Red Stripe Spider (Castianeira crocata)

Physical description and identification

The red stripe spider is a small, glossy, ground-running hunter in the family Corinnidae and the ant-mimicking genus Castianeira. Seen from above it looks like a narrow black ant with a bright line painted down its back, and that line is the whole identification: among eastern North American members of the genus, a lengthwise orange-red stripe on a black abdomen is distinctive.

Adults

• Size: body length is roughly 5–10 mm (0.2–0.4 in). Mature females sit at the upper end of that range and are noticeably broader; males are smaller, narrower and longer-legged, with swollen pedipalp tips that look like tiny boxing gloves.
• Colour: the carapace and legs are blackish to very dark brown with a lacquered shine. The abdomen is black and carries a single orange to orange-red stripe running lengthwise along the upper surface, usually broadest toward the front and tapering back. The stripe is formed by coloured hairs and scales, so it looks brighter in direct sun and duller in shadow.
• Eyes: eight small eyes of similar size in two curved rows of four. No pair is enlarged; that alone rules out the jumping spiders, whose ant-mimicking species can otherwise look similar in a photograph.
• Other characteristic features: the body is elongated and sparsely haired, the abdomen is slightly narrowed where it meets the carapace, and the hind legs are the longest pair. In life, the front legs are usually raised and waving rather than walking, which is the quickest way to spot the genus.

A correction to a claim repeated in older material: the red stripe spider is not closely similar to a true black widow, even if the colours suggest it. The two are not related, do not share a body shape, and are not confusable once the outline is seen. The comparison below explains what to look for.

Eggs

The female encloses her eggs in a white, roughly circular silk sac and fastens it in a sheltered place — the underside of a rock or slab, a fold of loose bark, a board lying on soil. She may stay with it for a time but does not carry it, and the spiderlings receive no care after they emerge.

Spiderlings

Young are independent from the moment they leave the sac. They are paler and less clearly striped than adults, so half-grown individuals can be hard to name to species; the stripe strengthens with successive moults.

The web and how it hunts

This is another point that older accounts get wrong. The red stripe spider builds no web near its resting place or anywhere else. It is a cursorial hunter: it runs prey down on soil, litter, logs and low plants and overpowers it directly. Silk is used for two purposes only — a safety dragline paid out as it moves, and a flattened sac-shaped retreat tucked into a crevice, where the spider rests during the day, moults and shelters with an egg sac. For how this compares with true web builders, see our guide to spider webs.

Where the red stripe spider lives

Castianeira crocata is native to North America. It is recorded widely through the eastern and central United States, from New England and the Great Lakes states south to the Gulf and west into the prairies, and it reaches southern Canada. Older summaries that limit it to "the United States" are too narrow; the northern edge of the range crosses the border into Ontario and neighbouring provinces.

The preferred habitat is open, sunlit ground with plenty of surface cover: deciduous woodland edges and clearings, old fields, prairie remnants, hedgerows, parks and gardens. The spider spends its time under and between stones, logs, loose bark, boards and leaf litter, emerging to hunt across open soil. Like most of the genus, it is closely associated with ant activity, which is a large part of why it looks the way it does. It is not a house spider and does not establish indoors, although a wandering male may come in through a gap at floor level during the summer.

Is the red stripe spider venomous?

Yes, in the same limited sense as nearly all spiders: it injects venom to subdue the insects it catches. That venom is tuned to small arthropods and is not medically significant to people, pets or livestock. No antivenom exists because none is required, and no serious envenomation has been documented for the species. It is unrelated to the recluse spiders and the widow spiders, the two North American groups that matter medically.

Can a red stripe spider bite?

It can, but records are very few, because the spider's reflex on meeting anything large is to sprint for cover. Bites occur only where escape is impossible — a spider caught inside a glove, pressed under a hand on a log, or trapped in clothing left on the ground. The effect described is a short, sharp pinprick, followed by slight redness and perhaps a little local swelling that settles within a day. Wash the area with soap and water and hold a cold pack against it if it aches. Expanding redness, an ulcerating wound or fever warrant medical advice, as those indicate infection rather than venom.

Ecological importance and behavior of the red stripe spider

The red stripe spider is a wandering predator with an ant's manners. It walks on its rear six legs and holds the front pair up, waving them like antennae; it moves in short jerky bursts and pauses, and its glossy, constricted-looking body finishes the impression. This is myrmecomorphy, and for the spider it is primarily a defence: many visual hunters — birds, wasps, and especially other spiders — avoid anything that reads as an ant, because ants bite, sting and arrive with reinforcements.

Diet: small ground-living arthropods — ants, flies, small beetles, springtails, leafhoppers, crickets and other small spiders. A single spider works through a steady stream of such prey across a season, so a garden population is a real contribution to pest control.

Natural predators: older summaries name only birds and wasps, but the list is longer. Birds and parasitoid wasps — mud daubers stock their cells with spiders of exactly this size — are joined by lizards, toads, centipedes, ants when the disguise fails, and larger spiders. See what eats spiders.

Daily and seasonal rhythm: the species is active mainly in the warm months, in daylight and after dusk alike, though it keeps to cover in the hottest hours. Mature males wander in search of females through summer and are the individuals most often seen crossing open ground. The population overwinters largely as eggs and juveniles tucked into litter and crevices.

Lifespan: about a year is typical for a temperate Castianeira, with individuals hatching in one season, overwintering part-grown and maturing in the next. Claims of three-year lifespans for this species are not supported.

Relationship with humans: harmless and useful. No control is justified; one found indoors can be cupped and put outside.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The species was described in 1847 by Nicholas Marcellus Hentz. He is often called a French arachnologist, which is only half right: he was born in France but emigrated to the United States as a young man and did all of his arachnological work there, which is why he is generally credited as the founder of American arachnology.
• The genus Castianeira was erected later, by Eugen von Keyserling in 1879, so the species was originally published under a different genus name and moved afterwards — a normal piece of taxonomic housekeeping.
• Holding the front legs up as imitation antennae costs a spider real running speed, since it is effectively travelling on six legs. The disguise is worth more than the speed.
• The stripe is made of pigmented hairs and scales rather than the cuticle itself, which is why it looks vivid in sunshine and can appear dull brown in a flash photograph.

In summary, the red stripe spider is a harmless, hard-working little predator that survives by pretending to be an ant. Look for the slim black body, the raised waving front legs and the single orange-red line down the back, and remember that no widow runs across open ground without a web.