The Yellow Sac Spider

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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.  

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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. Some will have a dark stripe down the middle of the abdomen, but some will simply be a solid yellowish color.

Key Characteristics

  • Tan to yellow color, often with a green tinge
  • Top center of abdomen occasionally has dark marking
  • Legs are slender and always the same color as cephalothorax
  • Front two legs extend much longer than others
  • Tips of legs have a dark, sock-like appearance
  • Pedipalps look like smaller legs, including sock-lis tips
  • Two rows of four evenly sized eyes

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. Eggs are most frequently deposited in June and July, resulting in frequent reports of large juvenile populations in the summer months. 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.

After mating, the female lays between 30 and 50 eggs in each of her 5 silk sacs.  ➥  The female creates a silken retreat where she and her eggs will hide from predators.  ➥  Young spiderlings will remain in the silken retreat with the mother for a short time before venturing off on their own.  ➥  Smaller spiderlings will sometimes return during daylight hours to hide in the silken retreat in the beginning stages of their development.  ➥  Average time to full maturity is between 100 and 150 days.  ➥  Average lifespan is up to 2 years, and adults spend most of their lives in their retreats during the day and hunting 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. 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 when they are hunting or evading a predator.

Mature spiders that survive until winter will create a warmer, sturdier silken retreat by incorporating organic matter into the structure. 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.

Below are a few more interesting facts on yellow sac spiders and how they use their silk strands.

  • They cover a lot of ground when hunting by riding air currents while dangling.
  • While dangling, smaller bugs get caught on their sticky silk for an easy meal.
  • Yellow sacs ascend quickly via their silk strands to avoid predators.
  • The creature’s palps detect vibrations on its silk house when prey approaches.
  • Yellow sac spiders destroy and rebuild their silk homes every single day.

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. 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, muscle cramps, and even necrosis, but these are not common reactions. Most symptoms will dissipate within a couple hours, and medical attention should be sought immediately if they persist.

We hope you enjoyed this blog post on the yellow sac spider. Another common spider in America is the hunter spider known as the wolf spider wolf spider. Our last blog will teach you all you need to know about this arachnid and our latest blog will cover all the interesting facts on the brown recluse brown recluse spider.

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