The Zarn was a creature that greatly influenced me in my early gaming days. It came from Gamma World and in the first edition it seemed invincible. Mind you, nothing was soft in Gamma World. Subsequent editions of Gamma World have reduced the überkill nature of these beasts and increased their size for some unaccountable reason.
I’ve spent a lot of mental horsepower over the years thinking about the ecology of such a creatrue. Why didn’t they overtake the whole planet? What was the biological control that kept their numbers in balance? I wrote short fiction about them. God knows where those writings have got to now.
Here is the description from gamma World 1e:
The Zarn is a dangerous little orange parasitic beetle that, when full grown, attains a length of only 10 centimeters. It will attack any living creature, regardless of size. On one melee turn it spits intensity 16 paralytic contact poison, up to a maximum range of 5 meters. On the next melee turn, although only semi-intelligent, it teleports to a different location (maximum range 200 meters). After teleporting, it will spit poison again, continuing this routine until a victim has been paralyzed. Note: the spittle is so toxic that the saving throw for poison must be made each melee turn for each area of spittle that is not washed off the victim’s skin. Once its victim is paralyzed the Zarn bores through the skull and lays 4-24 eggs in its brain. Paralysis lasts one full week. The eggs will hatch in four days, consume the brain, and emerge on the fifth day as little Zarn. Zarn eggs may only be removed surgically.