The Silithids are a race of insect-like creatures inhabiting various parts of southern Kalimdor.
Before the Great Sundering, Kalimdor, the ancient continent of eternal starlight, was a nurturing mother to all of its creatures. At the continent's center was a mysterious lake of incandescent energies, the Well of Eternity. It was the true heart of the world's magic and natural power. From this magical ether the Silithid were born. As the fallen Old God C'Thun recognized their appearance, he attempted to sunder the world that it once held in its merciless grasp. The Old God created avatars from the Silithid in its own image. These avatars were eventually known as Qiraji. Sapient and with thousands of years the Qiraji worked feverishly to build a force capable of laying waste to the world that would betray their god with the Silithids as their minions.  Biology
-Their chitinous plating is interlaced in such a way that makes it incredibly strong. These creatures are very powerful and highly evolved.
-Silithid can range in size from a normal insect to gigantic. Size seems to depend on how long the Silithid are allowed to develop in their egg stage. It appears that size is also directly correlated with colony size.
-It can be inferred that caste is dependent on how the Silithid are "instructed" to develop, since larva forms have been found.
-Each of the three Hives in Silithus has a crystalline prism in it. They are apparently a means for the Qiraji and other unknown entities to communicate various messages to the silithid.  Locations
The Silithids burrow their underground hives all over southern Kalimdor. All of Silithus is under their control, and they have hives in Feralas, Tanaris, Un'Goro Crater, and the Shimmering Flats. They've even been sighted in small numbers in The Barrens.  Castes
Over the centuries, many castes developed:
* Silithid Larva * Silithid Worker * Silithid Scarab * Silithid Wasp * Silithid Tank * Silithid Colossus * Silithid Sand Worm * Sandreaver * Silithid Light * Silithid Pill Bug * Silithid Egg Layer * Riding Silithid
In the field of giants, there are other castes which resemble scorpids, surf crawlers, and grubs.
* Silithid Creeper (Resembles a teal Scorpid) * Silithid Swarmer (Resembles a teal Scorpid) * Silithid Swarm (Small Minions) * Silithid Protector (Resembles a Crab)(Can only be found during a quest, after digging into an egg mound.) * Silithid Grub
A part of the Rustmaul Digsite Silithid Hive A part of the Rustmaul Digsite Silithid Hive
* Hive'Zora - Silithus * Hive'Ashi - Silithus * Hive'Regal - Silithus * Hive'Zara - Ruins of Ahn'Qiraj
* Gorishi - Un'Goro Crater * Zukk'ash - Feralas * Centipaar - Tanaris * Hazzali - Tanaris * Rustmaul Digsite - Thousand Needles * Field of Giants Hives - The Barrens