Holmes Lawn & Pest Blog

The Yellow Sac Spider

Yellow sac spider

The yellow sac spider is one of the most commonly identified species of spiders in Utah. Though it is small, it is one of the most aggressive and feared spiders in the state. Their aggressive nature is the reason their bites are among the most common in the United States. Their venom is not as dangerous as that of a brown recluse or black widow, but a bite from a yellow sac spider is always cause for concern and should be taken seriously.  


Like most spiders, the male of the yellow sac species is smaller than the female, but males will typically have longer leg spans. Yellow sac spiders can grow up to ⅝ of an inch long, but many are even smaller. Aside from a darker color around the mouthparts, yellow sac spiders do not typically have distinctive markings on their bodies, instead presenting a more uniform color and appearance. 


Yellow sac spiders can be found in a range of tan to yellow in color, with many spiders even having a greenish tinge to them. Some yellow sacs will have a slightly darker mark at the top center of the abdomen, but this is not always a reliable indicator, as it is not exhibited on all yellow sac spiders. Their legs are the same color as the cephalothorax, with the front two legs being longer than the rest. The tips of their legs commonly have a dark, sock-like appearance.


The eyes of yellow sac spiders are distributed in two rows of four eyes that are all similar in size. As nocturnal creatures, yellow sac spiders do not rely much on their vision. The pedipalps of yellow sac spiders are small appendages at the front of the cephalothorax, and they are the primary sensory receptors used by the spiders.

Life Cycle

The species of yellow sac spiders commonly found in North America is Cheiracanthium inclusum, and the females of the species mate only once in their lives. After mating, the female lays up to five egg sacs that typically contain between 30 and 50 eggs, but that number can vary. The female creates a silken retreat where she and her eggs will hide from predators.


Eggs are most frequently deposited in June and July, resulting in frequent reports of large juvenile populations in the summer months. Once hatched, the young spiderlings will remain in the silken retreat with the mother for a short time before going off on their own. Even once the spiderlings do leave, they will sometimes return during daylight hours to hide in the silken retreat in the beginning stages of their development.

Like many spiders, the mother may eat some of her offspring, and other spiderlings will die from environmental or other factors. Those that survive will often be found hiding outdoors underneath leaf litter or high up in trees where they feel safe and protected from predators. Average time to full maturity is between 100 to 150 days, and the life expectancy of C. inclusum is up to two years. They will spend the majority of their lives in their silken retreats during the day, overwintering in their retreats when it gets cold, and hunting for food at night.


Yellow sac spiders are active and aggressive nocturnal hunters. They do not spin webs to catch their prey; they only create silken spheres in which to retreat during the daytime and wait for nightfall. A new retreat is built every day by the creature. At night, yellow sac spiders will break out of their protective cocoon in order to hunt.


The most common prey of yellow sac spiders are smaller insects, bugs, and other spiders. Yellow sacs enjoy the cover of leaf debris and shrubbery, but they are also commonly found in trees. They travel quickly between various environments by creating a thin strand of silk that is used for riding air currents to their next destination. 


The type of silk strand used by yellow sacs for traveling can also be used for hunting, as smaller airborne prey can be caught by the spider while it is dangling. This strand of silk is also used to quickly ascend and get away from predators. Yellow sac spiders are well known for their ability to move vertically, as well as their ability to cover a lot of ground while hunting via their silk strands. 


Mature spiders that survive until winter will create a sturdier silken retreat in which to overwinter and survive until the following spring. Occasionally, the spiders will briefly leave their retreats in the wintertime if easy prey is near, but they will quickly build a new protective structure after hunting. Prey is sensed with the use of the creature’s pedipalps, which can detect vibrations even when it is in its retreat. The pedipalps are the main tool used by the yellow sac for nocturnal hunting.

Are Yellow Sac Spiders Dangerous?

The majority of spider bites in the United States are attributed to yellow sac spiders. The high frequency of bites from this species is due to the prevalence of yellow sacs in the country and their unusually aggressive nature. If yellow sacs get near humans, they will not hesitate to bite and inject venom, even if the attack is unprovoked. 


As great climbers with exceptional vertical mobility, yellow sacs are often found in corners of walls and ceilings; however, the silk retreats created by the spiders are often hard to notice and can easily be confused for tiny dust bunnies. People frequently come into contact with the structures that are housing yellow sacs, causing the spiders to attack in self defense. While these spiders do prefer outdoor living, they also wander inside homes and build retreats under furniture, in clothing, inside shoes, and many other dark places. It is very common to encounter a yellow sac by simply putting on a shirt that has been hanging in the back of your closet. 


The venom of a yellow sac spider will likely not result in life-threatening injuries, but a bite should always be taken seriously. In particular, C. inclusum bites are initially very painful and have been known to cause more serious effects. Most bites from this species will include a burning sensation that lasts up to an hour, followed by the development of blisters near the bite. Other symptoms can include fever, dizziness, nausea, and muscle cramps, but these are not common reactions.


Severe effects of yellow sac bites include the formation of ulcers and necrosis, somewhat similar to the bite of a recluse spider. It is important to note, however, that while the symptoms may seem similar to a recluse bite, they will be much less severe and should dissipate in a matter of hours. Some people may have very mild reactions that include little more than swelling and irritation, but medical attention should be sought immediately if any of the more serious symptoms begin to develop.


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