Your well is contaminated by surface water. There is a leak somewhere near the top of the casing or at the top. You need to get the water tested for bacteria ASAP. If surface water is getting in, it could possibly be bringing in bacteria. I work for a water company, and we test all the wells we use to make sure they aren't getting surface water in there. The presence of bug parts is one of the criteria on the list.
Are you on a community well, or is it your own personal well? If its a community well (that's what I'm on) you could contact your local water authority about this. They're supposed to provide you with yearly test results on the well water quality.