I think that was definitely the hardest, or perhaps the cruelest level. You really have to have a long path to solve it. As I've mentioned in previous comments, its difficult to gauge the difficulty of puzzles when you know all the solutions. The ghost image idea is really clever. It could work great as an optional toggle for people who are stuck. My only concern is if it would go from being too hard to being too easy.
The spiders movement is just too difficult to read, and when the puzzles get bigger it becomes too much to keep in your head. I've thought about some other things that could help, like keeping the line in between runs and having it be editable, or displaying a counter that shows how many squares you've covered as you move.
Regardless, thank you so much for playing the game and the kind words. And thank you for the excellent suggestion!