Irkens are a race of green-skinned humanoids, with three fingers and two toes. Irkens have a skeletal structure not unlike those of humans, but are much more resilient and heal much more quickly. An Irken's muscular system has never been fully explored, however.
Irken skin is hairless and green, with some slight variation in its shade from individual to individual, like Humans; the skin color of both Tallests is ever so slightly lighter than Zim's, whereas Tak's is exceptionally pale in comparison. Their eyes have no whites or pupils (much like "Grey" aliens, which are the classic sci-fi aliens), and are only one color; ruby being the most common, but other colors such as red, pink, purple, green, and even yellowish-brown exist as well. It was also mentioned in Walk of Doom that Irken Invaders have surgically-installed ocular implants - since the eyes he was born with are not visibly different from subsequent appearances, it can be assumed that these implants are subcutaneous.
Their tongues are long, segmented and almost worm-like, while their teeth are white like human teeth but with a barely-perceptible pinkish tinge (presumably due to there being more blood vessels and nerves than human teeth) and are interconnected to form what resemble two long bands or a zipper (smeets, however, have only one small tooth similar to a human child's baby tooth, suggesting that the other teeth grow and interconnect at a later time).
Just like humans, Irkens grow taller; however, it was never conclusively explained how an Irken grows tall to be named the planet's ruler, though it is possible that young Tallest-to-be start the same size as the average developing Irken, and then suddenly go through a massive growth spurt that puts them at the forefront of the Irken Empire.
Door to Door seems to confirm that an Irken's saliva is similar to that of humans, as Zim licks the camera lens during one scene. (Backseat Drivers from Beyond the Stars was going to also have a similar scene with Almighty Tallest Purple; he was going to lick donut crumbs off the "camera lens", likely salivating on the lens in the process.)
Although Zim claimed in the cancelled episode Roboparents Gone Wild that Irkens do not require sleep, Larb's Great Assigning picture showed him sleeping on the Universe's Most Comfortable Couch; this seems to suggest that perhaps the Irkens hold sleep and dreams in a much different regard than humans do.
The antennae on the tops of their heads are unusual in structure, bending back at the ends at a sharp angle; the manner in which they bend, however, differs between genders: males' antennae end in simple perpendicular angles, whereas female antennae curl at the ends (females also have eyelashes, and their lashes and antennae often vary in shape, size, and style). It is unclear if these are actual genetic differences or just socially acceptable fashion practices. It was never confirmed exactly what the Irkens' antennae are meant to do. They are mainly seen to be used in the same manner as eyebrows to convey emotion, but it is heavily implied in Battle of the Planets that they also pick up vibrations. These antennae also seem to be extremely sensitive, as Zim always seems to avoid touching them when placing on headgear. He also squirmed away uncomfortably when GIR poked at one in Walk of Doom. As a show of respect to the Almighty Tallest, Irkens are to show respect by wiggling their antennae.
Interestingly, an alien bearing a slight resemblance to an Irken appeared in Jhonen's comic Squee; this alien had an Irken-like outfit and appeared to be wearing part of Zim's human disguise. Its eyes and head structure, on the other hand, are more similar to those of the infamous Roswell Greys.
As confirmed in the episode Mortos der Soulstealer, Irkens apparently have souls and life energy.
All Irkens are fitted with a hemispherical multipurpose device called a PAK, which is surgically attached to the upper back at birth and holds the Irken's memory, knowledge, and countless tools and weapons, such as spider legs, a communication device, etc. In effect, the PAK is the individual Irken, and the body is simply something to carry the PAK around. It was mentioned in cancelled episode The Trial that the Control Brains absorb the knowledge and experiences of Irkens when they die, and re-use the PAK for later generations, assigning the individual their personalities and traits. On the other hand, if an Irken is deemed defective, his or her memories and knowledge will be removed from the collective knowledge passed to a new Irken at birth.
If a PAK is forcefully detached from an Irken's spine, the Irken will die in ten minutes. If this happens, their PAK can be reused by other Irkens. Despite the assertion in the commentary that PAKs also circulate blood, Zim's PAK was shown to be detachable in NanoZIM, and Zim was also seen tinkering with it in his lab then reattaching it without any discomfort, let alone the side effects that were seen in Ten Minutes to Doom, where he languished after having his PAK forcibly removed by Torque Smackey. Based on this information, it can be concluded that the side effects only occur if the PAK is detached without the owner's consent, or that there is a five-minute grace period before the side effects kick in.
