Thanks for the responses! (More are still welcome, too.)
It's a difficult decision because I have never found a bug like that in anything before and I don't know the food safety rules about bugs and dried legumes (I think if you find weevils in any dried goods you are supposed to throw them out, but that's all I know) -- and because I got so grossed out. I find bugs (mites, flies, aphids, assorted little beetles, the occasional spider, a caterpillar) in vegetables and fruit quite often, and I just wash them off (or, for native bugs that are still alive and harmless I might put them outside) and it's fine; But those are usually whole, non-decayed bugs, and smaller, and more familiar to me so in my mind they belong where I find them ...this one in the lentils was completely unfamiliar, about 1/2 inch long, missing all it's appendages, and looked squashed and partially decayed (best guess is that it may have been some kind of beetle).
Returning them to the store is a good idea if dead bugs in lentils is a food safety problem, but it wouldn't be worth it for me (going to the store is a fairly big deal because of sensory difficulties + the time and effort of planning the trip and getting there...plus the bus fare to get there would cost almost as much as new lentils.)
I also think the Sainsbury's thing is funny (the comment is funny, not the dead frog).
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