Larvae guns were the living weapons of the Zarbi. They were also mentioned as being Zarbi larvae.
The larvae ejected jets of venom from a pointed orifice located beneath their eyes. A plated shell covered their backs, protecting that area from attack. They were helpless if overturned.
As the larvae guns served the Zarbi, they were in turn subservient to the Animus. They served in the war against the Menoptera. When the Animus was defeated, the Zarbi larvae set about replenishing the planet. (TV: The Web Planet)
When the Zarbi attempted to kill a group of Menoptera, the Doctor's landing ended up saving them from said fate. (COMIC: On the Planet Vortis)
Behind the scenes
Doctor Who and the Zarbi, the Bill Strutton novelisation of his script for The Web Planet, refers to the creatures as Venom Grubs. They do not receive this name in the televised story.
In their commentary for The Web Planet, Verity Lambert and William Russell agreed that the "venom grubs" were rather cute. (DCOM: The Web Planet)