At dusk across highways in south-eastern Colorado, thousands of Oklahoma brown tarantulas are beginning their annual trek searching for mates. The hairy eight-legged beasts, about 3 inches long, migrate in large numbers near La Junta, Colo.

The males are about 10 years old and have only one chance to mate with females who wait in burrows in the Colorado grasslands, said Whitney Cranshaw, entomology professor at Colorado State University.

"They'll be dead by Christmas," he told USAToday.

Colorado's tarantula migrations begin in late August and last through September, with many crossing the Comanche National Grassland, especially along Highway 109.

Tarantulas are sometimes sold in pet stores as pets, but catching one is against wildlife regulations, wildlife officials say.

Although the spiders are usually harmless, tarantula bites contain a mild venom that can cause an allergic reaction similar to a bee sting.