Until just a few weeks ago, I had no idea that leps could be sexed as early as late larval instars. Here I basically modified a black and white figure from Miller & Machotka (1980) to demonstrate how to sex fully grown fifth instar Polyphemus larvae, based on the ventral aspect of the ninth abdominal segment. Males have a single distinct dark pit, a cuticular modification associated with the male genital histoblast, on the venter of S9. In female caterpillars the area is unmarked. The difference is less clear on prepupal caterpillars, which have expelled the dark gut contents that would otherwise enhance the contrast of the male histoblast relative to the surrounding area. Other saturniids as well as some noctuids can apparently be sexed based on markings in the same general anatomical area. Both caterpillars in this picture were raised on multiple Quercus spp.

Miller & Machotka (1980) give more information on the histological basis of the trait for Antheraea polyphemus and also describe techniques for sexing Hyalophora gloveri gloveri. Miller et al. (1977) describe techniques for sexing Hyalophora cecropia, Callosamia promethea, and Eupackardia calleta as well as A. polyphemus.

Let me know if there're any issues with the picture.