They aren't. They don't. DE kills exoskeletal insects via dehydration, not appreciably because of any abrasive effect. The dehydration occurs by the DE stripping away the molecules-thin oil/wax coating that is part of the exoskeletal biology, allowing their inner fluids and moisture to escape...
Sighted, there is no science that supports that opinion and much that directly contradicts it. I wrote the Chicken Chick a few months back when she had a previous version of her DE page up. She was wrong then and she is still wrong now that she has changed that page, presumably as a result of...