What most people get right about "The Zarbi" is that it's really weird. It's not just weird, it's disorienting and confusing--everything about this episode feels like something you've never seen on television before. What people get wrong is insisting that this is an accident or a fault--this is actually a deliberate aesthetic choice, meant to evoke the same feelings the TARDIS crew has at being trapped on an utterly alien world.
To that extent, everything works together almost seamlessly. The aliens are more alien than anything we've seen before, even more than the Daleks--sure, there are superficial resemblances to insects, and yes, the costumes do look a bit "stagey" and not at all what we're used to in a modern era of CGI. But it feels like the show has made a conscious effort to create not just an alien look, but an alien mindset--the Zarbi don't speak in anything comprehensible to the audience, a risky decision that pays off in making them feel harsh and inhuman on a level that the costumes alone can't quite pull off. The Menoptra, who are the closest we get to relatable characters, nonetheless behave in a way quite different from anything we've seen before in the series. Their strange and eerily graceful movements, deliberately choreographed by Roslyn de Winter, give the impression that they're not quite from the same series as the rest of the cast. Their bizarre, flat, affectless speech continually reminds you that you're not dealing with creatures who see the world the way we do. Best of all, they examine Barbara's hair with the tentative curiosity of a race who aren't quite sure what the strings of protein dangling from this weird creature's head are, or what they're used for. It's a clever, tangible reminder that we are on an alien world and can take nothing for granted.
And more than that, that we are trapped on that alien world. The episode opens with the TARDIS being stolen, and although the sight of it creeping along the alien landscape as though it's tiptoeing is downright ludicrous, it's another reminder that we don't understand any of the rules here and the one route to safety for us (well, for our proxy selves onscreen) is being dragged off to an unknown fate. The very air seems strange, inhuman, surreal with its odd gleams and refractions of light, and Ian and the Doctor quickly learn that it feels different in their lungs as well. There's a sensation that this is a wrong turn, as though the TARDIS has been dragged not just to a planet but to a narrative where it doesn't belong.
And then we get the ultimate violation--a Zarbi enters the TARDIS. It's the first time any monster has ever gotten into the Ship, and it remains one of only a handful in the series history; the violation feels complete and horrifying, to the point where even the Zarbi itself seems to recoil at its own action. Or else the TARDIS itself repels the invader, both seem equally likely. When the Doctor reacts in cold fury to the intrusion, we completely understand.
The whole episode is just one peak after another of inhuman alienation, reaching perhaps its crescendo when Barbara is forced to watch as the Zarbi rip the wings off a Menoptra to prevent it from escaping. Even without knowing anything about this world, we can instinctively sense the cruelty of the act along with her. The reasons for the act are lost on us, but that only contributes to the sense of dislocation and fear that pervades the whole story to this point. When the Animus finally speaks at the end, it's almost a relief...at long last, something here is communicating to us on a level we can understand.
But of course, it's the villain...