The Hawaiian Drosophilidae are one of the most spectacular cases of diversification in nature. The ~700 described species, and hundreds of known species that remain undescribed, are placed in the genera Hawaiian Drosophila and Scaptomyza, and are all descendants of a single colonization event. Most species are found on a single island, have become ecologically specialized, and present high morphological variation. While the Hawaiian Drosophila exhibit remarkable variation in the external morphology and complex courtship behaviors, the most variable trait of the genus Scaptomyza is the male terminalia. It comprises genital and anal segments, located at the tip of the abdomen, which are often the most rapidly evolving traits in insects with internal fertilization. In the first chapter, I reviewed the terminology used in Scaptomyza species descriptions and other publications that are broadly relevant to Drosophilidae and Diptera taxonomy. I compiled all the English synonyms, proposed a standardized terminology, and prepared a visual atlas of the external morphology and terminalia. The proposed terminology was adopted on the taxonomic revision in the second chapter. I focused on the subgenus Elmomyza, the largest within the genus Scaptomyza, with 86 of the 274 described species. I described nine new species, redescribed S. exigua, expanded the collections records for the S. cyrtandrae species group, and created two new species groups based on morphological similarity. In the third chapter, I used ancestral states reconstructions to study the patterns of differentiation of two non-intromittent components of the male terminalia, responsible for grasping the female terminalia during copulation and securing a tight genital coupling. As a result, the terminology revision connects historical publications with modern literature, and the visual atlas helps recognize each morphological character in Scaptomyza, especially in the male terminalia. This trait was used to describe new species, which increased the number of Elmomyza from 86 to 95 described species, ~30% of which are now placed into species groups. Despite high variation on male terminalia among closely related species, our findings reveal morphological constraints between genital components, highlighting the essential role of sexual selection in the process of reproductive isolation and the subsequent species divergence.