Disclaimer: Sometimes I cite Jhonen Vasquez and the DVD commentary as evidence. Sometimes I outright contradict Jhonen Vasquez. This is because Jhonen is full of lies and will either say whatever the people want to hear most or whatever the people want to hear LEAST, depending on his snark levels at the moment. Your fan experience is what you make it.
In many ways, the Irkens were designed to fit the classic "little green men" archetype: short, smooth green skin, large bug eyes and antennae. I like to regard these characteristics from an insectoid point of view rather than a pop-culture alien one. (Zim fanart that makes him look like an odd green human child bothers me: it's not that he can't be cute, but that he shouldn't have to be human-cute. He should be cute like a ladybug. He IS cute like a ladybug.) 🐞
The Irkens' bright, bulging, chitinous eyes and their antennnae are wonderfully expressive. Irkens never seem to use their antennae for any sensory function, though they might've had that ability before the Irkens became a cyborg race (more on that later). Expressively, the antennae move much like cat ears, flat back in fear, straight up in surprise or excitement, cocked when curious. Irkens seem to have, but rarely use, conscious control over their position (like learning to wiggle your ears). Too bad. I love imagining Zim rubbing his antennae diabolically together.
The biggest hole in the insectoid interpretation is that Irkens have internal skeletons, not exoskeletons. I'll give the animators the benefit of the doubt that this is to preserve the clarity of cartoon tropes like x-rays, electrocution, expressive teeth, and slapstick injury, not just a failure of imagination. Actually, regarding teeth, Irken teeth are NOT part of the skeleton. They appear rounded, pink, and fused together, unlike the teeth of humans in the show. This implies weird fleshy-cartilaginous appendages (maybe something like waterfowl "teeth"??), which is... a weird and unexplored design choice. I feel sorta cursed by that knowledge.
The visceral humor of the Jhonenverse actually gives us a pretty good idea of what's inside an Irken. They definitely have a brain and a nervous system, and likely a heart, but are mostly digestive system ("squeedlyspooch"). This seems a little pointless, as all Irkens ever eat is junk food, the PAK converting this to useful energy anyways. Zim tends to scream "AAAUGH MY SQUEEDLYSPOOCH" when something whacks him anywhere, but that proves nothing- give any friend a good kick in their internal organs and they'll probably yell "OW, WHY, MY STOMACH" whether or not that was the specific organ injured.
An organ Irkens DON'T have are lungs. Without the human weaknesses of wet, soft eyeballs or easily-ruptured interal air sacs, the Irken race takes a rather cavalier attitude towards the vacuum of space. Smaller spacecraft can open the windshield during flight while the pilot is inside, but larger Irken vessels have airlocks. The Tallest sometimes throw bad Irkens out the airlock, so they definitely do need to breathe eventually, but their PAK seems to give them a lot of leeway (breathing room, one might say).
Nickelodeon really had a thing against showing blood so it's never pictured in the series, but Jhonen has said that Irken blood is a clearish-pink, which is a ROCKIN' character design choice from a color theory standpoint. To mesh this with injuries shown in-show, Irken blood is probably dark green under the skin but turns pink on contact with air (like the common myth about human blood being blue). Dark green is a perfectly valid blood color and is also really cool.
The mysteries of Irken biochemistry are a little beyond me, and not just because the cartoon itself seems to forget what things Zim can or cannot eat or touch. Jhonen's explanation for Zim's inconsistent weakness to water is that chemical contaminants, not the water itself, are what hurts Zim. The Jhonenverse is so thoroughly disgusting that it's a wonder everyone doesn't get chemical burns.
Zim's reaction to earth meats seems more arbitrary, but mad as it is, I have a theory. Zim's bad reaction is characterized as an "allergy", but as someone with food allergies, I have never had something I'm allergic to FUSE to my SKIN. Irkens have remarkable healing abilities, facilitated by the PAK. It may be that some earth meats react badly (or far too well) with the Irken immune system, accepting and attempting to graft the foreign flesh onto the Irken's skin.
