It is true that search engines regularly "spider" the internet and "crawl" your site, to gather content for their search results. It is also true that there are well over 1,023,492,818 and climbing websites in existence today. Check InternetLiveStats.com for more details. With that many websites to crawl, it is impossible for the various search engines to crawl every single website regularly enough to encompass any changes you make to your site, quickly. For that reason, most major search engines have manual submission pages that you can post your URL to whenever you make changes to your site. *This should be used sparingly, as google and others will ban you from submits if you overuse the feature.