There are a few different types of spiders that can have blue coloring on their backs. The most common spiders with blue backs belong to the genera Cyclosa and Araneus. These orb-weaving spiders are found throughout North America and get their vibrant blue coloring from specialized hair-like setae on their abdomens. The blue color serves as camouflage, helping the spiders blend in with the sky when they sit in their webs. Other less common spiders with blue backs include some jumping spiders in the genus Menemerus and lynx spiders in the genus Oxyopes. Identifying the exact species requires looking at characteristics beyond just the blue coloration. But in general, spiders with blue backs use their coloring as a form of protection through camouflage or warning coloration.
Blue-Backed Orb Weavers
The most commonly encountered spiders with blue backs are orb-weaving spiders in the genera Cyclosa and Araneus. There are several different species that display vibrant blue coloration on their abdomens:
Cyclosa conica
– Known as the barn spider or blue-backed spider
– Medium-sized orb weaver found throughout North America
– Females have round blue/green abdomens
– Often build their webs on buildings, fences, and trees
Cyclosa turbinata
– Called the flat-backed orb weaver
– Smaller cousin of C. conica with a flatter blue abdomen
– Makes its web in shrubs and other vegetation
Araneus gemmoides
– Known as the cat-faced spider
– Medium-large orb weaver native to North America
– Females have blue-green bellies with white dots
– Builds its web in gardens, between buildings, etc.
Araneus marmoreus
– Called the marbled orb weaver
– Small-medium orb weaver found across North America
– Has bright teal-blue patterned abdomen
– Makes its web near human habitats
| Species | Common Name(s) | Size | Web Location |
|---|---|---|---|
| Cyclosa conica | Barn spider, blue-backed spider | Medium | On buildings, fences, trees |
| Cyclosa turbinata | Flat-backed orb weaver | Small-medium | In shrubs and vegetation |
| Araneus gemmoides | Cat-faced spider | Medium-large | Gardens, between buildings |
| Araneus marmoreus | Marbled orb weaver | Small-medium | Near human habitats |
Blue Color Adaptations
So why do these spiders have blue backs? The vibrant blue coloration is an adaptation that serves multiple purposes for the spiders:
Camouflage
The most common reason for the blue abdomens is camouflage. When the spiders sit in their webs and are backlit by the sky, the blue hues help them blend in and avoid detection by predators like birds and wasps. The color essentially disguises them as a piece of sky, making it harder for predators to spot them in their webs.
Warning Color
In some species like C. conica, the blue back may also serve as a warning signal. Blue colors are often associated with toxicity in the animal kingdom. So even though orb weavers are harmless to humans, the bright blue tells predators to stay away or risk getting bitten.
Temperature Regulation
Some researchers also think the blue hue can help regulate body temperature. The setae traps heat from sunlight, allowing the spider to warm up quicker in cool conditions. This may aid with energy levels and metabolism.
Attracting Prey
Additionally, some experts believe the blue color may attract insect prey to the web, acting like a bullseye target. However, more research is needed to determine if color directly impacts prey capture.
So in many cases, the blue back gives these spiders a survival advantage. It’s an evolutionary adaption that has developed over time to help the spiders avoid predation, regulate temperature, and potentially catch more prey.
Other Spiders With Blue Backs
While orb weavers are the most common blue-backed spiders, some other groups have blue coloration as well:
Jumping Spiders
Some species in the genus Menemerus have bright blue abdomens that distinguish them from other jumping spiders. Most notably, the striking blue hue of Menemerus bivittatus has earned it the nickname “azure jumping spider.” Found in the tropics, the vibrant blue may serve as a warning signal.
Lynx Spiders
The lynx spider Oxyopes salticus has greenish-blue sets of spots on its abdomen. Lynx spiders don’t build webs for catching prey, so the function of the blue spots is likely camouflage while the spider hunts on plants.
Crab Spiders
There are some tropical crab spiders that have blue abdomens, such as species in the genus Philoponella. As ambush predators, the blue most likely functions as camouflage against foliage.
So while not as common, blue coloration does pop up in other spider groups beyond orb weavers. Depending on the species, it may match function as camouflage, warning coloration, or other purposes.
Identifying Blue-Backed Spiders
If you come across a spider with a blue abdomen, how do you identify what species it is? Here are some tips:
Take Note of the Web
– Orb weavers like Cyclosa and Araneus will always be found in a large, intricate circular web. Jumping spiders and lynx spiders do not build webs to catch prey.
Look at Body Shape
– Orb weavers have a round, plump abdomen. Jumping spiders appear compact and flat. Lynx spiders are elongated and spindly.
Watch How it Moves
– Orb weavers will be stationary in the web. Jumping spiders have quick, darting movements. Lynx spiders move more slowly when stalking.
Consider the Habitat
– Cyclosa often build webs on human structures like buildings and fences. Araneus webs are commonly found in gardens and woodlands. Menemerus are tropical jumpers. Oxyopes frequent shrubs and grass.
Use a Field Guide
– Guides like the Audubon Field Guide or National Geographic’s Field Guide to Spiders can help narrow down species based on location, web, and physical details.
Getting a clear view and taking note of key characteristics will help pinpoint exactly what kind of blue-backed spider you’ve found. With practice, identification becomes quicker and easier.
Conclusion
While multiple spider species can have blue coloration on their abdomens, the most common blue-backed spiders belong to orb weaver genera like Cyclosa and Araneus. The vibrant blue hues serve an important purpose, providing camouflage against the sky to help the spiders avoid predation. Blue backs may also regulate temperature, attract prey, or act as a warning signal. Other spiders like jumping spiders and lynx spiders can have blue spots or patterns, but orb weavers are the definitive blue-backed spiders found throughout North America. With close observation and the use of field guides, identification of these colorful spiders is straightforward. The next time you come across a web glistening with blue, take a closer peek – it just may be one of these brilliantly colored arachnids.
