The Tyranids have been a threat to the Warhammer 40,000 universe for more than 20 years now. As many of you proud defenders are encountering these critters for the very first time the Inquisition has decided the time has come to give you all some more information about the history and evolution of these foul bugs.

The Tyranid models that you can see now on battlefields near you have been slowly evolving since the very first time they were mentioned in a Warhammer 40,000 game. Even the trusty, well-known Genestealers (the oldest core models continuously on sale) were quite different when they first sprung up from the fevered mind of that gaming genius, Rick Priestley.

So, if you are interested in how the greatest threat known to Mankind (no, not Britney Spears) was born than read the following text carefully and study it well, young warrior, as it might save your life in the future.

TYRANIDS

It all started on page 200 of that forbidden tome "Warhammer 40,000 Rogue Trader" under the header "Tyranids and the Hive Fleets". 

The very first picture of a Tyranid from Rogue Trader (1987)

Tyranids were described as:  
"They are centauroid in appearance, with six spindly limbs. The rear limbs have webbed appendages which serve as both hands and feet, helping to stabilise the creature in the low-gravity conditions aboard Hive Fleet ships. The front limbs are manipulative with double-thumbed hands, whilst the middle legs are hoofed and form the primary locomotion limbs. A Tyranid can move using its central legs alone, although when moving slowly they tend to use any free limb to gaps onto the irregular walls of the Hive Fleet ships. All Tyranids have excellent vision afforded by their secondary eyes which grow from stalks along their neck. These are often mistaken for ears by ignorant humans, but the creature's real ears are set into deep 'canals' along the Tyranid's neck. Having evolved in a low-gravity, low density atmosphere, the Tyranid's sense of hearing is extremely poor, but they are remarkably sensitive to changes in atmospheric pressure and vibration. Tyranids are entirely carnivorous, extremely intelligent, and totally merciless."

The first Tyranids had access to all kinds of advanced weaponry like lasguns and flak armour and the only other troop type that they could field during battles were Zoats, a separate race that were enslaved by the Tyranids.

Clearly very different from the Tyranids as we know them now. In fact these early Tyranids became later known as Tyranid Hunter Slayers and after that Termagants.

Fleshborer 1987	Termagant 1992	Gaunt 2001
Tyranids 2001

GENESTEALERS

The first mention of Genestealers was again in Rogue Trader and its purpose was more like a random encounter during a scenario then a real troop type. It wasn't even associated with Tyranids yet. It was described as:

This enigmatic monster originates from one of the moons of Ymgarl, but has spread throughout space and is threatening to become a real menace. In its native state it is blue, has two pairs of clawed arms, a long muscular tail and a tough leech-like body. Its head is a simple tube full of slobbering tendrils and sharp inward-pointing teeth. What makes the Genestealer truly unique is its deadly and extremely rapid reproductive cycle. Genestealers are of a single gender, and are incapable of reproducing among themselves. To reproduce they require another creature, any creature will suffice, of any gender. 

For all the Genestealer requires is the basic genetic material which is contained in every living cell of the mate. A Genestealer implants its own genetic material in the form of an egg capsule, which is forced into the body of the host by an extendible ovipository normally secreted inside the head. The implantation does not kill the mate immediately, but the implanted egg quickly grows until ready to burst out as an immature Genestealer.

Needless to say, this kills the parent/host. Interestingly enough, newly born creatures, although fundamentally Genestealers, will have characteristics inherited from the host parent. Thus a Genestealer/human may have a vaguely humanoid head, or only two arms instead of the usual four, and perhaps its tail will be shortened or missing. A Genestealer of four or more generations consistent human parentage would pass for a human on cursory inspection, although a closer look would reveal a bluish skin, sharp pointed teeth and rather disturbing stare. On their home world, Genestealers exploit a large leech-like creature called a Csith which is the usual mate. Matings with a Csith always produce natural Genestealers, no matter what its parentage otherwise. "

Clearly very different from the Genestealers as we know them today.

The first Genestealer plastics from 1988. Still popular today.

They have only one purpose and there is nothing they will not do to accomplish this, no matter how vile or loathsome it might be. These abominations mean to destroy everything proud and noble, everything we hold dear and have fought so long to achieve.
Inquisitor Agmar on Tyranids