I really dig the overall concept, it's something I haven't seen before. A few ideas I had to make people play it longer: More backgrounds and a new song for each stage (NG has plenty of music). COMBOS - the game realy needs something like this - the more bugs in 1 web, the higher the bonus score. Power ups: such as a bigger web, multiple webs over a period of time or immortal spider, you know, the usual. The bug models could deal with looking more distinct and, well, better.
Okay, these are my thoughts, I hope you'll keep developing the game because I really like the idea.