Colorado hosts several hundred different spider species. While encountering a spider can cause alarm, the vast majority are harmless to humans and play a beneficial role by controlling insect populations. Understanding the differences between common house spiders and the few that pose a medical risk promotes informed coexistence. This knowledge is useful because many harmless species are often mistaken for their more concerning counterparts.
Medically Significant Species
Only a small fraction of Colorado’s spiders have venom harmful to humans. The most commonly encountered species requiring caution is the Western Black Widow, Latrodectus hesperus. Mature females are shiny black with a bulbous abdomen, measuring about a quarter to a third of an inch long. Their most recognizable feature is the red or reddish-orange hourglass marking on the underside of the abdomen.
Black Widows build irregular, messy webs in dark, undisturbed areas close to the ground, such as woodpiles, sheds, and crawl spaces. The venom is a neurotoxin that can cause severe muscle pain, cramping, and nausea. While fatalities are rare, bites are serious, especially for young children and the elderly.
A less common concern is the Brown Recluse spider, Loxosceles reclusa. It is not native to Colorado but is sometimes accidentally transported into the state via shipments or moving boxes. The Hobo Spider, Eratigena agrestis, is a light brown funnel-web weaver often found in basements. Its medical status is controversial, as the Centers for Disease Control and Prevention removed it from the list of venomous spiders in 2015. The severe necrotic effects previously attributed to its venom are not widely supported by current science.
The Most Common Harmless Spiders
The majority of spiders encountered are harmless hunters or web-builders that control pest insects.
Wolf Spiders
Wolf Spiders (Lycosidae family) are among the largest spiders in the state, with thick, hairy bodies that are often brown or gray, often leading them to be mistaken for dangerous species. They are active hunters that stalk prey on the ground rather than building webs. The Carolina Wolf Spider (Hogna carolinensis) is the largest species found here. Females carry their egg sacs attached to their spinnerets and later carry their young on their backs.
Jumping Spiders
Jumping Spiders (Salticidae family) are small, fuzzy, and often brightly colored, featuring black, gray, or metallic blue and green markings. They are identified by their large, forward-facing eyes, which provide excellent vision for hunting during the day. These spiders stalk and pounce on prey, possessing the ability to jump several times their body length. The Bold Jumper (Phidippus audax) is a common species found on walls and plants.
Cellar Spiders
Cellar Spiders (Pholcidae family), often called “daddy-long-legs spiders,” have extremely long, thin legs and small, oval bodies. They construct irregular, tangled webs, usually in the dark, damp corners of basements and crawl spaces. When disturbed, they exhibit a distinctive bouncing behavior within their web. Their venom is not considered a threat to humans.
Orb Weavers
Orb Weavers (Araneidae family) are responsible for the large, geometric, wheel-shaped webs seen in gardens and between structures in the late summer and fall. Common species include the Banded Garden Spider (Argiope trifasciata) and the Cat-Faced Spider (Araneus gemmoides). Orb weavers are beneficial predators of flying insects. While a large individual may pinch if handled, they are not venomous to people.
Understanding Spider Diversity and Habitat
Colorado’s varied landscape, including plains, foothills, and mountains, supports a diverse spider fauna of over 400 species. Spiders are classified into ecological guilds based on hunting strategies: hunters and web-builders.
Hunting spiders, such as Wolf Spiders (Lycosidae) and Ground Spiders (Gnaphosidae), dominate many natural ecosystems. These hunters rely on speed and keen senses to capture prey without using silk. Web-builders construct intricate silk structures tailored to their environment, ranging from messy cobwebs to symmetrical orbs. The distribution of these types is tied to local microclimates and available shelter, with many species moving indoors as temperatures cool in the fall.
Coexisting and Safe Removal
Spiders enter homes primarily in search of shelter, warmth, and prey. Minimizing these attractants is the most effective form of prevention. Sealing cracks and gaps around doors, windows, and utility entry points with caulk or weatherstripping prevents easy access.
Reducing clutter in storage areas like basements, attics, and garages removes the dark hiding places spiders prefer for building webs. Managing exterior lighting also reduces the food source that attracts spiders, as bright outdoor lights draw in insects. Switching to yellow or sodium vapor bulbs, which are less attractive to insects, limits the number of prey near your home. For safe removal, a simple cup and piece of paper can be used to catch and release harmless spiders outdoors. Where medically significant spiders like the Black Widow are a concern, professional pest control intervention is the most prudent course of action.