Herbie Popnecker?

Okay, so I was on pdsh and stumbled across the Herbie Popnecker page. A very very unique and fascinating character. However, I'm not exactly clear on the limitations he has...his first appearance is in the public domain. Along with several others (a few of which I've read), so...my question is, I guess. Can I make derivative works of the character based on the characters and elements that ARE in the public domain? i.e. the way the Flaming Carrot utilized him? And by the way, so the work on pdsh doesn't go unappreciated, I greatly look forward to seeing the recent additions of all these new characters. Many I hadn't even known about. Keep up the great work!
-Mattd43

Whatever characters or ideas first appeared in a public domain comic, are essentially public domain. So yes, you can use Herbie and his friends from PD issues. I haven't personally had a chance to read those stories yet, so I'll have to do that in the near future....