There's a wire that goes there, it works like a retaining ring...most of the little plastic ridges in the hub cap (Plastic thing that caps your hub....) get bent/knocked off, so they don't have a good hold on the retainer....hence its loss.

Look at the inside of the hub cap, see if there's anything left to hold the retainer...you can bend a piece of spring wire to replace it if there's enough mounting ridge left.

If you just want to buy the spring wire, I doubt they are available w/o a cap...so, you'll most likely just replace them.



PS - I was looking at the blow up of your hub above...and based upon the available magnification, it appears that 3 of your molecules are out of place. Might want to have that looked at.