I wondered at the insectoid babies... they looked like fully formed insects from the get-go. That's not how RL insects on earth work. They come out as caterpillars, maggots, general wormy-things, and then eventually metamorphose into insects with legs (and in some cases wings). I guess the insectoids wouldn't be exactly like earth insects though, being an alien species and all that. Their respiration system would have to be different for example - real life insects are size-limited by their inefficient system for oxygenating their bodies. The largest insect in the world is the Giant Weta of New Zealand, it's about 2 - 3 times the size of a mouse. Insectoids that were human sized pretty much would need to have something like lungs somewhere, and a more developed circulatory system, to distribute the oxygen carrier as needed.
Good points. I never thought about what they *should* have looked like as babies. And even the giant insect of 300 million years ago were not the size of the Xindi insectoids. Earth once has dragonflies with a wingspan of a hawk, and scorpions that were 2 feet long, yet even those animals do not compare in size with the Xindi insectoids. Its believed that Earth's atmosphere had a richer oxygen content when the giant insects lived, and they would not be able to live in our atmosphere today. But in the Star Trek universe, every planet is exactly the same as Earth - same atmosphere, temperate climate, same gravity, and same humanoids. So it really isn't much of a stretch to believe giant insects could cohabitate with primates in the same world. After all, everyone else seems pretty interchangeable.
Not all of them, actually. Quite a few species of insects have what are called nymphs, and they look just like miniature adults. Source.
I can imagine a yeoman bringing Archer's uniform to laundry chief (I assume the Enterprise had a laundry?) and them saying: "Gawd, what it is this time? Does the captain know how long it took for me to get the stains out from that thing the Kreetassans left behind?"
I liked the Sloths, they seemed like the most friendly. The Primates were simply like us and the Aquatics, while looking interesting, weren't in the spotlight too often and suffered from being defined less by their actions and words and more via the descriptions of other Xindi.
My favorites are the ones all over the place in later series, all getting along great with the rest of the Federation in preparation for the big allied war with the sphere builders. Oh, wait...
Plus, the reptilian leader was almost comically evil; I half expected him to demand that Archer hand over the Omega-13.
Avians - Every time I watch CSI and they reconstruct a face from a skull they found I think how neat it would be to do that to that Avian skull so we would know what they looked like.