Leaf lettuce – head lettuce – lots of small insects inside – what to do?

I bought some nice fresh lettuce yesterday. I didn't quite finish it, though, so I put it in the vegetable drawer of the fridge yesterday for today. I put a paper towel underneath it and put it in a plastic bag that I left slightly open. Yesterday I checked all the leaves for insects, etc., and there was nothing 👍🏻 Today, however, as I moved closer to the middle of the lettuce, I noticed there were a lot of insects on it (about 2-4 per leaf). However, they were no longer alive.

Then I threw the salad away.

And I checked my fridge to see if there were any little creatures running around – nothing.

They were small white ones.

Is this normal, and if so, what should I do if something like this happens again?