This study describes the larva and pupa of the monotypic darkling beetle Idiobates castaneus (Knoch) (Coleoptera: Tenebrionidae) in the tribe Tenebrionini, from eastern North America. The species inhabits forested regions where adults and associated larvae have been discovered under dry loose bark of dead standing trees where cavities or pockets of frass have formed, apparently following infestations of other insects and rodents. Identity is confirmed by rearing of adults and pupae and preservation of associated exuvia. Distinctive characters of the larval abdominal apex and the armature of pupal ‘gin-traps’ are illustrated. A key to the larvae of North American genera currently placed in Tenebrionini is included.