Castianeira Descripta – Red-Spotted Ant Mimic Spider

Red-Spotted ant mimic spider Castianeira descripta

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.

Picture or red-spotted and spider
Photographed by: Justin Meyerholtz – Big Rapids, Mi.

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.


A normal female red-spotted ant mimic spider grows to around half an inch (13 mm). The male are smaller than the female spiders.

Red-Spotted ant mimic spider size
The red-spotted ant mimic spider compared to a USD quarter. Photo by Jennifer Wolf – Berrien Springs, Mi.


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.


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.

Castianeira descripta spider
Photographed by: Jill Chaskey from Harrison, Michigan

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

castanieira descripta ant mimic spider usa range

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

Castianeira Descripta – Red-Spotted Ant Mimic Spider

37 thoughts on “Castianeira Descripta – Red-Spotted Ant Mimic Spider

    1. 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.

      1. 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.

  1. 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.

  2. 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?

    1. 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:
      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.

  3. I live in Central Oregon. A small, about half an inch, black shiny spider with an orange looking fiddle shaped marking on its back was crawling on the foot path. I took a picture but he was hiding under a piece of grass.

  4. Hi, I live in MN and have found several of these spiders inside. I have looked and looked and can’t identify it. I thought it was the bold jumping spider, but when I have caught them they have not jumped, and this type of spider looks to have a bigger or clunkier, more exposed mouthpiece too. Almost pincer-looking, or stout like a bulldog’s jowls. The picture is the underside of the spider. The top of the spider has a few white stripes on it’s body in the back, almost like a zebra spider does. These are about a little over twice the size of a zebra spider.

    1. Hi Libby, this is a hacklemesh weaver (family Amaurobiidae). They are not medically significant.

  5. Please identify this spider found in Denver, Colorado.

    Small, around 1/2”, dark brown or black. Legs seem about the same length, rear ones a little long maybe. Uniform leg color (dark), small fine hairs. Red mark on its rear, top, not underneath.

  6. I just caught this spider in my bathroom after it scared the crap out of my daughter. After searching Google, we think we identified this spider as a Castianeira Descripta. Can someone verify?

    I am using this opportunity to teach her, and help her overcome her fear of bugs!

  7. Hi, we found a solid yellow spider that looks to be pretty large in size. We are not sure what kind of spider it is.

  8. Just saw one of these hanging out next to a line of ants with its front legs in the air just like described. Very cool website, I just scrolled down the list for Texas until I saw a spider that looked similar, and now I know all about what I saw.

  9. Found this in our kitchen in Minnesota today. We think it’s a red-spotted ant mimicking spider based on our Google
    research. Was afraid it was a northern black widow at first but now know the difference.

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