Tarantulas, large and hairy spiders of the family Theraphosidae, often spend the vast majority of their lives hidden in underground burrows. The sudden, visible appearance of many of these arachnids across certain landscapes each year leads many people to believe they are witnessing a true migration. While the movement is significant and happens annually, the term “migration” is a biological misnomer for this phenomenon. Tarantulas do exhibit extensive annual movement, but the behavior is highly specific in its purpose and the demographic involved.
Defining Tarantula Movement
True migration involves a round trip, where animals move seasonally from one habitat to another and eventually return, often to follow food sources or avoid harsh weather. The visible, mass movement of tarantulas does not fit this definition because the spiders do not make a return journey. This annual event is more accurately described by biologists as a “dispersal,” “wandering event,” or “walkabout.” The movement is directional only in the sense that the spider is seeking a mate, not a specific distant geographical location, moving in various directions across their home habitat.
The Annual Male Wandering Event
The large-scale movement that captures public attention is undertaken almost exclusively by sexually mature male tarantulas. After spending many years, often between five and ten, living a solitary life within their burrows, the males undergo a final molt signaling their reproductive readiness. Once this final molt is complete, the male leaves his burrow permanently to search for a female. This reproductive drive is the singular purpose of his wandering, overriding the typical secretive, sedentary lifestyle and making the males highly visible.
The quest to find a mate is a perilous, one-way journey for the male, who typically does not eat or drink during this time. The male is driven by pheromones released by females in their burrows, which he follows to locate a potential partner. To avoid being mistaken for prey, the male will tap and vibrate the silk near the female’s burrow entrance, signaling his identity. He uses specialized hook-like structures on his front legs to secure the female’s fangs during mating before transferring his sperm package. Even if he escapes being consumed by the female, the mature male’s remaining lifespan is short, often only a few months after leaving his burrow.
Timing and Regional Visibility
The annual wandering event is a seasonal phenomenon linked to the reproductive cycle and local environmental conditions. In the Southwestern United States, where the event is most visible, movement typically begins in the late summer and continues through the early fall, generally from late August through October. The specific timing is triggered by cooler evening temperatures and is often preceded by seasonal moisture, creating favorable conditions for surface travel.
This phenomenon is most frequently reported and observed in arid and semi-arid regions of the Southwestern US, including states like California, Arizona, Texas, and southeastern Colorado. The visibility of the wandering is directly related to the local population density of tarantulas, such as the Aphonopelma species. The sheer number of males emerging simultaneously can create the illusion of a mass movement, with spiders often observed crossing roads and trails during dusk and nighttime hours.
Sedentary Habits of Females and Juveniles
The high visibility of the wandering males stands in stark contrast to the sedentary lifestyle of the females and juveniles, who remain largely unseen. Female tarantulas are long-lived, with some species surviving for twenty years or more, and they exhibit strong fidelity to their burrows. A female spends her entire adult life within or very close to her silk-lined burrow, emerging only briefly to ambush prey or interact with a searching male.
The females wait for the mature males to find them. They continue to molt periodically even after reaching sexual maturity, allowing them to grow throughout their long lives. Juvenile tarantulas also live in small burrows and do not participate in the visible wandering event, though young spiderlings disperse from the maternal burrow after hatching.