This is a kleptoparasitic spider called a dewdrop spider,
Argyrodes bonadea. Kleptoparasitic spiders are tiny little things that hide in the orb webs of the big spider webs, such as Argiope or Araneus. If you see some of these big webs look carefully for the little spiders on the margins, which stand out because of their silvery color. Apparently they're overlooked by the big spider, whose web they inhabit and steal their prey, thus the term, kleptoparasitic.
