Venom is nothing more than modified saliva. While some colubrids have venoms that are just as toxic drop for drop as elapids, they not only lack the delivery system but most produce very tiny quantities of it. As for cribos, I believe Dr. Fry had said that the indigos/bird snakes had reduced, very small venom glands and have resorted to being "bully feeders" using their size and power to overcome prey. Apparently the only colubrids that lack venom are the North American clade including kingsnakes, ratsnakes, pituophis, etc. Have a look at this site and forum at www.venomdoc.com. Extremely informative.