Recognizing Venomous Spiders
Rest assured that most spiders aren't dangerous. All spiders have a small amount of venom, but many spiders that look decidedly creepy won't actually cause a dangerous reaction when they bite. If you’re in North America and the spider you see can't clearly be identified as a brown recluse, black widow, or hobo spider, it probably won't cause you any harm. In fact, having some spiders around is beneficial, because they trap insects that actually can harm you. Keep in mind that most spiders are non-aggressive biters, meaning they will typically only bite if they feel threatened. If you live outside of North America, consult one of several widely available guides to learn about the dangerous spiders in your locale.
Determine if you've got a brown recluse on your hands. When it comes to spider identification, the first one you'll want to rule out (if you live in North America) is the brown recluse. This is the most dangerous spider in the U.S., and is found most often in the Southeast and Midwest. A brown recluse bite can cause infection, and medical attention is usually necessary. Here's how to know if the spider is a recluse: Look for the tell-tale violin shape on its back. The brown recluse spider has a medium brown body and legs, with a slightly darker brown violin shape on the back. Count the eyes; if there are 6, it might be a recluse. The recluse's eyes are arranged in pairs, with 1 pair in front and a pair on either side. Analyze where you found the spider. If it's in a warm, dry place, like a shed or woodpile, it might be a recluse. The brown recluse is known to be aggressive, rather than hanging back when it encounters someone.
See if it's a black widow. Black widows are common in the Southern and Western states in the U.S. When they bite, they release a neurotoxin that causes pain and other severe effects, especially in children and the elderly. Here's what to look for: These aptly-named spiders have a shiny black body, with a bright red hourglass shape on the bulbous abdomen. Black widows commonly dwell in woodpiles and under eaves. They have long legs that taper into points.
Find out if it's a hobo spider. This is the third major venomous spider in the U.S. It's commonly seen in the Pacific Northwest. It is not quite as dangerous as a brown recluse or black widow, but medical attention is still necessary when a bite occurs. Hobo spiders are brown with chevron-shaped yellow markings. They make their webs in cracks, corners, and holes, and can be found in woodpiles and other sheltered places.
Evaluating Its Appearance and Behavior
Notice the spider's coloring. Take note of the spider's primary color—usually a shade of brown (like the mouse spider), black (like the cobweb spider), or grey. Also look for color variations (as with the jumping spider), subtle or not so subtle, that will give you a clue as to what type of spider you're looking at. Does the spider have spots, or coloration in a striking shape like the brown widow’s hourglass marking or the brown recluse's classic violin? Are the legs the same color as the body? Note that many species of spider have similar coloring, so looking at this characteristic alone won't be enough to give you a definitive answer.
Look at the shape of the legs. If possible, examine the spider’s 8 legs a bit more closely. Look at the size of the legs in proportion to the body. Some spiders have long, thin legs (like the yellow sac), while others have stocky, thick legs (like the wolf spider). Some spiders (such as the lynx spider) have tiny hairs on their legs, while others have spikes (such as the funnel spider) or fur.
Examine the eyes. The number and location of eyes varies widely among different spider species. If you can, get close enough to look at the spider's eye size, count, and arrangement. It’s often easiest to take a picture of the spider, then zoom in to see the eye count and pattern. While the spitting spider is one of the exceptions, most spiders have 8 eyes, sometimes equal in size (like the nursery web spider), but other times with 2 (like the portia) or 4 of the 8 much more prominent; the eyes can also be arranged in many different patterns.
Notice the orientation of the fangs. The orientation of the fangs (either side-to-side, resembling a crab-claw, or vertically oriented, as most people commonly think of a spider) will tell you what infra-order the species belongs to. This helps rule out rather long lists of species you don't need to further consider. Most spiders with the side-to-side orientation belong to the infra-order Araneomorphae and spiders with vertical fangs belong to either infra-order Mygalomorphae or, rarely, infra-order Mesothelae. Chances are you're not dealing with infra-order Mesothelae, because only 116 species belong to this infra-order, making it the rarest infra-order of spiders. These 116 species are also restricted in terms of geography to Southeast Asia, China, and Japan, meaning you will not find these in the wild in most places in the world. This is important as an identifying feature, because, for example, while a wolf spider might be taken as a small tarantula, so use clues to tell the differences, we know that tarantulas are Mygalomorphae species and that wolf spiders belong to Araneomorphae. Thus, the fang orientation will always be a dead giveaway between those 2 species.
Estimate the size of the spider. Some spiders get a lot bigger than others, so looking at factors like body length or leg span can be helpful. Even if you can’t measure the spider exactly, try to get a general idea of its size. For example, an adult brown recluse is about the size of a U.S. quarter when its legs are spread out. The harmless wolf spider, on the other hand, is closer in size to a U.S. silver dollar when full grown.
Take a picture of the spider if you can. If you're insistent on figuring out exactly what species of spider you've spotted, take a picture of it. If possible, try to capture a picture of the spider's underside as well as the top. Since so many spiders look alike, it's impossible to know for sure what you're seeing until you compare the spider's image to those of other spiders that have been definitively classified by scientists.
Assess the spider's behavior. Some spiders are aggressive (like the Brazilian wanderer), and will start moving toward you when they notice your presence. Others hide (like the crab spider), and still others stay put instead of moving. Pay attention to any other behaviors you notice that could help you figure out what type of spider it is.
Do research to figure out what species it is. If you have a moment after taking note of the spider's appearance and habits, you can use the World Spider Catalog, the online Journal of Arachnology, and other resources put together by experts to look up the spider and compare it to pictures of different species. WikiHow is also a handy source for spider info — check out the identification guides on, for instance: Garden Spider; Barn Spider; Redback Spider; Tarantula
Considering Other Basic Factors
Make sure it's really a spider. A lot of people think spiders are insects, but they're actually very different. To make sure the creature you've encountered is actually a spider, look for the following qualities: A body divided into 2 segments. Unlike insects, which have 3 body segments, spiders only have 2. 8 legs. Don't mistake an insect's antennae for legs. No wings. No species of spider has wings. No matter how spider-like the creature may otherwise appear, if it has wings, it's not a spider.
Know what species are common in your region. If you live in New York City, you probably won’t encounter the fearsome banana spider (although it's certainly not impossible). Familiarizing yourself with the species that live in your area is a good way to narrow down the possibilities when it comes to identifying spiders. There are many excellent resources you can use to learn more about spiders. Check out a book from your library or look at spider databases online. You can search according to region.
Look at the spider's habitat. You can learn a lot from the place where you encountered the spider, as well as the web or burrow in which you found it. Answer the following questions regarding the spider’s habitat: Where did you find the spider? Was it in the bathroom, the basement, the shed, outside, etc.? Was it in a dark place (as preferred by the baboon spider), or a place with plenty of light? Was it a wet place (or even in the water, like the water spider) or a dry one? What type of web does the spider have? Is it a classic-looking spider web (the type built by orb weavers), a tunnel web (as made by the purse spider), or just a few strands of webbing that aren't neatly woven?
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