Castianeira Longipalpa – Long-Palped Ant Mimic Sac Spider

Castianeira longipalpa long-palped ant mimicking spider information

Castianeira longipalpa, sometimeys referred to as the long-palped ant mimic sac spider is found along the Eastern United States as well as in the Pacific Northwest. The small black spider with at least four white lines along its abdomen is not medically significant. Castianeira longipalpa is often confused with its close relative,

Long-Palped Ant Mimic Spider Description

Despite its name, the long-palped ant mimic sac spider does not look much like an ant. However, all spiders of the ant-mimicking spider genus Castianeira behave and move similar to ants. Like its close relative, the red-spotted ant mimic spider (C. descripta), it is often observed holding its two front legs up in order to appear like a six-legged ant with antennae. The reason for this behavior is not entirely clear yet. It is believed that this behavior should help them get close to ants in order to prey on them.

C. longipalpa can have some different appearances, depending on age and sex. The abdomen of the spider is always black with at least four lateral white or light gray stripes. The cephalothorax (head area) can be black, brown or grayish, almost white. The legs are completely translucent brown in juveniles. In adults, the two front leg pairs are usually brown towards the end and black at the beginning close to the body.

Castianeira longipalpa long-palped ant mimicking spider in eastern USA gray black spider with lateral gray stripes
Castianeira longipalpa. Photo by: Judy Gallagher in Edgewater, Maryland

The two hind leg pairs of adult specimens can be black with white bands, completely black or also bicolored in black and brown.

Note: Castianeira variata which occurs in a similar range in the Eastern United States. Exact identification can be impossible without examining the spider under a microscope. Therefore, there is no guarantee that all images on this overview actually reflect C. longipalpa. C. variata is also not medically significant, so an exact species level ID might not be necessary for the purpose of reader safety.


Despite the much larger appearance on the images here, C. longipalpa are relatively small spiders. An adult female can reach a body size of maximum 1/2 in (13 mm). Males and juveniles are often much smaller with body sizes between 3 mm to 6 mm.

Castianeira longipalpa found in Wisconsin size comparison with quarter dollar coin
This C. longipalpa found by Julian in Milwaukee, WI is about the size of half a dime (0.3 in or 8 mm). Including its legs, it reaches about 0.7 in


As other spiders of the Corinnidae family, the long-palped ant mimic spider is a hunter spider. It does not build a web to catch its prey but actively hunt ants and other small critters. They only use their silk producing abilities to wrap their eggs in protective egg sacs and to build a nest to place the egg sacs in.


C. longipalpa is a fast spider and may seem somewhat aggressive due to its speed. However, their first instinct when approached by much larger humans or pets, their first instinct is generally to flee. Therefore, bites are relatively uncommon. Due to their size, they may even have difficulty even piercing through human skin. In the rare cases where an actual bite might occur, the symptoms are usually limited to minor localized discomfort such as swelling or some pain comparable to a bee sting.

Castianeira longipalpa long-palped ant mimicking spider small spider orange legs black abdomen
The cephalothorax (head) and the legs of C. longipalpa can also be brown/reddish, almost translucent. Photo: Don Loarie

Scientific Classification of Castianeira longipalpa

  • Kingdom: Animalia
  • Phylum: Arthropoda
  • Subphylum: Chelicerata
  • Class: Arachnida
  • Order: Araneae
  • Infraorder: Araneomorphae
  • Family: Corinnidae
  • Genus: Castianeira
  • Species: Castianeira longipalpa
Castianeira longipalpa long-palped ant mimicking spider black with gray white lateral stripes - Kopie
A beautiful Castianeira longipalpa found by David Hill in South Carolina.

Distribution of the Long-Palped Ant Mimic Spider in the USA

The long-palped ant mimic spider is found throughout the Eastern United States, east of the Rocky Mountains. Independent populations also exist in the Pacific Northwest in Oregon and Washington. Hence, it is found in the following U.S. States: Alabama, Arkansas, Connecticut, Delaware, Florida, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Nebraska, New Hampshire, New Jersey, New York, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Vermont, Virginia, Washington, West Virginia and Wisconsin.


Castianeira Longipalpa – Long-Palped Ant Mimic Sac Spider

22 thoughts on “Castianeira Longipalpa – Long-Palped Ant Mimic Sac Spider

  1. Milwaukee, WI. Not happy with the results from the from the tool so here you go. My wife found it in the cabinet I sprayed it with glass cleaner and took the pictures.

    1. Hello Julian, I’m sorry that the tool didn’t work for you. Unfortunately, it’s not perfect yet to identify all types of spiders but we are constantly working to improve it.
      The spider you found is an ant mimicking spider of the genus Castianeira. Here is a similar-looking specimen on bugguide:
      It’s most likely Castianeira longipalpa, given your location.

  2. Very fast, maybe 1 cm body length. No web. Seen (and photographed) in Boulder, Colorado, today, inside an enclosed patio. It hung around long enough for a couple of portraits, then took off. I went thru your rigamarole, which was very thorough indeed, but can’t figure it out. The closest I can come is the ant-mimic spider, which I am pretty sure it is not. Many thanks! Matt

  3. I looked through all kinds of websites to see if I can find this spider and I could not find not one picture of it. Please help me identify this spider 🙂

  4. Slightly over .5 inches. Front legs mostly a silvery gray, forward facing. Back legs striped with the same gray. Not easily startled, moved pretty quickly. Seen on the ground, not in or near a web.
    Candler NC. 7/16/22

  5. In my last comment I failed to mention where the spider was found. It was found in Midlothian, Virginia. It moved very quickly and it was difficult to capture a good picture as it moved across the pavement.

  6. I found it in my house in Eastern Washington state. Very quick mover! Would his bite have left an itchy welt?

  7. I know this isn’t a spider but can you tell me what kind of bug this is? I found several of this same kind outside in Michigan

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