Coluber constrictor flaviventris... The Eastern Yellow-Bellied Racer
Also called the blue racer, these quick little critters are a subspecies of the coluber or racer family. If you've ever run across one out in the wild you know they have well earned their name. They are extremely fast.
The topside of eastern yellow-bellied racers is solid gray or gray with a blueish or greenish tint. Their bellies up to their mouth is bright yellow. One thing that is really interesting about these snakes is that they look extremely different when they are little. Juvenile eastern yellow-bellied racers actually have a camouflage pattern of tan or cream with brown spots. No one is exactly sure at what age they change, but sometime after they get through their teen angst they change to the gray color with yellow bellies. That was a dumb joke, they've narrowed the time frame down to between a year and half and three years after birth the snake changes colors.
The eastern yellow-bellied racer is very thin wiry snake, and gets fairly long. An adult can range from around 25 to 50 inches, though it's not uncommon for them to get up to 60 inches. They've even been found in the 70 inch range.
Eastern yellow-bellied racers can be found throughout the middle of the United States. To the west they span as far as New Mexico and up to Montana. To the east they span to Louisiana and up to Iowa. They go as far south as Texas and as far north as Canada.
Because of their speed and I would also assume their color, eastern yellow-bellied racers prefer to run around in tall grass. They like fields, and prairies, and brush.
Confusingly enough, despite having 'constrictor' in it's scientific name, the eastern yellow-bellied racer is not in fact a constrictor. Like coachwhips, the eastern yellow-bellied racer uses it speed to run down and over take it's prey. They consume rodents, amphibians, other reptiles, and insects.
Interactions with humans
While I have no evidence whatsoever to support this, I have a sneaking suspicion that the phrase, “Yellow bellied coward” comes from the eastern yellow-bellied racer. I have personally stumbled across a few racers out in the wild, and they have all responded the exact same way. They take off with blinding speed. You barely have time to notice they are there then BAM!!! They shift gears, kick in the nitrous, and disappear!
I've never handled one myself, but I have heard they do have a tendency to bite when handled. They are over course nonvenomous so there is little danger from the bite, but the are apparently strikers when scared. I've also come to understand that they do not do well in captivity and very few are ever kept.