H. haydeniana reach a length of 4–5 centimeters (1.6–2 in) when mature. The upper surface of the body is black to olive green, and is distinctively marked along the sides with patches of a yellowish colour. H. haydeniana has approximately twenty body segments, bearing a total of 30 (males) or 31 (females) pairs of legs. The difference between males and females is due to one pair of legs on the seventh segment in males being modified to form gonopods used for sperm transfer. Individuals may live for 2–3 years.
H. haydeniana occurs from southeast Alaska south to Monterey County, California, as far east as the Sierra Nevada mountains.