Structure of the ptiliid female spermatheca (sperm storage organ) is of such unique design that nothing compares to it within other groups of Coleoptera. It has proven to be the most critical taxonomic character when determining species of featherwing beetles. This is especially helpful, as most species of ptiliids within a genus generally appear identical when making identifications based on external morphology. However, spermatheca design is so varied between genera, that it has proven to be a poor character when determining higher groups of taxa.
The spermatheca is generally composed of a sac-like structure of variable form, possessing a spermathecal pump, a structure unique to Ptiliidae. In properly prepared specimens, the spermathecal pump muscles are visible laterally to the pump apparatus.