In sawfly taxonomy the structure of the lancet or the saw is an important character for identification of females. Because adult females use the saw to place eggs into the host plant its teeth may wear and their shape may thus be altered. We investigated the pattern of wear in a species of nematine sawfly that we also describe for the first time. Saw wear of females that deposited known numbers of eggs was examined. Saw wear was not strongly related to the number of eggs laid, despite some saws being considerably worn. The range of variation in saw profile illustrated by Lindqvist (1956) in the closely-related species D. virididorsata (Retzius) was not observed in our study. Despite the shape of the saw being altered to some extent with use, other structures on the saw do not change and provide good taxonomic characters in the genus Dineura, as evidenced by our detection of the cryptic (= sibling) species on the basis of differences in ctenidial structure. We describe this species under the name Dineura pullior sp. n.