You’re weeding the flower bed or maybe just walking to the mailbox when you see it. A flash of primary colors. It’s small, but those jarring shades of red and blue stop you cold. Most people immediately think of two things: "Is it a Black Widow?" or "Wait, is that a real-life Spider-Man?"
Nature is rarely that simple.
Finding a red and blue spider usually means you’ve stumbled upon one of a few very specific species, and honestly, they are some of the most fascinating predators in the arachnid world. We aren’t talking about the dusty brown house spiders that hide in your shoes. These colorful characters have specific jobs, weird hunting habits, and, luckily for you, they generally couldn't care less about humans.
Why Do These Spiders Have Such Loud Colors?
In the wild, bright colors are usually a billboard. They scream "I taste terrible" or "I am incredibly dangerous." This is called aposematism. But with the red and blue spider varieties we see in North America and beyond, the colors often serve a dual purpose of mating displays and camouflage against specific flowers.
Take the Mimetus genus, often called Pirate Spiders. They aren't always bright blue, but certain variations and lighting can make their variegated patterns look strikingly metallic. Then you have the Phidippus jumping spiders. Some of these guys, like Phidippus johnsoni (the Red-Backed Jumping Spider), have deep red abdomens that, when hit by the sun, reflect a bluish, iridescent sheen on their cephalothorax. It's nature's way of being flashy while staying functional.
The blue is often structural color. It’s not a pigment like the red; it’s the way light hits microscopic scales on the spider’s body. Think of a soap bubble or an oil slick. That’s why a spider might look grey in the shade but transform into a vivid red and blue spider the moment it hops into a sunbeam.
Meet the "Pirate": The Mimetus Species
If you see a spider that looks a bit spindly, with long legs covered in black spines, and a mottled red-and-dark-blue or purple-ish abdomen, you might be looking at a Pirate Spider. These guys are heavy metal.
They don't build webs to catch flies. No. They are "araneophagous." That is a fancy way of saying they eat other spiders.
The Pirate Spider wanders onto another spider's web—usually a harmless orb weaver—and begins to pluck the silk strands. It’s a lure. It mimics the vibrations of a trapped insect or even a potential mate. When the resident spider comes over to investigate the "meal," the Pirate Spider strikes. It bites the other spider on the leg, injecting a highly specific venom that paralyzes other arachnids almost instantly.
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It’s a brutal, effective strategy. Seeing a red and blue spider like this in action is a reminder that the backyard is basically a tiny, colorful gladiator arena.
The Jumping Spider: Nature’s Tiny, Colorful Tank
The most common "red and blue" sightings usually involve the Salticidae family—jumping spiders. Specifically, species like Phidippus californicus or even the more exotic Saitis barbipes (though you'll mostly find those in Europe).
These are the puppies of the spider world. They have huge, forward-facing eyes and a curious temperament.
Why the colors matter for them:
- Courtship dances: Males use their red and blue patches to signal to females. It’s a high-stakes dance. If the dance is bad, the male becomes dinner.
- Depth perception: Their vision is incredible. They see in high definition and can likely perceive colors we can't even name.
- Intimidation: To a bird, a bright red spot looks like a warning.
If you find a fuzzy, chunky red and blue spider on your windowsill, it's almost certainly a jumper. They don't make messy webs. They just hunt. They track their prey like cats, wiggle their abdomens, and pounce.
Is the Red and Blue Spider Dangerous?
This is the big question. Everyone sees red and thinks "danger."
The short answer? No.
The long answer is that while almost all spiders have venom, the ones sporting red and blue patterns in your garden are not medically significant to humans. A bite from a Phidippus jumping spider might feel like a mild bee sting at worst. They aren't aggressive. You'd have to basically pin one against your skin to get it to bite.
The Pirate Spiders are even less of a threat. Their fangs are tiny, designed for piercing the joints of other spiders, not human skin. We are just too big for them to care about.
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Compare this to the Black Widow (Latrodectus). While the widow has red, it’s a specific hourglass shape on a jet-black body. It doesn't have that vibrant blue or iridescent teal that characterizes the more "ornamental" spiders. If your spider has blue on it, you’re almost certainly safe.
Identifying by Habitat: Where Did You Find It?
Where you find your colorful friend tells you a lot about what it is.
