Carboniferous insect nymphs show development of ovipositor.

[Halichoeres]

A 'nymph' is an arthropod larva that is relatively similar to the adult; in insects, the term is used for immature stages that won't undergo complete metamorphosis (i.e., won't form a pupa). This paper describes several nymphs from the Carboniferous of Germany, belonging to the extinct order Paleodictyoptera. They seem to belong to the same species in different instars (larval stages), showing how the wings develop, and how the ovipositor develops at the caudal end of the abdomen. Really cool fossils.

Here are some CT scans compared with some photos:


More images in the paper (open access): http://www.geology.cz/bulletin/fulltext/1717_Kiesmuller_190331.pdf