How to Identify a Yellow Sac Spider

Yellow sac spiders are common spiders throughout Europe and North America. While they are venomous, their bites rarely result in serious medical consequences. Yellow sac spiders are also relatively easy to identify. By looking at the spider's physical characteristics, looking for its sleep-sac, and watching for it in its habitat, you'll be able to properly identify a yellow sac spider.

Part 1

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Identifying the Spider's Physical Characteristics

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The yellow sac spider's legs are relatively long – slightly longer than their body. In addition, their front 2 legs are longer than the rest of their legs. Their legs are also relatively thin and do not have a muscular appearance.^{[1]XResearch source}
The tips of the yellow sac spider's legs are black in color. The color comes from tiny black hair that cover their feet. This hair enables the spider to climb vertically – up trees and walls -- with ease. Since the spider doesn't spin webs, this is a huge help in enabling the spider to climb.^{[2]XResearch source}
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The yellow sac spider is a relatively small and slight spider. While females are somewhat larger, large spiders will only measure .4 inches (1 cm) (10 mm) in length (without their legs).^{[3]XResearch source}
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Look for the spider's eight similarly-sized eyes. The yellow sac spider's eight eyes are mostly equal in size and are arranged in two even lines. While the eyes are mostly black, the two middle ones might appear silver or gray.
- Their head and eyes may be covered with tiny black or silver hairs.^{[4]XResearch source}
Most yellow sac spiders are pale yellow or tan. However, some of them range from a light green, to orange, to a light brown. This is largely dictated by the spider's habitat and diet. As a result, be aware that the spider's color may vary based on the region you live in.^{[5]XResearch source}
Yellow sac spiders have a dark mark/line that runs up and down the middle of the spider's abdomen. The heart mark is named so because it crosses over the area where the spider's heart is located.^{[6]XResearch source}

Part 2

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Spotting the Spider's Sleep-Sac

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1
Look for a “sleep-sac.” Sac spiders spin a silk-like tube or sac. They use the sacs to sleep in during the day. They tend to spin these sacs around dawn. Ultimately, the spider's sleep-sac is one of its most characteristic qualities.
- Yellow sac spiders don't spin webs, they create sleep sacs.^{[7]XResearch source}
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Watch for sacs in rolled up leaves or under sticks and logs. When outside, you'll find sacs under sticks, logs, in rolled up leaves, or in any other area where the spider feels like it will be secure and out-of-the-way.
- You may run into sleep-sacs when you're gardening, racking leaves, or cutting hedges.
Yellow sac spiders tend to create their sacs where interior walls meet the ceiling, behind pictures, or around windows. These are out-of-the-way places where they won't be disturbed in the daytime.^{[8]XResearch source}

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Observing the Spider's Behavior and Habitat

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Yellow sac spiders are nocturnal. As a result, you can spot them hunting their prey at night time. They feed on flies and other small insects. As a result, you may be able to observe the spider hunting in an area where there are many flies or similar-sized insects.
In the outdoors, yellow sac spiders are commonly found on the margins of open spaces like fields, meadows, or even parking lots. They can be found in bushy trees and large shrubs.
Depending on your region, yellow sac spiders can commonly be spotted outside from April to October - warmer months when eggs hatch and young spiders mature.^{[9]XResearch source}
As the weather turns, yellow sac spiders tend to migrate into cars, buildings, dead trees, and other structures. There, they create sacs to in which they stay during the winter. They'll emerge fully mature in the spring.^{[10]XResearch source}

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How can I tell whether a spider is a brown recluse or a yellow sac?
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Brown recluses are bigger and have a violin shape on their back, not a straight line.
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Do yellow sac spiders ever look colorless and drop straight down on a web string?
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In my area (Northern Virginia) I have seen many yellow sac spiders around my house and just a couple nights ago I saw a spider that was slightly transparent drop straight down from my ceiling fan, so yes, I believe so.
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How deadly is the yellow sac spider?
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The toxicity of their bite itself isn't much of a problem, but it can cause other problems. Their bites have been linked to MRSA infections and they can be quite painful and lead to a necrotic lesion, which can cause a whole bunch of other complications. Some people react strongly, and some people don't. For your safety, it's just best to avoid them entirely.
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Warnings

The symptoms from a bite of a yellow sac spider mimic the symptoms from a bite of a brown recluse spider. If at all possible, capture the spider that bit you so that it can be positively identified. That way, you can get the appropriate medical attention.
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Yellow sac spiders are very aggressive and very invasive. Check or shake your sheets at night to ensure none have already started hunting in there. As they like to hide in crevices or pictures, once they feel threatened they'll drop to the ground and run. If your bed is pressed against the wall, move it slightly so none drop in. Additionally, spider repellant can be placed around the stands of your bed to ensure none climb up to go hunting while you are sleeping.
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