I noticed my largest 'kreb was being reclusive and trying to escape. Then I noticed her glare glands were in full display. The glare glands are on the tail and produce the glue that holds the eggs to the swimmerlets. The display is a light colored fan pattern. Once you see it in person, you know what it is.

This is a poor picture of a P, clarkii blue with the glare glands starting to show:


I isolated my wandering female in her own 20 gallon tank, and sure enough, next day she had a tail full of eggs.