Castianeira descripta, more commonly know as the red-spotted ant mimic spider gets its common name from the similar look and behavior to ants. It mimics the behavior of ants to get close enough to them for an attack.
Red-Spotted Ant Mimic Spider Description
The Red Spotted Ant Mimic Spider is a hunter spider. That means it does not catch its prey with a web but instead wanders around, searching for its meal. The red-spotted ant mimic spider will often walk with its two front legs held up in the air while twitching them. This gives it the appearance of a six legged ant with antennae. One belief is that it does so to mimic certain ants in hopes of getting close enough to one to pounce on.
The Specimen of the Castianeira Descripta can be easily confused with some species of the Black Widow as it is a black hairless spider with red markings on the back.
Though the differences are easily noticeable to those who are familiar to them, if you are not sure, it is best to be safe rather than sorry and not handle the spider.
Size
A normal female red-spotted ant mimic spider grows to around half an inch (13 mm). The male are smaller than the female spiders.
Web
As a hunter spider, the red-spotted ant mimic spider does not build a web to catch its prey. These spiders only use their spider silk to wrap eggs in a sac or to build a nest to rest in.
Bite
Even though the red-spotted ant mimic spider is a rather aggressive spider, it usually targets small insects or ants and not humans or pets. In case a bite occurs, it is not dangerous to humans. The area of the bite may turn red and it may cause some swelling, comparable to a bee sting. People with allergies should be more cautious.
Scientific Classification of Castianeira descripta
- Kingdom: Animalia
- Phylum: Arthropoda
- Subphylum: Chelicerata
- Class: Arachnida
- Order: Araneae
- Infraorder: Araneomorphae
- Family: Corinnidae
- Genus: Castianeira
- Species: Castianeira descripta
Distribution of the Red-Spotted Ant Mimic Spider in the USA
The red-spotted ant mimic spider can be found throughout the United States and Canada. However, it is more commonly seen on the East Coast. Alabama, Alaska, Arizona, Arkansas, California, Colorado, Connecticut, Delaware, Florida, Georgia, Hawaii, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska, Nevada, New Hampshire, New Jersey, New Mexico, New York, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Utah, Vermont, Virginia, Washington, West Virginia, Wisconsin, Wyoming
Photo Credit is wrong on one of your pics. It’s Jill Chaskey FROM Harrison Michigan no Jill Harrison.
Hi Jill – I am so sorry about that. I’ve just fixed the credit. And thanks for sending in the great picture. We are always happy to receive more if you find some spiders in your area.
This was in our house in TX.
Hello Chris, thanks for getting in touch. This is a red-spotted ant mimic spider, Castianeira descripta: https://usaspiders.com/castaniera-descripta-red-spotted-ant-mimic-spider/
It’s not a medically significant spider.
Found in my bathroom this morning in Central Ohio. You can see the web it was constructing overnight. It wasn’t there 7 hours prior.
Hello Ryan, thanks for getting in touch! It’s hard to make out the details on the picture but to me, it appears to be a nursery web spider (Pisaurina sp.). Compare the following pictures: https://usaspiders.com/pisaurina-mira-nursery-web-spider/
It’s definitely not a medically singificant spider.
Please identify for me. Thanks
Hello John, thanks for getting in touch! This is a red-spotted ant mimic spider (Castianeira descripta). They mimic the behavior of ants to get close to them and prey on them. It’s not a medically significant spider: https://usaspiders.com/castaniera-descripta-red-spotted-ant-mimic-spider/
This was in our house by Houston Tx. What is it? Thanks.
Hello Chris, thanks for getting in touch! This is a red-spotted ant mimic spider (Castianeira descripta). They mimic the behavior of ants to get close to them and prey on them. It’s not a medically significant spider: https://usaspiders.com/castaniera-descripta-red-spotted-ant-mimic-spider/
Found this in my sink this morning…what is it? TIA!
Black medium sized spider with a yellow rectangular mark on it’s back. I noticed it just sitting on my left knee and I quickly knocked it off my knee. I was looking for it after that and it was hiding nearby just a foot from where I was sitting. I killed it and crushed it beyond recognition so I can’t send a photo in. The rectangular yellow mark was orientated vertically on the back of it’s abdomen.
Found this walking across my kitchen floor in California. It’s length is about 3/8“. I didn’t notice the red mark on its back until it was out in the sun. Any idea what this is? Is it a parson spider?
Hello Sandy, this is some type of ant-mimicking spider of the genus Castianeira. As there are several similar-looking species, I can’t ID it deeper than genus level. Here is more information about the red-spotted ant mimicking spider – a close relative of your spider: https://usaspiders.com/castaniera-descripta-red-spotted-ant-mimic-spider/
If you are interested in learning more about spiders in California, check out our California spider identification and information book – available as an ebook here or as a printed paperback on Amazon.