The interactions of the Hessian fly with its host plants are characterized by plant defense via R genes and insect adaptation via avr genes. Because of the simple genetics underlying important aspects of this insect-plant interaction, a unique opportunity exists for integrating plant and insect molecular genetics with coevolutionary ecology. Yet, in spite of the relatively well-studied genetic aspects of this interaction, many fundamental questions remain. For example, little is known about how the virulent Hessian fly larva attacks wheat cells and establishes a feeding site. Within the first day of larval attack on leaves of a susceptible wheat seedling, cell walls and cytoplasm are disrupted. Three to eight days after the onset of larval attack, significant ultrastructural damage is apparent at the bases of several leaves. Understanding how virulent Hessian flies establish a parasitic relationship with the host plant will provide a strong foundation for understanding how R-gene-defended plants prevent parasitism.