Colorado is home to several hundred spider species, but only a handful show up regularly in homes and gardens. Most are harmless, and only one, the western black widow, poses a real medical concern. Here’s what you’re likely to encounter across the state and what to know about each.
Funnel Web Spiders: Colorado’s Most Common Indoor Spider
If you’ve spotted a spider in your Colorado home, it’s most likely a funnel web spider from the family Agelenidae. These are dark brown with black markings on the abdomen and distinctly striped legs. They build dense, flat mats of silk in corners of buildings, shrubs, and thick grass, with a funnel-shaped retreat where the spider waits for prey to stumble across the web.
Funnel web spiders look intimidating, especially when they dart across their webs at high speed, but they’re not dangerous to people. They’re often confused with the far more dangerous Australian funnel-web spider, which is an entirely different family and does not exist in North America.
Yellow Sac Spiders: The Nighttime Wanderers
Yellow sac spiders are pale, almost translucent hunters that don’t build traditional webs. Instead, they construct small silken tubes or sacs in upper corners of rooms, under window sills, or in wall cracks, where they hide during the day. On light-colored walls and ceilings, these retreats can be nearly invisible. At night, the spiders leave their sacs to hunt, and that’s when they run into people.
These spiders likely account for more human bites than any other type of spider. Bites typically happen when a spider gets trapped between your skin and bedsheets, clothing, or shoes. Despite their small size, yellow sac spiders will readily bite, sometimes without any provocation at all. The initial bite is usually painful, with a burning sensation lasting up to an hour. Redness, swelling, and blistering can develop over the next one to ten hours. Some people experience fever, muscle cramps, and nausea. The symptoms resemble a mild version of a black widow bite but are far less severe, and most bites heal without lasting problems or scarring.
Yellow sac spiders become especially common indoors during fall, when they move inside as temperatures drop. One species in particular breeds almost exclusively indoors, making it a year-round resident in many Colorado homes.
Orb Weavers: The Garden Architects
Two orb-weaving species are especially visible in Colorado yards. The cat-faced spider is brownish with a prominent, dimpled abdomen sporting a pair of distinctive humps that give it its name. These spiders build geometric webs among vegetation, on buildings, and near outdoor lighting, where the light attracts insects straight into their webs.
Banded garden spiders show up in late summer and early fall. They’re large and eye-catching, with silvery bodies marked by dark and yellow stripes. You’ll find them in gardens and shrubbery, often sitting in the center of an impressively large web. Both orb weavers are completely harmless and do excellent work controlling mosquitoes, flies, and other pests.
Western Black Widows: Colorado’s Only Real Danger
The western black widow is the only spider in Colorado with venom that can cause serious harm. Females are the ones to watch for: glossy black with the iconic red hourglass marking on the underside of the abdomen. Males are smaller, with fangs that rarely penetrate human skin.
Black widows build messy, sticky webs close to the ground in dark, undisturbed areas. Think underneath furniture, in garages, crawl spaces, woodpiles, low shrubs, and inside pipes or drains. They can overwinter indoors as adults or juveniles, though you’ll rarely encounter them outside during winter months. Bites are most likely near nests that contain egg sacs, where females are most defensive.
A bite feels like a sharp pinprick, sometimes followed by dull numbness. Some bites go unnoticed entirely at first. The venom contains a neurotoxin that attacks nerve endings in muscles, producing symptoms that can include severe muscle pain and cramping throughout the body, chest tightness, abdominal pain, nausea, sweating, a burning sensation on the soles of the feet, swelling of the eyelids, and difficulty breathing. Symptoms generally worsen during the first 24 hours. In healthy adults, they typically decline after two to three days but can linger for up to a month.
Bites are infrequent. Fatal outcomes are extremely rare and limited almost entirely to young children and older adults. If you suspect a black widow bite, wash the area with soap and water, apply ice wrapped in a cloth to slow the venom’s spread, elevate the area if possible, and get medical attention promptly. There’s no test that can confirm what species bit you, so doctors diagnose based on symptoms.
Brown Recluse Spiders: Extremely Rare in Colorado
The brown recluse is one of the most feared spiders in the United States, and many Coloradans worry about encountering one. The reality is that brown recluse spiders are extremely rare in Colorado. The Denver Museum of Nature and Science collection contains only a single confirmed specimen from the entire state, collected in Boulder County in 1996.
The occasional brown recluse that turns up in Colorado almost always hitched a ride in boxes or furniture shipped from heavily infested states in the south-central U.S. There are only two somewhat credible reports of recluse spiders establishing a breeding population in Colorado, one in Pueblo County and one in Prowers County, both isolated to a single building each. Colorado State University Extension is clear on this point: sustained breeding populations in Colorado are exceedingly rare, if they occur at all. If someone tells you they were bitten by a brown recluse in Colorado, the odds strongly favor a different spider or a different condition altogether.
The Tarantula Migration
Every fall, southeastern Colorado hosts one of the state’s most unusual wildlife events. From mid-September to mid-October, male Colorado brown tarantulas leave the safety of their underground burrows and walk across open ground in search of females. The area around La Junta is a particularly popular spot to witness the migration, which draws curious visitors each year.
The males are on a one-way trip. They approach a female’s burrow and send vibrations by tapping on the edge of her den, signaling that they’re potential mates rather than prey. As the female emerges, the male uses hook-like structures on his front legs to restrain her from attacking. During a brief wrestling match, he deposits a sperm package on her underside. The journey is dangerous: males face rejection, predators, and exhaustion. Most don’t survive the season. Despite their large size and hairy appearance, Colorado brown tarantulas are docile toward humans. Their venom is mild, roughly comparable to a bee sting.
Why Spiders Move Indoors in Fall
If you notice more spiders in your Colorado home between September and November, it’s not your imagination. As fall temperatures drop, spiders near the end of their annual life cycles are actively searching for mates and sheltered spots to lay egg sacs. Your home offers warmth, protection, and a steady supply of smaller insects.
Keeping spiders out starts with sealing the gaps they use to enter. Check weather stripping around doors and windows, seal cracks in your foundation, and repair torn window screens. Reduce outdoor lighting near entrances, since lights attract the insects spiders feed on. Inside, clear clutter in basements, garages, and closets where spiders like to set up undisturbed. Regular vacuuming in corners and along baseboards removes both webs and egg sacs before they hatch. Sticky traps placed along walls can help monitor and reduce indoor populations without chemicals.
For black widows specifically, wear gloves when reaching into woodpiles, storage boxes, or dark corners of sheds and garages. Shake out shoes and clothing that have been sitting in undisturbed areas. These simple habits dramatically reduce the already low chance of a bite.