Fleas

Hello and thanks for the fascinating question!

I’ve never read anything related to fleas or their offspring showing up under UV light. Here’s an image of a cat flea under fluorescent light. They don’t appear to show much in way of “fluorescence”. Though there is a slight effect.

Interestingly, there are some other arthopods, such as scorpions, which do show up well under UV light. The floursence in insects is caused by the nitrogen content in their cuticles, where external resilin (an elastic protein) is present.