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At first I assumed it was Rule of Cool, because its fething cool looking. But, looking at the art on the Building manual, it appears that smoke or some other gas is coming out of them.

Respiratory/waste removal? Both combined wouldn't surprise me.

Bugs have those holes on the thorax for respiration. They don't really breathe through their mouths like most animals.

Hive Fleet Cerberus wrote:At first I assumed it was Rule of Cool, because its fething cool looking. But, looking at the art on the Building manual, it appears that smoke or some other gas is coming out of them.

Those are miniature spore chimneys that most large Tyranids possess that emit spores for the furtherance of creating a Pro-Tyranid environment. While a Tyranid lifeform has been evolved as a hyper-efficient super predator, they also are give extra biological systems to further the goals of the Tyranid swarm in general.

Jape wrote:Respiratory/waste removal? Both combined wouldn't surprise me.

If I recall properly, Tyranids don't have waste removal systems, due to the way they were evolved into simple, high efficiency creatures. Most simply aren't alive long enough to actually use the restroom. They are born, then they kill and feed as much as they are able, and then when they are killed or their bodies shut down and die, the corpses are simply eaten and all biomass is digested into a reclamation pool to be reused.

They have two roles.

First, they vent excess heat as larger Tyranids get too high body temperatures otherwise.

Second, they spread spores into the air.

 Hive Fleet Cerberus wrote:
At first I assumed it was Rule of Cool, because its fething cool looking. But, looking at the art on the Building manual, it appears that smoke or some other gas is coming out of them.

Probably ventilation. Tyranids typically have ridiculous metabolisms and burn fuel more like a car than an animal for the simple amount of energy they need, the chimneys likely vent all of this heat into the air to keep the Hive Tyrant cool. Which is fairly sensible considering that Tyranids must support endoskeletons and exoskeletons, which is no small feat and requires exceptional strength and endurance. Which thus also requires energy, and lots of it.