Chitin is a long-chain polymer of N-acetylglucosamine, a derivative of glucose. This polysaccharide is a primary component of cell walls in fungi, the exoskeletons of arthropods, such as crustaceans and insects, the radulae of molluscs, cephalopod beaks, and the scales of fish and lissamphibians. Why are we describing the first sentence of Chitin in a wikipedia article? Because in the grimdarkness of the 41st Millennium, this simple chain of modified glucose is the most common form of armour of the Tyranids. Not Flak Armour, not Studded Armour, but the humble Chitin is the most numerous of the lot. To be fair, the Nids aren't the only ones with Chitinous armour, just about any invertebrate Xenos have Chitin like the Q'Orl and Vespids, but the Nids are the most infamous users.
As it is a natural form of armour, Chitin is part of any Tyranid organism and each species and superorganism are born with this basic, dense, hardened shell. These are tough, beetle-like carapaces which cover the creatures' vital spots. Chitin is tough enough to turn aside blades and even glancing small-arms fire, although direct hits from even Stubbers could penetrate it.
Larger and more powerful Tyranids are often bred with Reinforced Chitin, a series of carapace plates that exude a sticky resin-like substance which quickly hardens and create additional self-healing layers of ablative armour that offer the Tyranid greater protection against incoming attacks. Akin to natural chainmail or plated armour.
Despite being one of the weakest forms of armour, they are incredibly cheap, easy to grow, does not require that much biomass and come in large numbers, which is what the Nids need anyway.
|Armour of Warhammer 40,000|
|Imperials:||Flak Armour - Carapace Armour - Power Armour (Human Power Armour - Dragon Scale Armour |
Exo-Power Armour - Space Marine Power Armour - Artificer Armour - Aegis Armour - Auramite Armour)
Terminator Armour - Centurion Armour
|Chaos:||Flak Armour - Carapace Armour - Power Armour (Chaos Power Armour - Warp-Forged Armour) |
Chaos Terminator Armour - Fleshmetal Armour
|Asuryani:||Mesh Armour - Aspect Armour - Exarch Armour - Phoenix Armour - Rune Armour|
|Drukhari:||Kabalite Armour - Wychsuit - Incubus Warsuit - Ghostplate Armour|
|Harlequins:||Holo-Suit - Death's Legacy|
|Orks:||Studded Armour - Flak Armour - 'Eavy Armour - Mega Armour|
|Tau:||Kroot Armour - Recon Armour - Combat Armour - Battlesuit - Iridium Armour|
|Necrons:||Necrodermis - Sempiternal Weave|
|Tyranids:||Chitin - Carapace - Armoured Exoskeleton - Armoured Shell|