Sections of stem galls induced by Diplolepis triforma Shorthouse and Ritchie obtained by plant histological techniques were used to examine anatomical features of immature stages of the insect found in situ. First-instar larvae feed over the entire surface of larval chambers, whereas maturing larvae feed only at one end of the chambers. Last-instar larvae undergo two phases of development: feeding occurs in the first phase, whereas in the second, or prepupal phase, feeding has ceased, the mid- and hind-guts become joined, contents of the gut are voided, and compound eyes, gonads, and internal ovipositional apparatus begin to differentiate. The prepupal phase also consists of two phases: the first occurs in the fall and winter, and the second occurs in the spring prior to pupation. The spring phase exhibits external adult features within the integument of the last-instar and moults to the pupa. Sections of some maturing larvae contained endoparasitoid larvae of the genus Orthopelma Taschenberg (Hymenoptera: Ichneumonidae). none