Bark beetles (Coleoptera: Curculionidae: Scolytinae) are economically and ecologically the most important group of forest insects. They use several monoterpenes produced by their host plants to locate and colonize trees. Some of these compounds also are metabolized to produce sex, aggregation, or antiaggregation pheromones. Experimental studies have shown that certain terpenes are highly toxic to the insects, negatively affecting their development, reproductive success, and colonization of host trees. Nevertheless, the effects of these compounds on specific organs or anatomical systems are unknown. Based on relationship between bark beetles and monoterpenes of their host trees and the midgut morphological characterization performed previously, we studied the effects of α-pinene and myrcene on midgut cells of the red turpentine beetle, Dendroctonus Valens LeConte. Our results show that both α-pinene and myrcene induce an increase in the numbers of lysosomes and mitochondria. Outer and inner mitochondrial membranes were very conspicuous but were not disrupted. Both smooth and rough endoplasmic reticula were abundant and were found throughout the cytoplasm. Two kinds of secretory vesicles were observed: one associated with digestive material and the other associated with “flocculent” material. The Golgi complex was prominent but had no defined arrangement. Observed ultrastructural changes are indicative of intense cell activity and are associated with processes of digestion, synthesis, and excretion as well as of monoterpenes transformation but are not indicative of irreversible cellular damage or death.