The annual tarantula movement in Colorado occurs each fall as thousands of spiders emerge onto the prairie landscape. This event centers around the Oklahoma Brown Tarantula (Aphonopelma hentzi), the primary species observed. While often called a migration, the spectacle is a seasonal, mass movement undertaken almost exclusively by mature males. This emergence is a biological drive for the species’ continuation, making the spiders highly visible for a short period.
The Annual Timing and Duration
The timing of this movement is highly predictable, occurring annually in the late summer and early fall across the southeastern plains of Colorado. Activity generally begins in late August and continues through early October. The onset is triggered by the natural cooling of temperatures as summer ends, signaling to the male spiders that the breeding window has opened.
The peak viewing period typically occurs during the middle two weeks of September, when the largest number of mature males are actively searching. Daily activity is concentrated in the late afternoon and early evening. The best time to observe the tarantulas is generally the hour before the sun sets, as they become most active at dusk.
The Biological Reason for the Movement
The movement is not a true migration but rather a mass search for mates. Mature, nomadic male tarantulas embark on this journey, seeking out the sedentary females who remain hidden in their underground burrows. The females line their burrows with silk, which may contain pheromones that the males detect using sensitive hairs on their legs.
This difference in behavior relates directly to the species’ life cycle, where the two sexes have vastly different lifespans. Female Aphonopelma hentzi are long-lived, often staying close to their established burrows for their entire lives, which can span up to 40 years. Males spend seven to ten years reaching sexual maturity, and once they molt into their final, adult form, their lifespan is very short.
The males rarely survive for more than a few months after reaching maturity, meaning the mating trek is a one-time event for each individual. After a male successfully mates, the female may lay several hundred eggs, securing them in a silken sac inside her burrow. The male’s journey is a high-risk endeavor, driven by the biological imperative of reproduction before his short life ends.
Geographic Hotspots in Colorado
The tarantula movement is concentrated in the dry, short-grass prairie lands of southeastern Colorado. This habitat provides the undisturbed ground and soil conditions that females need to maintain their deep burrows. Areas near towns like La Junta, Pueblo, and Trinidad are the most well-known viewing locations.
The Comanche National Grassland is a vast, protected habitat for the spiders. Many of the best sightings occur along the county roads that cut through these grasslands, such as Colorado Highway 109 and U.S. Highway 350 south of La Junta. These roads provide flat, open surfaces where the traveling spiders are easily visible against the dusty ground.
For those hoping to observe the tarantulas, safety is a primary concern, especially regarding vehicle traffic. Observers should drive slowly and pull completely off the road onto a safe shoulder when stopping. Since the spiders are most active around dusk, caution must be exercised to avoid accidentally running over the slow-moving arachnids as they cross the pavement.