Identifying more common species of spiders, at least to the genus, is much more simple than trying to identify uncommon spiders. Some genera of spiders have distinct characteristics; examples include formation of the eyes, markings, web shape and reproductive traits. When trying to identify a spider based on a picture, it is important to remember that you should also take note of where you found the spider, what type of web it was in and any other details regarding its capture. These details can help you during the identification process. Also, remember that some spiders are so similar it may take a microscope to correctly identify them.
Collect the spider with a tissue, ensuring that you don't crush the spider. Gently place the tissue and your fingers around the spider and pick it up.
Place the spider in a clear, lidded container with a coin, ruler or a six-inch section of measuring tape on the floor of the container. The coin or measuring device is to determine the length and leg span of the spider.
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Observe the spider's characteristics. Look for distinct markings on the abdomen or cephalothorax (the cephalothorax is the first body segment that the legs are attached to, commonly known as the "head". Unlike insects that have three main body sections, spiders and arachnids have two -- the cephalothorax and abdomen). Look for hairs on the body or legs. Look to see if the spider has any bands on its legs. Look at the eye arrangement. Take note of all characteristics; write them down if you need to.
Look through a spider field guide or book; you may be able to find one specific to your state or region. Many of these books have pictures and detailed descriptions.
Search online for photos of spiders. To look through spider pictures from your state, consider a search like "site:.edu brown spiders of Kentucky images." You can also get more specific. Add words like "grass spiders" or "house spiders" into the search, depending on where you found the spider. The "site:.edu" pulls up only university and educational websites, often among the most reliable. You can also search websites like Arachnology.org.
Search online for venomous spiders, if you believe you have one. All spiders are venomous, but few are seriously threatening to people. Venomous spiders include the black widow, other widows, brown recluse and hobo spiders. You can look at the Centers for Disease Control and Prevention's website, CDC.gov; you can do a general Internet search; or you can do another "site:.edu" search for venomous spiders in your state. These websites show pictures and tell you how to identify venomous spiders. More likely than not, you haven't captured a venomous spider.
Take the collected spider into your local or state university's entomology department or into a local university cooperative extension. Experts will be able to identify your spider correctly; you may not be able to find an appropriate photograph.
All spiders can bite, although almost all of the North American species are non-threatening to people. Use caution.