Now, I'm an engineer, not a biologist, but I'm pretty sure every part of what I just said is ridiculous and wrong. But it amuses me and makes superficial sense, so that's what I'm sticking with.
Irkens don't sleep. Sounds nice.
The purpose and significance of the Irken PAK is, sadly, explored mostly in the cancelled episodes. What we DO know is that Irkens are a truly cyborg race that depend on their PAK for crucial life functions, interfacing with Irken tech, and storing their memories and personalities. Imagine a swiss army knife that's also a USB stick holding your entire identity, and also if you lose it, half your organs shut down. Like that!
The most flashy and probably original purpose of the PAK is to keep their advanced tech at their fingertips at all times. This includes insectoid robotic legs, communication, night vision, portable space helmet, and emergency weaponry. They're usually only used in emergencies, so it may be either socially unacceptable or physically draining to use these features for extended periods of time. After all, the biological body and the PAK keep each other recharged. Irkens do not seem to feel pain when the PAK's gadgets are damaged.
The PAK can be removed from its installation point in the Irken spine for modification and/or repairs. It just can't be removed for too long: the PAK compensates for the Irken's missing organs, with a "charging cell" (controlling metabolism), "atmospheric processor" (making up for lack of lungs), and blood/waste filtration (the commentary claims it also circulates their blood, which doesn't make sense, but this would explain Jhonen's claim that Irkens don't produce, uh, bodily waste). If the PAK is removed abruptly without consent you get a rapid, agonizing 10 Minutes To Doom deterioration. That should still make some sense from a human perspective: think of how long someone could stay conscious after taking a deep breath and diving underwater, vs. if someone yanked you under without warning.
The PAK also reportedly contains a universal translator. But Dib can intercept messages to the Tallest and other non-Irken aliens in plain English..... Actually, that's an entire other can of worms that I will leave for someone else to puzzle over. Let's get back to the PAK.
Not only can an Irken remain conscious and autonomous when the PAK is detatched or incapacitated, but the reverse is also true. Regarding Irkens as either "alien wearing a robot backpack" or "robot wearing an alien meatsuit" is oversimplification. I like taking the computer-sciencey perspective of the biological Irken brain being the RAM and the PAK's memory storage being the hard drive. Gir shorts out Zim's brain (and probably stops his heart) briefly in Plague of Babies and the PAK simply reboots him.
The division of body and mind enables the violent, ambitious, spacefaring Irken lifestyle. The PAK allows the extra memory storage to pass an IMPOSSIBLE amount of technical knowledge, the entire civilization's worth, to every Irken. The resiliency of the PAK counteracts the fragility of their bodies, and allows them to regrow skin, walk off broken bones, and replace dislodged eyeballs during the dangerous work of planet-conquering. As a result, Irkens are often asked to do (or decide to do) incredibly painful, dangerous things and have REALLY low injury empathy. Who cares if the body is screaming if the PAK fulfills its mission?
Much like the Irken can work on a damaged PAK, a PAK can likely use its tools to fix its own damaged body. Only one half of the Irken cyborg needs to still be functional to restore function to the other half.
Baby Irkens are called Smeets, and if that isn't the cutest thing you've ever heard, you can go jump out an airlock.
Irkens are made, not born, grown in rows and rows of identical pods tended only by robotic arms. They appear to have no brain function until their PAK is installed and an electric shock starts their heart/brain. Their PAK is then encoded with all the information they need, so they're ready to begin training from day one. We don't know much about their adolescence or when their roles are chosen, but their PAKs are encoded specially for their jobs. Hey, maybe one of the reasons that Zim is an idiot (other than the fact that he's just an idiot) is that he's encoded as a frycook. He's probably missing a lot of vital information that was erased when he was demoted from being Invader-class.
Irkens are a lot like worker bees, and like these insects are infertile and have no concept of romance. They're actually missing a LOT of organs other than just the reproductive ones, which suggests heavy genetic meddling to make them more compatible with their PAKs. I guess once your race has already started uploading their brains into machines, radical species body modification is hardly a hop, skip, and step over the ethical line.