If it was on a fence post or the side of your house, it’s likely a jumping spider. They love flat, sun-warmed surfaces where they can see prey from a distance. They are active during the day. If you see a red and blue spider at night, tucked into the corner of a messy, tangled web that it didn't build itself, you’ve found a Pirate Spider.
There's also the Red-Legged Purseweb spider (Atypus snetsingeri). These are much rarer and look like tiny, prehistoric tanks. While primarily black or deep brown, their legs can have a reddish hue, and in certain lights, their carapaces look deep, bruised blue. They live in silken tubes buried in the ground. If you see one of those, count yourself lucky—they are incredibly reclusive.
Misidentifications: It Might Not Even Be a Spider
Sometimes, what we think is a red and blue spider is actually a Velvet Ant.
Despite the name, a Velvet Ant is actually a wingless wasp. They are covered in dense, velvet-like hair and often come in brilliant shades of orange-red and blue-black. Do not touch these. Unlike the spiders we've discussed, Velvet Ants have a sting that is famously painful—earning them the nickname "Cow Killer."
How do you tell the difference? Count the legs. Spiders have eight. Velvet Ants have six and antennae. If it has antennae, stay away.
The Science of Spider Silk and Color
There is some fascinating research coming out of places like the University of Cincinnati regarding how these spiders perceive their own colors. They’ve found that some jumping spiders have a filter in their eyes that allows them to see the red spectrum, which is actually rare for invertebrates.
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This explains why they've evolved to be so colorful. They aren't just colorful to us; they are colorful to each other. When a male red and blue spider displays his colors, he’s showing off his health and genetic fitness. The more vibrant the red, the better he is at finding food.
What to Do If You Find One
First off, don't squish it. These spiders are massive assets to your local ecosystem. Jumping spiders, in particular, are excellent at keeping the fly and mosquito populations down.
If you want to get a closer look, use a clear glass and a piece of paper to relocate it. If you look at a jumping spider through the glass, you'll notice something amazing: it will look back at you. They track movement. They are one of the few bugs that actually seem to "interact" with humans.
Actionable Steps for Identification:
- Check the eyes: If it has two massive eyes in the middle of its face, it’s a jumping spider.
- Look at the web: No web? It’s a hunter. Messy, "stolen" web? It’s a Pirate Spider.
- Check for "fur": Fuzzy spiders are generally jumping spiders. Shiny, smooth spiders are more likely to be hunters or orb weavers.
- Notice the movement: Does it jump? If it moves in quick, jerky bursts and can leap several inches, it’s a Salticid.
Real-World Sightings and Citizen Science
If you’ve taken a photo of a red and blue spider, don't let it just sit in your camera roll. Platforms like iNaturalist or the "Spider ID" subreddit are full of experts—real arachnologists—who love identifying these things.
By uploading your photo, you’re helping scientists track species ranges. With climate change, many spiders are moving further north than they used to be. Your backyard sighting could actually be an important data point for a study on migratory shifts.
Ultimately, seeing a red and blue spider is a win. It means your local environment is healthy enough to support specialized predators. It means there’s a complex food chain happening right under your nose.
Next time you see those bright colors, take a second to appreciate the engineering. That little creature is a product of millions of years of evolution, fine-tuned to be a colorful, efficient, and mostly harmless neighbor. Keep your distance if you're nervous, but definitely take a photo. Nature doesn't often use that part of the color palette for spiders, so it's a rare treat when you catch one in the light.
Summary of Next Steps
To properly handle or identify a colorful spider in your vicinity, follow these practical steps:
- Photograph from a distance: Use your phone’s macro setting (the little flower icon) to get a clear shot of the "face" and the pattern on the back without getting too close.
- Observe the behavior: Watch if it’s active in the sun or hiding in the shade. Jumping spiders love the light; Pirate spiders love the shadows of existing webs.
- Use a catch-and-release jar: If the spider is indoors and you want it out, place a jar over it and slide a stiff piece of mail underneath. Relocate it to a bush or a stone wall.
- Check for an hourglass: Always verify there is no red hourglass on the underside. If there isn't, and the spider is multi-colored (blue, red, green), it is almost certainly harmless.
- Consult an app: Upload the image to iNaturalist. The AI and the community will give you a species-level identification within hours.