Technical Abstract: Gall midges constitute an important group of plant-parasitic insects. The Hessian fly (HF, Mayetiola destructor), the most investigated gall midge, was the first insect hypothesized to have a gene-for-gene interaction with its host plant, wheat (Triticum spp.). Recent investigations support that hypothesis. The minute larval mandibles appear to act in a manner that is analogous to nematode stylets and the haustoria of filamentous plant pathogens. Putative effector proteins are encoded by hundreds of genes and expressed in the HF larval salivary gland. Cultivar specific resistance genes mediate a highly localized hypersensitive reaction that kills avirulent HF larvae. Fine-scale mapping of HF avirulence genes provides further evidence of effector-triggered immunity against HF in wheat. Taken together, these discoveries suggest that the HF, and other gall midges, may be considered biotrophic, or hemibiotrophic, plant pathogens. They demonstrate the capacity the wheat-HF interaction offers to the study of insect-induced gall formation.