Absence of sexual dimorphism also means that gender really is nothing but a form of personal identification, like a name. Females often have curled antennae and visible eyelashes, but it seems to just be a cosmetic change to give cues on what pronouns to use. Zim could grab mascara and antennae-curlers any day and be exactly as much of a girl as Tak is. (Could Irkens be "transgender" if nobody seems to bother assigning them a gender in the first place?) Zim doesn't seem to have figured out human secondary sex characteristics yet, mixing up people from time to time.
Irkens age at a 1:10 ratio to humans, even Zim being older than any human alive. Conceivably, an Irken could die of old age, but I can't really see an Irken living long enough for that. Irken attitudes and customs around death go almost totally unexplored— this IS a kids show, after all. We do know that PAKs are a form of non-volatile memory that can be retrieved and added to the collective species memory even after the physical body's death. It's here, though, that the internal logic of Irken society gets muddy. If their entire identity is stored in the PAK, why not reattatch it to a fresh smeet? That's functional immortality!
Cop-out answer: Since the Irkens inhabit a capitalist hivemind hellworld where the only thing respected less than bodies is things, it may just be cheaper to upload the memories off the PAK, chuck it in the bin, and build a new one. Going out to retrieve dead PAKs and ship them all the way back across space to the smeeteries sounds like a lot of work for a race with exactly zero respect for individuals.
More interesting answer: This is only one of many inconsistencies that suggest that the Irken race is not what it claims to be.
What if the egg came before the chicken?
Irken society is rife with paradoxes. Why do Irkens place so much value on height when the identity of an Irken is so disconnected from their physical bodies? How are the Irkens so successful when the Tallests are dumb as bricks? To what end is their endless construction, production, and conquering, if there doesn't seem to be an Irken civilian population to serve? And WHY in the world would a society of communal hive-minded creatures all be such backstabbing bastards?
That image to the left shows a Control Brain. Let's talk about the Control Brains. Can we talk about the Control Brains? I've been dying to talk about the Control Brains all day. These massive supercomputers are used to make decisions about the highest levels of Irken society. Jhonen has gone on record as saying that they're incredibly advanced Irkens, evolved beyond the need for a body. (They do have a very PAK-like design.) But it seems equally likely that the Control Brains are something entirely different— and the Irkens are a race of slaves.
The Irkens are utterly dependent on their technology. Every organ function surrendered into the PAK was getting rid of a useful redundancy, which would be foolish if the PAKs were meant to be assistive devices.... but very useful if they were meant for control. New Irkens can only be grown by the Control Brains. New smeets immediately learn a history of the race, but a history that the Control Brains curate and approve. Everyone knows history is written by the winners. If the Control Brains really were an outside force, it would only take a generation of test-tube Irkens to forget that fact. For an Irken to question this knowledge would be treason to the species, and for them to abandon this knowledge, it would require removing the very thing that keeps them alive. Independent thought is death.
The presence of the Tallest is one of the biggest red flags. The Tallest are merely a fancy-looking figurehead. It's an empty but flashy position for up-and-coming Irkens to aspire to. The Irkens are a very race-loyal people, after all, who want to look up and see faces like their own. Even the Tallest have PAKs. The Control Brains prefer to be the man behind the curtain while the Irkens see themselves as working for themselves. It's only behind the scenes and in radical circumstances when the Control Brains give instructions directly. Why would they need to? They have the ability to program the mind of every Irken from death to birth.
If the Control Brains really once were Irkens, they've lost all remnants of the race they command. But it seems to me that a hive-like species would be a very easy target for a higher intelligence to take over and use as its own unquestioning army. It's an unprovable theory as original Invader Zim canon goes, and perhaps as paranoid as anything Dib's ever thought up, but I stand by it.
Do you think the Control Brains look a bit like PAKs because both are Irken technology? Or maybe, could PAKs look like the Control Brains because the PAKs weren't Irken to begin with?