Cecidomyiinae, by far the largest subfamily in the family Cecidomyiidae, may owe their success as plant feeders and predators largely to larval adaptations that were already present in the most primitive cecidomyiids that fed on fungi in decaying organic matter. These modifications include miniaturization of the head and body, the change from chewing to piercing-sucking mouthparts, and the spatula, a dermal structure unique to the family on the prothoracic segment of the mature larva. Presented here are examples that illustrate these attributes of cecidomyid larvae, chiefly in their role as plant feeders. Concurrent changes in pupal morphology are also noted.