When it heats up in the southern Utah deserts, you can stumble on a scorpion or two. Pretty easy to anticipate, as they generally make little lizard-like tracks in the soft dirt and sand.
They range in size from pretty tiny to as long 3 inches or so, maybe longer. A sting can vary from wasp-like pain to severe swelling and delirium. They hunt at night and then like to crawl into warm nooks and crannies as it gets light in the morning, so you'll find them in a shoe, or under your bivy, etc, as you're packing up. I don't worry too much about them at night and often sleep outside of the bivy or tent. I figure, they're active hunting, and staying away from me. I've hiked down there a lot without ever being stung, but I have also seen a lot of scorpions, and had a handful of hiking partners get stung.
Years ago, a friend of mine got stung in his crotch. He'd laid his pants out on a rock overnight, and the scorpion had settled inside, and stung him when he pulled the pants on in the morning. At first it was funny--he was hopping around screaming for help pulling off his pants. But we were at the second set of falls in Coyote Gulch near Stevens Arch. Getting him out proved to be a grueling experience.
And don't forget rattlesnakes--easy to avoid, but orders of magnitude worse than a scorpion.