It should be noted that the PAK is only compatible with Irken biology; if attached to another creature, the PAK will destroy them, but not before the personality inside of the PAK overwhelms that of the host.
The Irken diet seems to consist mainly of refined carbohydrates, and it is probable that Irkens gain all their nutrients from sugar; the Almighty Tallest constantly eat doughnuts and popcorn, and drink soda, yet never gain weight. There are very few overweight Irkens, with a few exceptions being Invader Skoodge, Invader Krunk, Invader Dooky and Sizz-Lorr. However, Sizz-Lorr is a Frylord, and has thus worked in a fast food restaurant all his life, so this might not be genetic. It can be assumed that Irkens thrive off of carbohydrates alone, and the only way for it to negatively affect them is to overeat. For example, concept art from Mopiness of Doom showed that Zim's laziness and attachment to junk food over a small span of time took a toll on his appearance and weight.
Throughout the series, Zim has been shown eating an Irken snack food resembling Fun Dip, consisting of a pale white, presumably candy stick dipped in a colorful substance, presumably flavored sugar. In Tak: The Hideous New Girl, Zim managed to obtain this unnamed snack food from a vending machine on planet Devastis. The snack also appeared in Zim Eats Waffles and Hamstergeddon.
Irkens have been seen eating non-fatty foods, though, as Irken sandwiches, burgers and hot dogs have appeared a few times in the series. Irken sandwiches look exactly like Earth sandwiches, except they have the Irken insignia on them. Additionally, Almighty Tallest Purple was eating a taquito around the time Zim was abducted by Mary and Fred, and one one scene from Hobo 13 showed several Irkens eating their version of pizza. Zim can also be seen casually eating an Irken sandwich in Battle of the Planets. In The Nightmare Begins, several Irkens can be seen walking to the Great Assigning holding their version of cotton candy as well.
In the Nicktoons Christmas commercial How the You-Know-Who Stole You-Know-What', it was implied that some form of chocolate exists on Irk; here, Zim's "chocolate" was in the shape of a candy cane, which Angelica tried to pry away from him as he slept. However, the canonicity of this is questionable, especially seeing as Zim is sleeping (although sleep may be optional for Irkens, as described above, as he was also seen sleeping in the pilot).
Organs and Anatomy
The details of the Irken inner anatomy are mostly unknown, save for their "Squeedlyspooch", which was mentioned throughout the series and confirmed in the commentary of Germs to be one large super-organ, filling an Irken's entire torso and performing most if not all bodily functions. It resembles a combined human stomach and intestines. It is possible that their equivalent of a heart is separate from the Squeedlyspooch itself, as Zim made reference to it in Parent Teacher Night, although it is never given on official name. Irken brains look almost exactly like human brains, but exist mainly to control the PAK, being unable to function normally without it. As the brain is part of a nervous system, it can be said that like all creatures, the Irkens also have nerves. As The Girl Who Cried Gnome and Invasion of the Idiot Dog Brain showed, an Irken's sense of hearing cannot tolerate high-pitched frequencies, causing them to cover the sides of their head (where a human's ears would be). Another example of an Irken's ears being in the same location as a human's occurs in The Wettening, where one scene shows Zim wearing a pair of headphones that cover the same places where a human's ears would be.
Irken blood has never been shed in the show, but there have been some hints in episodes such as Planet Jackers (when Zim tried to distract the Planet Jackers by fighting them hand-to-hand; after the battle, Zim's battered face was covered in dark green patches.) Additionally, whenever Zim sustains a burn injury, the damaged flesh is dark green. However, Jhonen mentioned at some point that Irken blood was a clearish-pink in color, though it is unclear if he was actually being serious or not. However, he later confirmed it for real at the previous InvaderCON at his panel that it is clearish pink "similar to a Klingon's blood".
An Irken's skeletal system is almost similar in structure to a human skeleton, but more flexible, durable, and can regenerate and heal much faster. The skeletal structure of several Irkens was seen in The Nightmare Begins as numerous Invaders marched past a series of X-Rays.
