Note: This article discusses material that was once considered canon but whose canonicity is now questionable.
The newly founded Space Marine Chapter known as the Yellow Jackets emerged as a distinct Chapter during the terror-filled era of the Horus Heresy. Cut off from the bulk of Imperial forces, the Chapter began a campaign of hit-and-run attacks launched from Thunderhawk gunships.
The sudden appearance of these warriors pinned down and paralysed vast enemy forces and fear of their retribution prevented many a Planetary Governor from going over to the service of Chaos. In the ages since, the Yellow Jackets have continued to serve the Emperor of Mankind with distinction, including their heroic stand against Warlord Graznik and his mighty Ork WAAAGH!
The Yellow Jackets primarily wear black Power Armour. The Aquila or Imperialis on the chest plate, as well as the top of the helm, shoulder plate trim and lower legs are yellow. Two distinctive black rings are painted on the lower legs, emulating the distinctive markings of their insect namesake. The inset of the right shoulder plate is painted red.
The yellow squad specialty markings -- Tactical, Devastator, Assault or Veteran -- are located on the right shoulder plate. Sergeants are designated by the skull symbol worn on their helms whilst Captains are designated by their red helms. A black Roman numeral is stenciled on the right knee guard, indicating squad number. The right knee guard is painted in the company colour in accordance with the dictates of the Codex Astartes -- i.e. White (1st Company), Yellow (2nd Company), Red (3rd Company), Green (4th Company), etc.
The Yellow Jackets' Chapter badge is a distinctive yellow wasp centred on a field of black.
The Yellow Jackets Chapter was created early in the history of Warhammer 40,000 at a time when the lore of the Horus Heresy was less defined than at present. At such, their background as printed in White Dwarf 177 claims that they were raised as a Chapter DURING the Heresy, which would have been impossible since only the 18 First Founding Space Marine Legions existed at the time, with the Chapters being created AFTER the Heresy following Roboute Guilliman's completion of the Codex Astartes.
- White Dwarf 177, "The Yellow Jackets", pg. 30 (Chapter created by 1994 Golden Daemon Chapter Design Competition Winner Jeff Durochia)
- Demon Winner - Golden Daemon Website, Golden Daemon 1994 Chapter Design Competition Winner, The Yellow Jackets by Jeff Durochia (Archived)