What are those maggots on the roots of my dead lettuce plant?

When one of my lettuce plants started wilting, I pulled on it and found I could just pull it straight out of the soil with no resistance. On the roots, I found these maggots. (You may need enlarge the picture a bit.)

Does anybody know what these are?

How likely is it that those are why the lettuce started dying?

Can they spread to other nearby potted plants?

What can you do against these?