Eww! That's why cats bug me, they do that kind of stuff. Can you set something heavy on top of the lid? Be careful..it might be your shoes next. Seriously. We had a cat once that didn't like a guest and took a pee in one of her shoes.
Add more litterboxes to your house. You should have two litterboxes per cat in the household. Move the mealworm tub to someplace kitty can't get to or isn't allowed, and replace the mealworm tub with a litterbox.
As an aside, make sure you are scooping the boxes after EVERY time they are used. Cats are clean by nature, and they will not use dirty litterboxes.
I have 3 cats and HIGHLY recommend the LitterMaid Elite auto litter box. It scoops out the waste 10 minutes after the eye senses a cat has left - places it in a plastic bin and then all you do is empty the bin every other day or so.
No one can tell we have cats - it stays so clean....
Usually around $100.00 but well worth it in my opinion - otherwise yes, I would add at least one more regular litter box