The Grex, known scientifically as Entomogrex impurus, or more commonly as 'Roaches', are a vicious race of extraterrestrial insects featured in the Swarm on the Somme timeline by BlackWave. Homaging famous science-fiction anthropoid beings like Tyranids or the Arachnids of Starship Troopers, the Grex, which refer to themselves as the 'Swarm', are depicted as a near-unstoppable interstellar threat, a scout force of which descends on Earth in early 1915, changing the face of World War One.

The Grex are driven by a gestalt, psychic hive consciousness that possesses a large degree of sentience, possessing a malign, sadistic personality that currently treats the conflict on Earth a a source of amusement.

Scarabs–Entomogrex impurus scarabaeus. The lowest form of Grex, Scarabs resemble large regular yellow-colored beetles. Despite not being very durable, they are nonetheless extremely vicious, and mainly serve as scouts. Their biggest threat lies in their ability to inject Grex ichor into human bodies, which then transforms their victim right down to the cellular level, allowing the Grex consciousness to reshape and control said body as it pleases, using them as infiltrators or mutating them into hybrids as it sees fit.

Warriors–Entomogrex impurus impurus. The most common type of Grex, warriors have carapaces tough enough to resist small arms fire, although their fronts are more vulnerable, and are strong enough to rend men limb from limb and peel open armor plating. They are also capable of fusing themselves with basic human weapons, firing organic projectiles.

Decabites–Entomogrex impurus macrognathus. Also nicknamed 'Jabberwocks' by British soldiers. Serving as the Grex answer to armor or fortifications, Decabites are capable of spitting out searing acid capable of burning through most surfaces.

Locusts–Entomogrex impurus volans. Grex flyers, Locusts are capable of dogfighting with aircraft and move at high speed, attacking ground forces from above or performing aerial recon.

Stingwraiths–Entomogrex impurus sicarius. Grex assassins, Stingwraiths are capable of flight, invisibility, and can psionically immobilize all people in a small radius. Nevertheless, they are not very durable, and are deployed carefully. 'Bull' Stingwraiths are termed 'Alpha Wraiths', utilizing further enhanced psionic abilities such as mind control or molecular disintegration, with only one such creature depicted thus far.

Bombardiers–Entomogrex impurus maximus. Literally serving as the heavy artillery of the Grex, Bombardiers are building-sized beetle like creatures, capable of shrugging off field gun shells and fusing themselves with artillery weapons, firing organic ordnance of varying kind.

Organic Bomb–This as yet unnamed type of Grex is apparently a Bombardier offshoot swelled up with acid, which it spreads over a wide area when it detonates itself. It is apparently incapable of defending itself and must be positioned by other Grex, limiting its use.

Organic mines–The Grex sometimes lay down organic landmines that can project spines or acid.

Wireworms–A rudimentary Grex sub-species, wireworms are long, thin tendril-like organisms lined with poisonous barbs that lurk in Grex-infested areas, resembling living barbed wire strands.

'Bull' Grex, also described as 'veterans', have been featured from time to time, and are essentially individual Grex that, as a result of their age and perseverance, are generally larger and tougher than the norm, sporting features and abilities such as tusks and projectable spines. Some of these earn reputations and names, such as a Bull Decabite at the Australian front nicknamed 'Big Edward', or an Alpha Wraith on the Western Front referred to as 'Sneaky Willy'.

Other Grex breeds have been implied to exist.

• English - Roaches, Roachy
• French - Les Cancrelats (“Roaches”)
• German - die Schaben (“Roaches”)
• Russian - Zhuki (“Bugs”)
• Japanese - Oni (“Demons”)

Swarm on the Somme