What is the primary nitrogenous waste product of arachnids?

The primary nitrogenous waste product of arachnids is guanine.

Guanine is a nitrogen-rich purine compound that arachnids commonly eliminate as a relatively dry, energy-saving waste. Conserving water is especially useful for terrestrial animals such as spiders, scorpions, mites, and harvestmen, many of which live in dry environments.

Arachnids may use coxal glands, Malpighian tubules, or both for excretion. Malpighian tubules empty into the gut, allowing waste to leave with digestive material. The exact excretory arrangement varies among arachnid groups, but guanine is the characteristic primary nitrogenous waste associated with the class.

A common mix-up is confusing guanine with urea or ammonia. Ammonia is highly toxic and requires substantial water for safe removal, while urea is more typical of many mammals and other vertebrates. Creatinine is a metabolic waste product in vertebrates, not the standard answer for arachnids.