Yes, there are spiders in Colorado whose venom may require medical attention, but the number of species considered a threat to human health is extremely limited. The primary concern comes from species that possess venom potent enough to cause systemic effects beyond localized pain at the bite site. In Colorado, this designation of “medically significant” applies almost exclusively to one established group of spiders. The vast majority of spiders encountered in the state are harmless to people, despite common fears and frequent misidentification. Understanding the differences between these spider groups is the most effective way to assess actual risk.
Medically Significant Spiders in Colorado
The only spider species established across Colorado that is considered medically significant is the Western Black Widow (Latrodectus hesperus). The adult female is easily identifiable by her shiny, jet-black, bulbous abdomen, typically measuring between one-quarter and one-third of an inch in length. The most recognizable feature is the bright red or orange hourglass marking located on the underside of her abdomen, though this mark can sometimes be faded or appear as two separate spots. The female’s venom is a neurotoxin that affects the nervous system, leading to systemic symptoms associated with a bite.
These spiders construct sticky, irregular, mesh-like webs, often referred to as cobwebs, in dark and undisturbed areas. Common outdoor habitats include woodpiles, under rocks, and abandoned rodent burrows. Indoors, they favor secluded spots like garages, sheds, crawl spaces, and basements. The male Western Black Widow is significantly smaller, usually light brown or gray with banded legs, and is not considered dangerous because his fangs are too small to penetrate human skin.
Distinguishing Between Threats and Harmless Spiders
Public anxiety often focuses on the Brown Recluse (Loxosceles reclusa), but extensive evidence shows this species is not native or established in Colorado. The state falls outside the recluse’s natural range, which is concentrated in the midwestern and south-central United States. While individual spiders are occasionally transported into Colorado via moving boxes or shipments, they rarely survive or establish breeding populations. Many common, harmless spiders are frequently misidentified as the Brown Recluse, leading to unnecessary fear and inaccurate self-diagnosis.
The Loxosceles spiders are known for the violin-shaped marking on their cephalothorax, but a more reliable distinguishing feature is their eye pattern. Unlike most Colorado spiders, which have eight eyes arranged in two rows, the Brown Recluse possesses only six eyes arranged in three pairs. Conversely, large, hairy spiders, such as Wolf Spiders (Lycosidae family), are often mistaken for threats due to their intimidating size. Wolf Spiders are ground hunters that stalk their prey rather than using a web, and their bites are not medically significant, causing only minor localized reaction.
Another group frequently mistaken for recluses are the Hobo Spiders and Funnel Web Spiders, which build dense, non-sticky, funnel-shaped webs. Hobo Spiders are light to medium brown and lack the distinct markings of the Black Widow or the unique eye pattern of the Recluse. While their bites can be painful, they are not associated with the severe systemic effects of the Black Widow’s neurotoxic venom. Recognizing the specific features of the female Western Black Widow—the single established threat—is the best defense against misidentifying harmless spiders.
What to Do If Bitten
If a Black Widow bite is suspected, immediate action should be taken to manage the wound and monitor symptoms. The bite itself may feel like a slight pinprick or go unnoticed, but symptoms typically develop within an hour. The venom’s neurotoxic effects can cause severe muscle cramps, often beginning near the bite and spreading to the abdomen, back, or chest. Other systemic reactions include nausea, vomiting, sweating, and weakness.
Immediate first aid involves cleaning the bite site thoroughly with soap and water to prevent secondary infection. Applying a cold compress or ice pack wrapped in a cloth can help slow the spread of the venom and reduce swelling. Keep the affected limb elevated and still to minimize the venom’s circulation.
Medical attention is necessary if severe symptoms develop or if the person bitten is a small child, an elderly individual, or someone with pre-existing health conditions. Physicians may administer pain relievers, muscle relaxers, or, in rare severe cases, antivenom to neutralize the neurotoxin. Even with prompt medical care, painful symptoms may persist for eight to twelve hours, with other effects lasting several days.