Although Irkens don't have any nostrils, they smell from the same area as humans; however, this reaction may be for comedic purposes for the show. Either that, or there is a permeable membrane in that area which can detect scents. In the episode Rise of the Zitboy, when Dib tried escape Pustulio's hypnotic control by closing his eyes, Zim stated that he would open them eventually, as he had to breathe sometime. It is unlikely that Irkens really breathe through their eyes; this particular incident can likely be attributed to Zim's lack of knowledge (at the time) concerning human physiology, or possibly a ploy by Zim to distract Dib.
Genders and Reproduction
Irkens are known to be produced through DNA merging in underground facilities many fans refer to as 'smeeteries' (a portmanteau of smeet and nursery). It is implied that since the discovery of this system, Irkens have either lost or no longer use the ability to reproduce naturally, since Smeeteries are more efficient in perpetuating their race. No Irken is ever mentioned as having a mate or offspring in the show. (Jhonen confirmed in the commentary to Dibship Rising that the Irkens do in fact have the capability to show love.)
The Irken race does have two separate genders, but the differences are purely physical. Males have straight antennae with a single kink at the end, while females have curled antennae and eyelashes. It is unknown whether Irken females have curled antennae naturally, or if they are styled that way for the sake of fashion. Irkens can also be defined by their voices, which are clearly either male or female.
However, there appear to be no societal differences between males and females: both genders are seen to be equally competent in every regard; both males and females are seen in the same jobs. Moreover, the unfinished episode The Trial also introduced a female Almighty Tallest, named Miyuki. Therefore, it can be said that all Irkens are equal in terms of gender.
Strengths and Weaknesses
Irken skin can kill parasites such as lice, possibly by secreting some sort of protective substance. However, while safe from such pests, their skin cannot tolerate Earth meat, water, or beans, the former burning them terribly and sometimes even fusing to their skin. They also have negative reactions when exposed to Earth liquids (namely water). As confirmed by Jhonen in the commentary for The Wettening, it is the pollutants in Earth water that causes the burns rather than the water itself. Paste suffices to completely nullify the effects of tainted Earth liquids. It seems that there is nothing that can protect them from Earth's meat, however.
They sweat similarly to humans, and can get pimples when the skin is scrubbed by an oily substance (such as cheese or bacon). However, unlike human zits, they can grow to enormous size (as big as a classroom when fully swollen), are translucent and filled with a vast amount of pus. Irken zits also have hypnotic powers when shaken. It may be just an allergic reaction to certain chemicals, such as a combination of cheese, bacon, and Acne Blast.
Most Earth foods will make an Irken sick, although foods such as waffles, rich in sugar and carbohydrates, seem to be fine if eaten in moderation.
It is seen in certain episodes that an appendage can fall off or be removed and that it has no real influence, as seen in Dib's Wonderful Life of Doom and Planet Jackers. They can heal at incredibly fast rates, which is why Zim said to Dib in Halloween Spectacular of Spooky Doom, "As soon as my skeleton stops being broken, I will destroy you!". Zim's eyes have been seen falling out of their sockets before, although they can usually be simply popped back in without irritation or injury.
While capable of sleep, Irkens do not seem to sleep, as confirmed in Roboparents Gone Wild, and their PAKs may just keep recharging even when functioning so that they don't have to.
Despite their decades of military training, few Irkens show much prowess in the way of physical strength; although he is only a child, Dib has been shown to be more than capable of pinning Zim down. Therefore, with this in mind, it is possible that Irkens rely on their wits, technology, and sheer numbers when it comes to battling, rather than by actual physical prowess. However, Zim has, on occasion, shown feats of uncanny strength, an example of which being when he bodyslammed a human in Attack of the Saucer Morons.
Irkens also are shown to have incredible agility on Earth, and this may be due to (a) Irk's massive size and gravity, (b) genetic enhancements, (c) all of the above, or (d) the way Zim and Tak are animated. Dib also seems to have this miraculous agility, but that might be due to the fact that he was created/built much like Irkens are, or the way he is animated.
In spite of their apparent weaknesses, however, Irkens have extremely long life spans compared to normal humans. Zim and Tak looked the same in a flashback that took place fifty years ago and today, and Jhonen confirmed that Zim is almost two centuries old. It has also been said that a year on Irk is equivalent to ten Earth ones.
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