Many species of spiders are native to Colorado. Some are easily recognizable while others are a bit less likely to be recognized. In general, spiders are beneficial arthropods that consume many other insects in and around homes and gardens. Colorado spiders typically have a lifespan of a single year; however wolf spiders and widows can live a few years and tarantulas can live for a decade or longer. Females lay eggs in sacs which are frequently covered in tough silk. Web spinning spiders often attach the eggsac near their web while other spiders carry the sac with them until it hatches. Upon hatching, many species of spiderlings climb to the top of a nearby object, produce long filaments of silk and are carried away be the wind. This method of dispersal is referred to as ballooning. Their ability to disperse in this manner in addition to other factors which affect their rate of survival can cause large variances in the number of spiders in a given area from year-to-year. Specific characteristics such as eye pattern can be utilized by entomologists to identify a specific species. Spiders have six to eight eyes depending on their species, which are usually arranged into two rows. True spiders all have venom although few spider bites require medical attention. Spiders bite humans only as a means of defense. Venom is intended for the immobilization of prey.
Below are a few of the species commonly found in Colorado homes and general information regarding each species:
Funnel web spiders – This species is the most common spider found within homes in Colorado. When funnel web spiders are outdoors, they build their dense mat style web around corners of buildings, shrubs and thick grass. Their common name comes from the funnel portion of the web where the spider hides waiting for prey to enter the surrounding web. Commonly mistaken for brown recluse (which is NOT native to Colorado), the funnel web spider lacks the characteristic violin or fiddle marking on the back, have four pairs of eyes instead of three, are much more rapid in movement and have striped legs.
Jumping spiders – Active hunters, they stalk their prey and pounce on it rather than utilizing a web to capture it. Their ability to jump several body lengths has a tendency to surprise most humans and can be used as a means of escape just as easily as it is used for hunting. Silk is used for covering eggs, laying down a trail and constructing temporary shelters. Most jumping spiders are brightly colored. Jumping spiders have seemingly large eyes in comparison to their size.
Wood Louse Hunter/Roly-poly hunter – Although their bite is painful, their venom is not known to cause medically related problems. These spiders live in a silk retreat and hunt at night. Feeding primarily on pill bugs and other hard-bodied prey, this species reaches a length of approximately ½ inch. Unlike many other spiders, this species has a smooth body appearance with the body being a cream to grey in color with distinct reddish colored legs.
Cellar spiders – Commonly mistaken for daddy-long-legs, cellar spiders are often found in dark corners with untidy webs that are fairly extensive. Females carry their egg in their chelicerae (jaws) in a loose silk sac.
Yellow Sac Spiders – Like jumping spiders, yellow sac spiders are active hunters. Their common name comes from the silk sacs in which the spend daylight hours. Typically found in homes during the fall, this species is suspected of being the source of most spider bites in humans although their bite does not cause known medical problems, the bite is fairly painful; however, the pain typically subsides within a short period of time. Sac spiders are typically pale in color and their sac-like retreats can be found in corners of rooms or cracks in walls when indoors.
Wolf spiders – Wolf spiders do not produce a web to capture prey, they are active hunters. Depending on their species wolf spiders can be rather large in size, which may cause concern in residents. Larger species are often mistaken for tarantulas. Females have an unusual habit of carrying her egg sac attached to her spinnerets. Spiderlings hatch and crawl up onto her back where they stay for the first few weeks of their lives. Wolf spiders are normally shy, although they will bite in defense and are not dangerous to humans. Coloration is typically browns, grays and black.
Ground spiders – As indicated by their common name, ground spiders are typically found at ground level, under rocks and logs, only emerging to hunt. Some species of this spider wander indoors during colder weather. Ground spiders are harmless to humans.
Widow Spiders – The spider species with the most potential for human harm in Colorado, widow spiders’ venom contains a nerve venom which at times may be of medical concern in humans. Although these are non-aggressive spiders, they will bite to defend themselves. The bite is painful and typically occurs after a female widow is provoked. The western widow is among the most common in Colorado. Their webs are built in dark undisturbed sites such as crawlspaces, window wells and cellars. A red or orange “hour-glass” shaped marking on the abdomen is characteristic of widow spiders; however, the shape is often varied and not perfect. Overall color of adult females is black while males and immature spiders of this species may have brown, red and white markings on the back. Cobweb spiders are likely to be the most commonly mistaken for widow spiders.