Larvae gun

Larvae guns were the living weapons of the Zarbi, at that time controlled by the Animus, in their war against the Menoptra. They measured approximately two meters in length and half a meter high. The grubs ejected jets of venom from a pointed orifice located beneath their eyes. (DW: The Web Planet)

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. Subsequently, the larvae guns have often gotten called these by fans even though this does not occur in dialogue in the televised story.