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Have you ever searched for something on Google and wondered, "How does it know where to look?" The answer is "web crawlers," which search the web and index it so that you can find things easily online. When you search using a keyword on a search engine like Google or Bing, the site sifts through trillions of pages to generate a list of results related to that term. How exactly do these search engines have all of these pages on file, know how to look for them, and generate these results within seconds? The answer is web crawlers, also known as spiders. These are automated programs (often called "robots" or "bots") that "crawl" or browse across the web so that they can be added to search engines. These robots index websites to create a list of pages that eventually appear in your search results. Crawlers also create and store copies of these pages in the engine's database, which allows you to make searches almost instantly.
It's also the reason why search engines often include cached versions of sites in their databases. So, how do crawlers pick which websites to crawl? Well, the most common scenario is that website owners want search engines to crawl their sites. They can achieve this by requesting Google, Bing, Yahoo, or another search engine to index their pages. This process varies from engine to engine. Also, search engines frequently select popular, well-linked websites to crawl by tracking the number of times that a URL is linked on other public sites. This is a file containing all the links and pages that are part of your website. It's normally used to indicate what pages you'd like indexed. Once search engines have already crawled a website once, they will automatically crawl that site again. The frequency varies based on how popular a website is, among other metrics. Therefore, site owners frequently keep updated site maps to let engines know which new websites to index.
What if a website doesn't want some or all of its pages to appear on a search engine? For example, you might not want people to search for a members-only page or see your 404 error page. This is where the crawl exclusion list, also known as robots.txt, comes into play. This is a simple text file that dictates to crawlers which web pages to exclude from indexing. Another reason why robots.txt is important is that web crawlers can have a significant effect on site performance. Because crawlers are essentially downloading all the pages on your website, they consume resources and can cause slowdowns. They arrive at unpredictable times and without approval. If you don't need your pages indexed repeatedly, then stopping crawlers might help reduce some of your website load. Fortunately, most crawlers stop crawling certain pages based on the rules of the site owner. Under the URL and title of every search result in Google, you will find a short description of the page. These descriptions are called snippets. You might notice that the snippet of a page in Google doesn't always line up with the website's actual content. This is because many websites have something called "meta tags," which are custom descriptions that site owners add to their pages. Site owners often come up with enticing metadata descriptions written to make you want to click on a website. Google also lists other meta-information, such as prices and stock availability. This is especially useful for those running e-commerce websites. Web searching is an essential part of using the internet. Searching the web is a great way to discover new websites, stores, communities, and interests. Every day, web crawlers visit millions of pages and add them to search engines. While crawlers have some downsides, like taking up site resources, they're invaluable to both site owners and visitors.
Black widow spiders are known for their potent venom, which is up to 15 times stronger than a rattlesnake's, but bites are rarely fatal to humans. These spiders prefer dark, dry places like wood piles, barns and basements, and are generally not aggressive unless disturbed. Female black widows are distinguished by their shiny black bodies and red hourglass markings, and they are more likely to bite than males. David Nelsen, an associate professor of biology at Southern Adventist University in Collegedale, Tennessee remembers sprawling on his belly under the slide at the elementary school playground, in search of the tangled web of the Latrodectus hesperus, aka the western black widow spider. He'd know it when he saw it, the sticky silk threads spun in messy snarls characteristic of such wondrous creatures. If he nudged the web with his long forceps in just the right place, he could catch the spider before it escaped and tuck it into one of his plastic bags where dozens of other black widows lay in wait.
It didn't matter that one bite from the shiny black spider could send his muscles into painful spasms within minutes; That even if he went to the emergency room writhing in pain, doctors likely wouldn't have the antivenom to treat him; That he'd have to wait out the burning, throbbing, and involuntary muscle contractions for hours or possibly days until his symptoms eventually subsided. They played a starring role in his doctoral research, and he wanted to understand them better. So how did the black widow spider get its name, and why do people find them so scary? Nelsen chose the black widow for his research because "they're mysterious and dangerous," he says. Indeed, the black widow is one of the deadliest spiders in the world, according to Encyclopedia Britannica. About 2,600 black widow bites are reported to the U.S. National Poison Data System each year. But its name comes not so much from the spider's ability to kill humans, but from a cannibalistic behavior noticed in the species during copulation.
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