Isa textula - Hodges#4681 (Crowned Slug Moth).
Size: approx. 10 mm. Photo: 10/13/11, 8 pm.

Possibly a crowned slug (Isa textula). The larvae are often found on oak and create zigzagging tracks on the underside of leaves. The caterpillar is flattened and about 5/8 inch long. It is a "stinger" with specialized nettling spines. These structures are hollow and contain toxins. The small photos show a different specimen that I found on 08/19/14 around 11 pm. 

bug guide (this photo):
Auburn U: