Schrellic gland

According to the Dalek Survival Guide, the schrellic gland was a part of the anatomy of the Dalek creatures.

When under control of a Minder, the Special Weapons Dalek Standard Battle type's creature often suffered from an overactive schrellic gland. It was discovered, during dissection of such a creature, that the schrellic gland became particularly active when a Dalek was in a battle situation. It excreted a noxious fluid, which was thought to attract the attention of other Daleks, potentially as a petition for reinforcements. In the case of this type of Special Weapons Dalek, the volume of schrellic secretion was such that it often leaked out of the Dalek casing around the gun area. This sticky fluid was often mistaken for some kind of lubrication oil, although it was by no means certain that the fluid did not fulfill this function as well.

In the Garazone campaign, scientists cleverly synthesised the schrellic gland's secretions, luring 50 squadrons of interplanetary Airborne Daleks into a trap. The potency of the schrellic substitute was such that the first 27 squadrons willingly flew into the Garazone sun. The Dalek Survival Guide counted this as the second greatest defeat of the Special Weapons Daleks. (PROSE: Dalek Survival Guide)