying the structure of things such as bee hives, as they prove to be very durable and efficient. Not only that, but they circulate air quite well. Perhaps the Xel'Naga respected insectoid creatures, for one reason or another...
Huh, an odd theory:
Zerg- Insectoid
Protoss- Reptoid
Terrans- Mammalian
Xel'Naga- Avian?